Monolithic PC audio circuit

[0002] This application is a continued prosecution application of application Ser. No. 08/934,969, filed Sep. 22, 1997 which is a continuation of application Ser. No. 08/333,451, filed Nov. 2, 1994, now abandoned.

[0003] 1. Field of the Invention

[0004] This invention relates generally to computer controlled audio systems and more particularly to an audio circuit for use with system boards and add-in cards for desktop and portable computers. The preferred embodiment of the present invention is particularly designed to be compatible with systems built primarily to run the MS-DOS, Windows, UNIX, and OS/2 operating systems, otherwise generally referred to IBM compatibles.

[0005] 2. Brief Description of the Related Technology

[0006] Typically, personal computers are manufactured with only limited audio capabilities. These limited capabilities provide monophonic tone generation to provide audible signals to the user concerning various simple functions, such as alarms or other user alert signals. The typical personal computer system has no capability of providing stereo, high-quality audio which is a desired enhancement for multimedia and video game applications, nor do they have built-in capability to generate or synthesize music or other complex sounds. Musical synthesis capability is necessary when the user desires to use a musical composition application to produce or record sounds through the computer to be played on an external instrument, or through analog speakers and in multimedia (CD-ROM) applications as well. Typical sound cards also provide MIDI interfaces and game ports to accept inputs from MIDI instruments such as keyboard and joysticks for games.

[0007] Additionally, users at times desire the capability of using external analog sound sources, such as stereo equipment, microphones, and non-MIDI electrical instruments to be recorded digitally and/or mixed with digital sources before recording or playback through their computer. To satisfy these demands, a number of add-on products have been developed. One such line of products is referred to in the industry as a sound board. These sound boards are circuit boards carrying a number of integrated circuits and other associated circuitry which the user installs in expansion slots provided by the computer manufacturer. The expansion slots provide an ISA interface to the system bus thereby enabling the host processor to access sound generation and control functions on the board under the control of application software.

[0008] Presently, the most common sound boards in the industry are the Sound Blaster, and Adlib. These boards include a monolithic FM synthesizer circuit for generating sound from data provided from system memory. Such boards also include a digital signal processing integrated circuit that carries out digital-to-analog and analog-to-digital conversions, processes commands from the host CPU under control of application software, generates control signals for the other circuits, processes MIDI data in and out, and provides data decompression on stored data. Other integrated or discrete circuits are necessary to interface with analog input or output ports, as well as separate circuits for system bus interface, among others.

[0009] These prior systems also have limited capabilities to produce audiophile quality sound, are high power consumers and are not suitable for use in system board applications where expansion slots are not utilized. Furthermore, such prior systems are not suitable for Plug-n-Play environments which require compliance with industry standard self-configuring methodology. Prior sound cards employed on-board jumper switches to provide configuration data for the host CPU.

[0010] Later versions of Sound Blaster, Sound Blaster Pro and Pro2, added stereo to sound output capabilities by providing upgraded FM synthesizer integrated circuits, stereo output jacks, stereo digital recording and playback, a separate mixer integrated circuit and a separate CD-ROM interface. The Pro DSP circuit provided record and playback at up to 44.1 KHz in mono or 22.05 KHz in stereo. The mixer allowed mixing sounds from the microphone, line-in, CD-input and the digital sound, and CD audio play in the background.

[0011] Such systems required multiple integrated circuits, did not provide Plug-n-Play compatibility, had limited mixing capabilities, were large power consumers, and were only useable in expansion-slot configuration. Furthermore, the synthesizer function was limited in the number of voices that could be processed and was FM-based, as distinguished from more advanced wave table synthesizers. Such systems had limited mixing, panning and control functions for providing effects and did not provide individual voice effects.

[0012] Furthermore, such prior systems did not provide a local memory interface for temporarily storing sound data, but required system memory access for all data transfers. This limitation required frequent DMA or programmed I/O cycles to provide sound data for recording and playback, thereby imposing significant processor overhead.

[0013] These prior systems were also limited by an 8-bit sample size which limited dynamic range to 256 steps, and produced more pronounced aliasing than larger bit sample techniques. The latest Sound Blaster product, designated Sound Blaster 16 ASP, provided 16-bit playback and record sampling and 44.1 KHz stereo sampling rate. This latest version was a multiple chip embodiment which included a wavetable synthesizer circuit or chip, a dedicated processor circuit or chip, a separate bus interface chip, separate ADC and DAC circuits, an analog amplifier and other associated circuitry on a expansion board. While this system offered enhanced programmability, higher sampling rates and a larger sample size, it was nevertheless a multiple chip embodiment, suitable primarily for expansion slot use and was a high power consumer. This latest version offered no local memory, was not Plug-n-Play compatible and included a dedicated processor to process application and synthesis instructions. The wavetable option required a separate daughter board which included, among other things, a four megabyte ROM for storing wavetable data.

[0014] Another prior art system was offered by Advanced Gravis and Forte under the name Ultrasound. This system was another expansion slot sound board embodiment which incorporated into one chip the synthesizer, MIDI and game interfaces, DMA control and Adlib Sound Blaster compatibility logic. In addition to this ASIC the Ultrasound card included on-board DRAM (1 megabyte) for wavetable data; an address decoding chip; separate analog circuitry for interfacing with analog inputs and outputs; a separate programmable ISA bus interface chip; an interrupt PAL chip; and a separate digital-to-analog/analog-to-digital converter chip.

[0015] None of the prior systems provided single chip implementation of the synthesizer, data compression/decompression, DAC/ADC, mixer, analog interface, system bus interface, interrupt and DMA control, and compatibility features. Multiple chip embodiments have obvious limitations relating to cost, size and speed, as well as power consumption.

[0016] Combining all the functions required and desired in a single chip embodiment, while avoiding unwanted noise and other signal denigration has been one limitation or obstacle to full integration. Another obstacle has been the unavailability of an efficient architectural design for the single chip embodiment. Still another obstacle has been the lack of an efficient way to control individual modules and to manage power to such a fully integrated system.

[0017] Furthermore, each of the prior systems had one or more limitations on compatibility with various industry standard software and/or hardware. None of the prior systems provided optional Plug-n-Play compatibility. The prior art systems either utilized the host CPU extensively for synthesizer functions, or provided a dedicated synthesizer processor thereby either increasing cost or slowing down the operation by requiring extensive host CPU overhead.

[0018] The system of the present invention solves each of these problems in a number of unique and efficient ways. The system of the present invention also provides enhanced capabilities heretofore unavailable.

[0019] The present invention provides a monolithic PC audio integrated circuit which includes a system bus interface which is AT ISA-compatible, a system control module providing Plug-n-Play compatibility, system control registers, system control logic and interrupt generation and compatibility functions for existing PC audio software. The system of the present invention further includes a coding and decoding module for providing analog-to-digital and digital-to-analog conversion, data compression, and analog mixing and muxing of audio signals. A wavetable synthesizer module and a MIDI and game port module are also provided. The circuit of the present invention further provides a local memory module which enables the circuit to interface with external DRAM, ROM and serial EEPROM for Plug-n-Play compatibility. Data transfer on the circuit of the present invention is facilitated via an on-chip register data bus and control circuit. The circuit of the present invention further includes noise reduction attributes, a facility for external address decoding, buffered input and output capability, as well as other features which conserve on-chip resources.

[0020] A better understanding of the present invention can be obtained when the following detailed description of the preferred embodiment is considered in conjunction with the following drawings, in which:

[0021] FIG. 1 is a schematic architectural overview of the basic modules of the circuit C;

[0022] FIG. 2 is a schematic illustration of the physical layout of circuit C;

[0023] FIGS. 3A and 3B is a table summarizing pin assignments for the circuit C;

[0024] FIGS. 4A and 4B is an alternative layout diagram for the circuit C; noise and a primary clock signal employed by the circuit C;

[0025] FIG. 5 is a table summarizing pin assignments for the circuit C grouped by module;

[0026] FIG. 6 is a schematic illustration of a typical full-featured implementation of a PC audio circuit C with associated circuits, buses and interconnections;

[0027] FIG. 7 is table summarizing pin assignments and functions that relate to local memory control;

[0028] FIGS. 8A and 10B comprise a table of register mnemonics with indexes and module assignments where appropriate;

[0029] FIG. 11 is a schematic diagram illustrating an example of multiplexing circuitry;

[0030] FIG. 12 is a block diagram schematic illustration of the system control module of the circuit C;

[0031] FIG. 13 is a schematic block diagram of the circuit C including modular interfaces to the register data bus;

[0032] FIG. 14 is a schematic diagram of implementation detailed for the register data bus;

[0033] FIG. 14ais a schematic diagram of a portion of the ISA bus interface circuitry;

[0034] FIG. 15 is a timing diagram illustrating worse case ISA-bus timing for the circuit C;

[0035] FIG. 16 is a timing diagram relating to buffered input and outputs for the circuit C;

[0036] FIG. 16ais a schematic diagram of a portion of the emulation logic for the circuit C;

[0037] FIG. 16bis a schematic block diagram of circuit access possibilities for application software and emulation TSR programs;

[0038] FIG. 17 is a schematic illustration of the Plug-n-Play state machine included within the circuit C;

[0039] FIG. 18 is a timing diagram relating to reading serial EEPROM data from external circuitry relating to Plug-n-Play compatibility;

[0040] FIG. 19 is a schematic illustration of a circuit for facilitating PNP data transfer from external circuitry to the circuit C via the register data bus;

[0041] FIG. 20 is a schematic illustration of a linear feed back shift register necessary to implement an initiation key for access to Plug-n-Play registers;

[0042] FIG. 21 is a flow chart illustrating the manner in which the Plug-n-Play circuitry associated with the circuit C transitions from isolation mode to either configuration mode or sleep mode;

[0043] FIG. 22 is a table summarizing resources required for programming the Plug-n-Play serial EEPROM;

[0044] FIG. 23 is a table providing data on all interrupt-causing events in the circuit C;

[0045] FIG. 24 is a schematic illustration of external oscillators and stabilizing logic associated therewith utilized by the circuit C;

[0046] FIGS. 24A-1 and 24A-2 is a schematic illustration of logic and counter circuits associated with various low power modes of the circuit C;

[0047] FIGS. 24B-1 and 24B-2 is a flow chart illustrating the response of circuit C to suspend mode operation;

[0048] FIGS. 24C-1, 24C-2 and 24C-3 is a flow chart illustrating the various register-controlled low power modes of the circuit C;

[0049] FIG. 25 is a schematic illustration of details of the clock oscillator stabilization logic of FIG. 24;

[0050] FIG. 26 is a table describing events which occur in response to various power conservation modes enabled via the status of bits in register PPWRI contained within the circuit C;

[0051] FIG. 27 is a timing diagram showing the relationship between various power conservation modes and signals and clock signals utilized by the circuit C;

[0052] FIGS. 28A and 28B is a table summarizing pins associated with the system bus interface included in the circuit C;

[0053] FIG. 29 is a block diagram schematically illustrating the basic modules which comprise the local memory control module of the circuit C;

[0054] FIG. 30 is a block diagram schematically illustrating the master state machine associated with the local memory control module of the circuit C;

[0055] FIG. 31 is a timing diagram illustrating the relationship of suspend mode control signals and a 32 KHz clock signal utilized by the circuit C;

[0056] FIG. 32 is a state diagram schematically illustrating refresh cycles utilized by the circuit C during suspend mode operation;

[0057] FIG. 33 is a timing diagram for suspend mode refresh cycles;

[0058] FIG. 34ais a timing diagram for 8-bit DRAM accesses;

[0059] FIG. 34bis a timing diagram for 16-bit DRAM accesses;

[0060] FIG. 34cis a timing diagram for DRAM refresh cycles;

[0061] FIG. 35 is a timing diagram illustrating how real addresses are provided from the circuit C to external memory devices;

[0062] FIG. 36 is a schematic block diagram of a control circuit for local memory record and playback FIFOs;

[0063] FIG. 37 is a diagram illustrating the relationship between data stored in system memory and interleaved in local memory via the circuit C;

[0064] FIG. 38 is a table describing data transfer formats for 8 and 16-bit sample sizes under DMA control;

[0065] FIG. 39 is a schematic block diagram illustrating circuitry for implementing interleaved DMA data from system memory to local memory via the local memory control module of the circuit C;

[0066] FIG. 40 is a schematic block illustration of the game port interface between external devices and the circuit C;

[0067] FIG. 41 is a schematic block illustration of a single bit implementation for the game input/output port of the circuit C;

[0068] FIG. 41ais a diagram illustrating input signal detection via the game port of the circuit C;

[0069] FIG. 42 is a schematic block diagram illustrating the MIDI transmit and receive ports for the circuit C;

[0070] FIG. 43 is a timing diagram illustrating the MIDI data format utilized by the circuit C;

[0071] FIG. 44 is a block diagram of the various functional blocks of the CODEC module of the present invention;

[0072] FIG. 45 is a schematic of the preferred embodiment of the left channel stereo mixer of the present invention;

[0073] FIGS. 45A-1 and 45A-2 is a table of gain and attenuation values.

[0074] FIG. 46 is a diagram of a partial wave form indicating signal discontinuities for attenuation/gain changes;

[0075] FIG. 47 is a block diagram showing zero detect circuits for eliminating “zipper” noise.;

[0076] FIG. 48 is a block diagram showing clock generation functions in the present invention;

[0077] FIG. 49 is a block diagram of serial data transfer functions of the present invention;

[0078] FIG. 49ais a block diagram of the serial transfer control block;

[0079] FIG. 50 is a block diagram showing internal and external data paths and interfacing with external devices, supported by the present invention;

[0080] FIG. 51 is a block diagram of the digital to analog converter block of the present invention;

[0081] FIG. 52 is a block diagram of the front end of the digital to analog converter block of the present invention;

[0082] FIGS. 53a-53fare graphs showing outputs of various stages of the DAC block, including frequency response;

[0083] FIG. 54 shows six graphs representing outputs and frequency response of various stages of the DAC block;

[0084] FIG. 55 is a schematic representation of the Interp.1 block, phase 1 of FIG. 52;

[0085] FIG. 56 is a schematic representation of the Interp.1 block, phase 2 of FIG. 52;

[0086] FIG. 57 is a schematic representation of the Interp.2 block of FIG. 52;

[0087] FIG. 58 is a graph of the frequency response of the Interp.2 block of FIG. 52;

[0088] FIG. 59 is a graph representing the in-band rolloff of the Interp.2 block of FIG. 52;

[0089] FIG. 60 is a schematic representation of an embodiment of the Interp.3 block of FIG. 52;

[0090] FIG. 61 is a schematic representation of another embodiment of the Interp.3 block of FIG. 52;

[0091] FIG. 62ais a graph of the frequency response of the Interp.3 block of FIG. 52;

[0092] FIG. 62bis a graph of the passband rolloff of the Interp.3 block of FIG. 52;

[0093] FIG. 63 is a schematic representation of the noise shaper block of FIG. 52;

[0094] FIG. 64 is a signal flow graph (SFG) of the noise shaper block in FIG. 52;

[0095] FIG. 65 is a plot of the poles and zeros in the s plane for the noise shaper block of FIG. 52;

[0096] FIG. 66 is a plot of the transfer function magnitude of the noise shaper block of FIG. 52;

[0097] FIG. 67 is a plot of the poles and zeros in the z plane of the noise shaper block of FIG. 52;

[0098] FIG. 68 is a graph of the transfer function of the noise shaper filter of FIG. 52;

[0099] FIG. 69 is a plot of the ideal and realizable zeros of the noise filter block of FIG. 52;

[0100] FIG. 70 is a plot comparing two embodiments of noise transfer functions for the noise shaper block of FIG. 52;

[0101] FIG. 71 is a plot of the noise and signal transfer functions of the noise shaper block of FIG. 52;

[0102] FIG. 72 is a plot of the signal transfer function magnitude in phase and passband of the noise shaper block of FIG. 52;

[0103] FIG. 73 is a graph of the group delay (sec.) of the noise shaper block of FIG. 52;

[0104] FIG. 74 is a graph of the constant attenuation/gain contours of various embodiments of the noise shaper block of FIG. 52;

[0105] FIG. 75 plots A_maxversus noise gain k for an embodiment of the noise shaper block of FIG. 52; and

[0106] FIG. 76 is a graph of an embodiment of the noise gain k versus band width for g=−90 dB of the noise shaper block of FIG. 52.

[0107] FIG. 77 is a graph showing the impulse response of the D/A FIR filter;

[0108] FIG. 78 is a graph showing the frequency response of the D/A FIR filter;

[0109] FIG. 79 schematically illustrates one embodiment of the D/A conversion circuit of the present invention;

[0110] FIGS. 80 and 81 schematically illustrate another embodiment showing the differential D/A conversion circuit of the present invention;

[0111] FIG. 82 is a block diagram of the CODEC ADC of the present invention;

[0112] FIG. 83 is a block diagram of the front end of the CODEC ADC;

[0113] FIG. 84 is a graph illustrating the sigma-delta modulator output spectrum-range and phase for the ADC of the present invention;

[0114] FIG. 85 is a graph illustrating the sigma-delta modulator output spectrum, in detail;

[0115] FIG. 86 is a graph illustrating the output spectrum of the sinc6 Decim.1 filter output;

[0116] FIG. 87 is a graph illustrating the output spectrum of the half-band Decim.2 filter output;

[0117] FIG. 88 is a graph illustrating the output spectrum of the 16-bit Decim.3 filter output;

[0118] FIG. 89 is a block diagram of the Decim.1 filter;

[0119] FIG. 90 graphically illustrates the frequency response of the Decim.1 filter;

[0120] FIG. 91 graphically illustrates a detailed frequency response of the Decim.1 filter;

[0121] FIG. 92 is a block diagram of the half-band Decim.2 filter-direct form;

[0122] FIG. 93 is a block diagram of the half-band Decim.2 filter-transposed form;

[0123] FIG. 94 graphically illustrates the frequency response of the Decim.2 filter;

[0124] FIG. 95 is a detailed frequency response graph of the Decim.2 filter;

[0125] FIG. 96 is a block diagram of the compensation filter of the CODEC D/A conversion circuitry;

[0126] FIG. 97 graphically illustrates the frequency response of the Decim.3 filter;

[0127] FIG. 98 graphically illustrates, in detail, the frequency response of the Decim.3 filter;

[0128] FIG. 99 graphically illustrates the compensator circuit frequency response (un-compensated);

[0129] FIG. 100 graphically illustrates the total frequency response of the compensator circuitry in passband (uncompensated); and

[0130] FIG. 101 graphically illustrates the total frequency response of the compensator in passband (compensated).

[0131] FIG. 102 is a block diagram of the synthesizer module of the present invention;

[0132] FIG. 103 illustrates signal flow in the synthesizer module of the present invention;

[0133] FIGS. 104a-104fare graphs illustrating addressing control options in the synthesizer module of the present invention;

[0134] FIGS. 105a-105eare graphs illustrating volume control options in the synthesizer module of the present invention;

[0135] FIGS. 106aand 106bare graphs of low frequency oscillator waveforms available for the synthesizer module of the present invention;

[0136] FIGS. 107A and 107B is an architectural diagram of an address controller of the synthesizer module of the present invention;

[0137] FIGS. 108aand 108bare timing diagrams of the operations performed by the address controller of FIG. 107;

[0138] FIGS. 109A-109C is an architectural diagram of a volume controller of the synthesizer module of the present invention;

[0139] FIG. 110 is a timing diagram of the operations performed by the volume controller of FIG. 109;

[0140] FIG. 111 is an architectural drawing of the register array of the synthesizer module of the present invention;

[0141] FIG. 112 is a timing chart of the operations of the register array in FIG. 111;

[0142] FIG. 113 is an architectural drawing of the overall volume control circuitry of the synthesizer module of the preset invention;

[0143] FIG. 114ais a logic diagram of a comparator illustrated in FIG. 113;

[0144] FIG. 114bis a timing chart of the operations of the comparator in FIG. 114a;

[0145] FIG. 115 is an architectural drawing of the LFO generator of the synthesizer module of the present invention;

[0146] FIGS. 116A and 116B is an architectural diagram of the signal path of the synthesizer module of the present invention;

[0147] FIG. 117 is a timing diagram of the operations performed by the signal path of FIG. 116;

[0148] FIG. 118 is an architectural diagram of accumulation logic of the synthesizer module of the present invention;

[0149] FIG. 119 is a timing diagram of the operations performed by the accumulation logic of FIG. 118; and

[0150] FIGS. 120A-120C is a timing diagram of the overall operations performed by the synthesizer module of the present invention.

[0151] FIG. 121 is a block diagram of a signal processing circuit employed in an integrated circuit of the present invention.

[0152] FIG. 122 is a schematic illustration of a block diagram of a signal processing circuit employed in an integrated circuit of the present invention.

[0153] The following description sets forth the preferred embodiment of a monolithic PC audio circuit, including system architecture, packaging, power management, system control, timing and memory interfacing, as well as significant implementation details. Various options for circuits suitable for use with the present invention are disclosed in the following United States patent applications, the contents of which each are incorporated herein by reference. An alternative technique for reducing power consumed by clock driven circuits is described in U.S. patent application Ser. No. 07/918,622, entitled “Clock Generation Capable of Shut Down Mode and Clock Generation Method,” assigned to the common assignee of the present invention. Throughout the specification where it is required to affect the status of single bits within a register or field, the preferred method and apparatus for performing such single-bit manipulations are set forth in U.S. patent application Ser. No. 08/171,313, filed Dec. 21, 1993, and entitled “Method and Apparatus for Modifying the Contents of a Register Via a Command Bit,” assigned to the common assignee of the present invention.

[0154] Throughout this specification where reference is made to various timers, gating and other control logic, unless otherwise specified, the precise logic circuit implementation details may not be provided. In such instances the implementation details are considered trivial given the state of the art in computer-assisted logic design and layout techniques available for VLSI logic circuit design.

[0155] Under the current state of the art, such details are implemented from selectable, programmable logic arrays or blocks of standardized logic circuits made available for such purposes on VLSI circuits. Timers, for example, can be readily implemented by providing a clock signal derived from external oscillator signals to an appropriate logic circuit. An 80 microsecond clock signal can be provided by dividing the 16.9 MHz oscillator signal by 1344 for example. The generation of control signals which respond to the status of various bits of data held in registers throughout the circuit C is a simple matter of providing control inputs to blocks or arrays of gate circuits to satisfy the required input/output or truth table requirements. Consequently, these details, where not considered significant to the claimed invention, need not and have not been provided since such matters are clearly within the level or ordinary skill in the art.

[0156] Referring now to FIG. 1, an architectural overview of the basic modules of the circuit C is provided. The circuit C includes five basic modules: a system control module 2; a coder-decoder (CODEC) module 4; a synthesizer module 6; a local memory control module 8; and MIDI and game port module 10. These modules are formed on a monolithic integrated circuit. A register data bus 12 provides communication of data between modules and between circuit C and a system bus interface 14. Timing and control for circuit C is provided by logic circuits within system control module 2 operating in response to clock signals provided by one or both oscillators 16 and 18 depending upon the particular system requirement. Control of circuit C is generally determined by logic circuits included within module 2 which are in turn controlled by the state of various registers and ports provided throughout the circuit C.

[0157] FIG. 1 is a functional block diagram and does not correspond directly to a physical layout for the integrated circuit embodiment. Various circuits, interconnects, registers etc. which provide or facilitate the functions specified in FIG. 1 may be formed in several locations spread throughout the integrated circuit as needed or as dictated by manufacturing processes, convenience or other reasons known to those of ordinary skill in the art. The circuit of the present invention may be fully integrated using conventional integration processes such as are well known in the industry. The circuit of the present invention is packaged in a 160 pin plastic quad flat pack (PQFP), as will be described in more detail below.

[0158] A. Physical Layout Features and Noise Reduction

[0159] It is a feature of the present invention that the physical layout of the various modules and the pin-out arrangement have been designed to isolate analog circuits and inputs/outputs from the noisier digital circuitry and pins. Referring now to FIG. 2, an example of the desired physical layout relationship among various portions or modules of the circuit C is schematically illustrated. To minimize digitally induced noise in analog circuits, the most noise sensitive elements of circuit C, e.g., those associated with the analog aspects of the CODEC, specifically the mixer block, are located near the circuit edge opposite the largely digital local memory control and synthesizer modules.

[0160] To further isolate digitally induced noise in the analog circuitry, the pin-out arrangement of the package isolates analog mixer input and output pins in a group 20 as far removed as possible from the noisiest digital output pins and clock inputs, which are located on the opposite side 22. Furthermore, the most sensitive pin group 20 is flanked by less noisy inputs in regions 26 and 28. Representative pin assignments are given in FIGS. 3A and 3B where pin names correspond to industry standard designations, such as the ISA Plug-n-Play specification, version 1.0, May 28, 1993, available from Microsoft Corporation and the industry standard ISA bus specification as set forth in AT Bus Design by Edward Solari, published by Annabooks, San Diego, Calif.; ISBM 0-929392-08-6, the contents of which are incorporated by reference herein. An alternative pin assignment is provided in FIGS. 4A and 4B, which likewise maintains the desired physical relationship among the various modules.

[0161] Since it is a feature of the present invention to provide compatibility with existing standard or popular hardware and software such as the ISA Plug-n-Play specification, AdLib, Sound Blaster and Graves Forte Ultrasound applications, references throughout this application to certain signal and register mnemonics such as ISA, PNP, AdLib, GUS, generally refer to compatible configurations for the circuit of the present invention. It also should be noted that a # sign following mnemonics for signals, or bit status flags and the like, indicates such are active low.

[0162] Referring now to FIGS. 3A and 3B, analog pins generally include those in the range of 96 through 113, including a plurality of analog power (AVCC) and ground (AVSS) pins. It is a noise reduction feature of the present invention to provide individual VSS and VCC pins for the majority of individual analog pins. Pins 82-95 and 114 are less noisy inputs. Other layout features include placing the external oscillator pins XTAL1[I,O] and XTAL2[I,O] near the clock block of the system control module. This system control module clock block should also be placed near the CODEC clock block 30. It is also important that all 16.9 MHz clocks used throughout the circuit C are implemented to minimize the skew between them. Minimizing internal clock skew is important for timing purposes as well as noise reduction in the present circuit.

[0163] It is a feature of the present invention to minimize noise in the analog signals ensuring that analog sampling and digital circuit activity be clocked independently. In the preferred embodiment, separate analog and digital clock signals with different frequencies are provided from a common oscillator. The analog clock signal is not derived from the digital or vice versa, so there is no defined phase relationship between the two. Furthermore, an analog clock skewing circuit is provided to reduce the possibility that digital and analog clock driven events overlap.

[0164] Referring now to FIG. 5, further explanation of the pin assignments according to a general functional group is given. Those pins associated with System Control Module 2 are listed under that heading with a mnemonic and number of pins provided. Likewise, those pins associated with the CODEC, local memory and ports and miscellaneous function appear under those headings respectively. Note here, as well as elsewhere in this specification, reference to “CD” pins or functions, such as CD_DRQ, should be considered equivalent to “EX,” such as EX_DRQ which generically designates a pin of function associated with an external device.

[0165] B. Typical System Implementation

[0166] Referring now to FIG. 6, a typical full-featured implementation of a PC audio circuit C with associated circuits, buses and interconnections is described. The configuration of FIG. 6 is exemplary of how the circuit C would be utilized in a PC audio card, taking advantage of all available RAM and EPROM resources and being fully compatible with the ISA Plug-n-Play specification.

[0167] In FIG. 6, the circuit C is interfaced to host computer system (not shown) via system bus interface module 14 and the industry standard AT/ISA system control, address and data connections. These include: system data (SD); system address (SA); system byte high enable (SBHE); interrupt request (IRQs); input/output channel check (IOCHCK); direct memory access request (DRQ) and acknowledge (DAK); input/output read (IOR); input/output write (IOW); reset; address enabled (AEN); terminal count (TC); input/output channel ready (CHRDY); and input/output chip select 16 (IOCS16). These connections provide standard communication and control functions between the circuit C and the host computer system.

[0168] In a typical embodiment, the following input/output lines and associated circuitry and/or devices are interfaced to the CODEC module 4. Provision is made for four sets of stereo inputs via standard jacks, 42, and a stereo analog output (line out L, R) 44 with external stereo amplifier 46 and jacks 48. A monophonic microphone/amp input 50 and monophonic output 52 via external amplifier 54 are provided. An external capacitance, resistance circuit 56 is provided for deriving reference bias current for various internal circuits and for providing isolation capacitance as required. A general purpose, digital two-bit flag output 60, controlled by a programmable register, is provided for use as desired in some applications. Game/MIDI ports 62 include 4-bit game input 65, 4-bit game output 66 and MIDI transmit and receive bi-directional interface 68.

[0169] The system control external connections include the 16.9344 MHz and 24.576 MHz clocks 16 and 18 and a 32 KHz clock or suspend input 70. Input 70 is used for memory refreshing and power conservation and is multiplexed with other signals as described in detail elsewhere in this application.

[0170] The interface for local memory control module 8 includes frame synchronize (FRSYNC) and effect output 72 which is used to provide a synchronizing clock pulse at the beginning of each frame for voice generation cycles and to provide access to an optional external digital signal processor 74 which may be used to provide additional special effects or other DSP functions.

[0171] Plug-n-Play chip select 76 enables an external EPROM 78 for providing configuration data over a 3-bit data bus 80 during system initialization sequences. For data and address communication between local memory control module 8 and external memory devices an external 8-bit data bus 82 and an 8-bit address bus 84 is provided.

[0172] ROM chip select 83 and 2-bit ROM address output 85 are used to address one-of-four, two-megabyte by sixteen bit EPROMs 86 which are provided for external data and command sequence storage, as described in more detail elsewhere in this specification EPROMs 86 interface with circuit C via 4-bit output enable 88 which is a one-of-four select signal multiplexed with ram column address strobe 90 to conserve resources. One aspect of addressing for EPROMs 86 is provided by a 3-bit real address input 92. The address signals on line 92 are multiplexed with multiplexed row-column address bits for DRAM cycles provided on DRAM input 94. Pin 96 of circuit C (MD[7:0]) is an 8-bit bidirectional data bus which provides data bits for DRAM cycles via data [7:0] lines 98. Pin 96 is multiplexed in ROM cycles as real address bits 18:11 to EPROMs 86 and input data bits 7:0 (half of RD 15:0) to circuit C. Data communication from EPROMs 86 is via 16-bit data output 100 (RD[15:0]) which is split and multiplexed into circuit C via 8-bit buses 82 and 84 during ROM cycles. EPROM data input is carried over bidirectional line 96 and bidirectional line 102 (RD[15:8]) during ROM cycles. Line 102 also provides 8-bit ROM addressing (RLA[10:3]) during multiplexed ROM address and data transfer cycles. Line 102 also is multiplexed to provide row-column address bits (MA[10:3]) for DRAM cycles.

[0173] Output 104 is a ROM-address hold signal used to latch the state of 16-bit ROM addresses provided via outputs 96 and 102, buses 82 and 84 and 16-bit address input line 106 during ROM accesses by the circuit C. A 16-bit latch 108 is provided to latch ROM addresses in response to the ROM-address hold signal.

[0174] The circuit C supports up to four, 4-megabyte by 8-bit DRAMS 110 used for local data storage. Circuit C interfaces with DRAMS 110 via various address, data and control lines carried over two 8-bit buses 84 and 86, as described above. Row address strobing is provided via RAS output pin 112. Output 112 is provided directly to the RAS inputs of each DRAM circuit 110. For clarity, in FIG. 6, output 112 is also shown as providing the write enable (WE) output control signal which is provided to the write enable input of each DRAM circuit 110. In the preferred embodiment, the write enable output is provided on a separate output pin (see FIGS. 3A and 3B) from circuit C. DRAM column address strobe (CAS[3:0]) is provided via BKSEL[3:0] output pin 114 during DRAM cycles. 3-bits of DRAM row and column addressing are provided via output 116, and an additional eight address bits are multiplexed via bidirectional pin 102, bus 84 and DRAM input 118 during DRAM cycles. A summary of all local memory interface terminals is provided in FIG. 7.

[0175] Referring again to FIG. 6, the circuit C provides seven interrupt channels 130 from which up to three interrupts can be selected. In the preferred embodiment, two interrupts are used for audio functions and the third is used for the CD-ROM or other external device. Also shown at line 130 (a group of eight lines) is the ISA standard IOCHCK output, which is used by the circuit C to generate non-maskable interrupts to the host CPU.

[0176] The circuit of the present invention provides general compatibility with Sound Blaster and AdLib applications. When running under MS-DOS a terminate and stay resident (TSR) driver sequence must be active with the host CPU to provide compatibility. One such driver sequence is that provided by Ultrasound and called Sound Board Operation System (SBOS). When application software, typically a game, sends a command to a register in circuit C designated as a Sound Blaster or AdLib register, the circuit C captures it and interrupts the processor with the IOCHK pin. The non-maskable interrupt portion of the SBOS driver then reads the access and performs a software emulation of the Sound Blaster or AdLib function.

[0177] The circuit C also provides six DMA channels 136 and DMA acknowledge lines 138 from which three DMA functions can be selected. The three DMA functions include: wave-file record transfers and system-memory transfers; wave-file playback transfer; and a DMA channel required by the external CD-ROM interface. The availability of local memory DRAMs 110 and the provision of large first-in/first-out data registers in the DRAMs, as is described herein below, reduces the requirement for wave-file DMA functions, and in some instances can eliminate the need for wave-file DMA channels altogether.

[0178] Referring to FIG. 6, for use in those systems which include a compact disc drive, the circuit C provides necessary signals or hooks to facilitate the use of an external PNP compatible device driver such as external CD interface 125. The circuit C provides separate interrupt request and direct memory address request pins for external interface 125, which are schematically shown as a single line 124. In the preferred embodiment, a separate input pin is provided for each (see FIGS. 3A and 3B). External device chip select and DMA acknowledge outputs are provided by circuit C via separate output pins (FIGS. 3A and 3B); shown collectively as line 126 in FIG. 6. Data exchange between circuit C and the external device drive is provided via the ISA standard 16-bit bidirectional data bus 128.

[0179] Circuit C is, in general, a register controlled circuit wherein various logic operations and alternative modes of operation are controlled by the status of various bits or bit groups held in various registers. Complete descriptions and definitions of registers and their related functions are set forth in the charts and written description included elsewhere herein. Circuit C also includes several blocks of input/output address space, specifically, five fixed addresses and seven relocatable blocks of addresses. In the register description given herein, register mnemonics are assigned based on the following rules:

[0180] 1. The first character is assigned a code that specifies the area or module to which the register belongs;

[0181] 2. The middle two to four characters describe the function of the register.

[0182] 3. The final character is either R for a direct register, P for a port (to access an array of indexed registers), or I for an indirect register.

[0183] A. Relocatable Address Blocks

[0184] The seven relocatable address blocks included in the circuit C are referenced herein according to the mnemonics set forth in Table I below, wherein PNP refers to industry standard Plug-n-Play specifications:

[0185] B. Direct Address Summary

[0186] There are eight groups of functions in circuit C that utilize programmable registers. The status of programmable registers are subject to control in response to instructions executed by the host CPU system. The eight groups of functions and their associated direct addresses are listed in Table II below. Two of the addresses for Plug-n-Play registers are decoded from all twelve bits of the ISA address bus (SA[11:0]). The remaining addresses are decoded from the first ten bits (SA[9:0]).

[0187] A complete listing of all input/output programmable registers and ports is given in FIGS. 8A-10B wherein all address numbers are in hexadecimal format. Index values provide alternative function addresses using a common basic address.

[0188] C. External-Decoding Mode

[0189] In addition to the ten and twelve-bit address spaces used for internal input/output mapping, the circuit C also provides an optional external-decoding mode wherein four system address bits (SA[3:0], FIGS. 3A, 3B and 6) and two chip-select signals, implemented as SA[5,4], address registers within circuit C. This mode is selected by the status of address pin RA [20] at the trailing edge of the system reset signal.

[0190] If RA [20] is low at the trailing edge of the reset signal, then normal input/output address decoding is implemented, where system address inputs SA[11:0] address all the registers in the circuit C. If RA[20] is high at the trailing edge of system reset, then external decoding mode is implemented:

[0191] This multiplexing can be provided in the manner discussed below with regard to other multiplexed pins and functions.

[0192] The following table shows how direct addressed registers and ports are accessed in external decode mode. Indexed registers are accessed the same way as in internal decoding mode (see preceding register table), except that the direct addresses change to the ones shown in Table III below.

[0193] Note: It is not legal to assert both SCS[0]# and SCS[1]# at the same time.

[0194] D. DMA Accesses

[0195] A number of registers and defined first-in/first-out address spaces within circuit C are accessible via DMA read and write cycles. These are listed in the following tables, where LMC and CODEC refer to the module within circuit C where such registers and FIFOs reside:

[0196] Note that in the table above, DMA Group is a register defined term and does not refer to ISA standard DMA channels or request acknowledge numbers.

[0197] External decoding mode is utilized in those systems which are non-PNP compliant to provide access to internal registers and ports via external decoding logic circuits.

[0198] E. Multiplexed Terminals

[0199] To conserve resources, several groups of external terminals or pins, in addition to the ROM/DRAM multiplexed address and data transfer pins described above, are multiplexed between alternate functions. Four of the groups are multiplexed based upon the status of external pins upon the trailing edge of the reset signal, which occurs upon power up or other system resets.

[0200] Referring now to FIG. 11, it is desired to multiplex pins 139 and 140 which correspond to the suspend # and C32KHZ inputs in one state, with the FRSYNC# and EFFECT# outputs in the alternate state. The functions served by these signals are discussed elsewhere herein. In the circuit C, multiplexing is provided for these pair of pins by sensing the state of terminal RA[21] (see FIG. 6) at the trailing edge of the reset signal. By providing a pull-up resistor 142 on the RA[21] pin or not providing such a resistor, the D-input to latch 144 can be set to a low or high value. Latch 144, upon being clocked by the trailing edge of the reset signal will provide at the Q output a corresponding low or high output. This latch output is provided to a 4:2 multiplex circuit 146. Multiplexor 146 assigns pins 139 and 140 to the SUSPEND# and C32KHZ function if the Q output is high, and alternatively, assigns pins 139 and 140 to the EFFECT# and FRSYNC# output function of the Q output is low.

[0201] Multiplexing or selecting between Plug-n-Play compatible expansion card mode and system board mode is provided in the same manner, by latching the state of input pin PNPCS on the trailing edge of the reset signal. Plug-n-Play mode is selected by a low value, and system board mode selected by a high value. The selection is made depending on whether the circuit C is being used in a Plug-n-Play compatible system, or a system board, non Plug-n-Play compatible system.

[0202] A summary of the pins that are multiplexed based on modes selected at reset is provided in Table V below.

[0203] To further conserve resources, the circuit C includes multiplexing between external pins relating to compact disk drive control and serial port synchronization, clock and data transfer used when an external digital signal processing circuit or other external, serial format circuits are utilized. This is more fully discussed in the CODEC module description below.

[0204] Referring to FIG. 6, control of an external device, such as a CD drive, is provided within the system control module via the EX_IRQ (interrupt request), EX_DRQ (DMA request), EX_DAK# (acknowledge) and CD_CS# (chip select) pins. These four pins are illustrated schematically as lines 124 and 126 in FIG. 6. When circuit C is functioning in a serial transfer mode (as discussed more fully in the CODEC description below), multiplexing of these four external device control pins is controlled by the status of bit seven of register ICMPTI. External serial transfer mode is enabled when ICMPTI[7] is high. In that case, the external device control pins are multiplexed as follows:

[0205] The ESPSYNC, ESPCLK, ESPDIN and ESPDOUT functions correspond to synchronization pulse, clock, data in and data out, respectively.

[0206] The following is a table of all the pins in the circuit C, sorted by I/O pin type:

[0207] A. System Control Functions

[0208] Referring now to FIG. 1, the system control module 2 includes numerous registers, compatibility logic, Plug-n-Play ISA implementation logic, interrupt and DMA channel selection logic, and miscellaneous control functions such as clocks, resets, test logic, etc. System control module 2 is shown in greater detail in FIG. 12.

[0209] Referring now to FIG. 12, system control module 2 includes a system bus interface block 150, industry software compatibility logic block 152, interrupt and DMA channel selection logic block 154, a Plug-n-Play logic block 153, a register data bus 12, and a miscellaneous logic and timing block 158. The system control module in general controls the functioning of the circuit C in response to various timing, and control signals as well as enables responses to control functions held in various registers which serve to change the modes of operation, power consumption levels, and other control features.

[0210] 1. System Bus Interface

[0211] System bus interface 150 provides the hardware links between the processor-controlled system bus 156 and the various modules and portions of circuit C. Circuit C is designed to be fully compatible with the Plug-n-Play ISA system bus specification. One aspect of the Plug-n-Play ISA specification is a requirement for an interface to a serial EEPROM where the system configuration data is stored and available during system initialization to provide configuration data to the host CPU. The system bus interface also has to comply with the ISA portion of the EISA bus specification. These two specifications are industry standards and commonly available.

[0212] The ISA bus interface 150 provides interface compatibility with a 16-bit data bus, which when in 5 volt mode has a drive capability selectable to be either 24, 12 or 3.2 miliamps. The power up default is 24 miliamps. In output mode, the data bus is edge-rate controlled and the delay between lines is mutually skewed to reduce the effects of ground bounce.

[0213] ISA bus interface 150 also provides interface for a 12-bit address bus and support for two audio interrupts and one CD-ROM or external device interrupt chosen from seven interrupt request lines 130. Support is also provided for use of the IOCHK signal to generate non-maskable interrupts to the host CPU.

[0214] The interface 150 also provides support for three DMA channels chosen from six sets of DMA lines 136 and a corresponding set of DMA acknowledge lines 138. In accordance with the ISA DMA specification, the six channels available are 0, 1, 3, 5, 6 and 7 (channel 0, 1 and 3 are 8-bit DMA channels and channels 5, 6 and 7 are 16-bit DMA channels). The three DMA functions supported are: wave-file record transfers and system-memory transfers; wave-file play transfers; and DMA channel required by the CD-ROM or external device interface. A mode is provided whereby both record and play functions can be mapped to the same DMA channel, although only one can be enabled at a time.

[0215] 2. The Register Data Bus

[0216] Data distribution between the ISA bus and the circuit C is provided via register data bus 12. Register data bus 12 facilitates system bus input and output and DMA accesses to registers provided throughout the circuit C. Referring now to FIG. 13, register data bus interfaces via a plurality of bi-directional data bus transceivers 160 to synthesizer registers 162, local memory control registers 164, system control registers 166, MIDI and game ports and registers 168 and CODEC registers 170. The purpose and function of these registers is described more fully elsewhere in this specification. A bi-directional data bus transceiver 160 is also provided between register data bus 12 and ISA data bus 172, which is the data portion of ISA system bus 156 shown in FIG. 12. Register data bus 12 also interfaces with various local memory latches 173, 174 and 175 and CODEC FIFOs 176 and 178, as will be described in detail elsewhere in this specification.

[0217] Circuit C supports either eight or 16-bit data transfer to or from the system data bus. In the case of input/output accesses from the ISA data bus, the alignment of the data to and from the register data bus is defined by the least significant bits of the ISA address bus. These are designated SA[0] and SBHE#, as shown in FIG. 6. These two bits are decoded as shown in the following Table VI for accesses to other than the general input/output data ports (I8/16DP):

[0218] Note that all 8-bit quantities are passed over the lower half of the register data bus 12. The condition of both SA[0] and SBHE# high, which is not allowed by the ISA bus specification, is used to specify a high-byte access from an 8-bit card. For an 8-bit card, the card designer would pull the SBHE# bit high.

[0219] Referring now to FIG. 14 details of register data bus control are illustrated. Register data bus 12 is formed of two 8-bit busses 180 and 182. Low byte bus 182 interfaces via data bus transceiver 184 to the low byte of system data bus 128 (see FIG. 6). High byte bus 180 interfaces to high byte of system data bus 128. Controlled bus driver 186 transfers data between buses 182 and 180 to effect data translation set forth in the table above, in response to control and decoding logic 190. Control logic 190 responds to input SBHE#, and SA[0] to generate control signals via lines 192, 194, 196, 198 and 200 to implement the data translation set forth in the table above.

[0220] An 8-bit latch 202 is provided to latch the low byte data until the high byte is active to provide 16-bit input/output accesses. Controlled driver 204 responds to control signals from control and decoding logic 190 to effect simultaneous low and high byte input/output accesses.

[0221] Control logic 190 also receives ISA bus signals IOR# and IOW# which enable read or write accesses respectively. While these inputs are shown as a single line 206 they are provided on individual pins to circuit C, as are the input signals illustrated on lines 208 and 210. PNP data transfers under the control of logic circuits 190 and 212 are provided via bus 182 and controlled bidirectional transceiver 214. PNP logic functions and registers are described in detail elsewhere in this specification. Control and decoding logic 190 may be implemented in any suitable conventional method to provide industry standard ISA system bus interface control and address decoding and to implement the data handling protocol set forth herein. Likewise, PNP logic circuit 212 maybe implemented with conventional circuits to provide an industry standard PNP complaint interface.

[0222] Control and decode logic 190 provides conventional handshake, decoding and bus interface circuitry to interface with industry standard ISA system bus.

[0223] Accesses to all PNP registers use odd, 8-bit addresses. Since IOCS16# is not asserted for these accesses, the lower 8 bits of the ISA data bus are used. These are passed from/to the lower 8 bits of the register data bus. IOCS16# is an industry standard interface signal asserted via an external pinout (see FIG. 6).

[0224] I8DP located at P3XR+5 and I16DP located at P3XR+(4-5) are used to access 8 and 16-bit, indexed registers included in the circuit C. I16DP is the only port on the circuit C that is capable of 16-bit I/O accesses. IOCS16# is asserted for all accesses to these general data ports. The general I/O data port accesses are translated is a follows:

[0225] System bus interface 150 is responsible for translating 16-bit I/O writes that are broken up by software into two 8-bit writes (even byte first, then odd byte). For this, the even-byte write is latched in the latch 202 and provided over the low half of the register data bus during the subsequent odd-byte write. The register data bus will provide whatever was last latched in an even-8bit-I/O write during odd-8-bit-I/O writes.

[0226] For DMA accesses, the data width is determined by the DMA channel used as follows:

[0227] During 8-bit DMA and I/O reads, the appropriate byte is driven on the ISA data bus 128. The other byte is not driven; it will remain in the high-impedance state.

[0228] It should be noted that to make sure the register data bus' voltage does not drift into the transition region when it is not being driven, weak feedback inverters (“keeper” or “sticky-bit” circuits) are provided in accordance with conventional, well known methods. Such circuits provide a weak feedback path that drives the node voltage back on itself to keep it from floating.

[0229] ISA Data Bus Drive Considerations

[0230] There are three special ISA-data-bus design facets built into the IC for the purpose of reducing the peak return current required when the data bus is driving out. The first is that the output drive capacity is selectable, via a programmable register, to be either 24, 12 or 3.2 milliamps (when VCC is at 5 volts). The second is that there is a special current restriction circuit built into the output buffers that slows the edge rates; this circuit is implemented in the same way as that used by the PC Net ISA chip, 79C960/1. The third design aspect is that the data bus is broken up into a few groups, each of which is skewed from the others, as shown in the FIG. 14a.

[0231] 3. Register Data Bus I/O Decoding

[0232] There are seven relocatable and four non-relocatable blocks of address space decoded. They are:

[0233] The notation “PPWRI[SD]” in the above table indicates the circuit C is in shut-down mode, initiated by a specific I/O write to PPWRI.

[0234] The 2XX and 3XX decodes are further broken down as follows:

[0235] AEN

[0236] The decodes above are only enabled when AEN is low.

[0237] IOR and IOW

[0238] Along with the above decodes, the SBI 150 provides IOR and IOW from the ISA data bus. The worst case ISA-bus timing that must be assumed when interfacing to these signals is illustrated in FIG. 15. Note that this diagram shows the fastest I/O cycle possible which will only occur during accesses to the single 16-bit port that is addressable in the IC, P3XR=(4-5). All other ports will follow 8-bit timing (IOCS16# is not driven active), which is much slower, IOR# and IOW# stay active for about 530 nanoseconds for 8-bit I/O cycles.

[0239] IOCHRDY Control

[0240] Only accesses to P3XR+2 through P3XR+7 are capable of extending the ISA-bus I/O cycle by causing IOCHRDY to become inactive; accesses to all the P2XR, ports, CODEC, and Plug-n-Play ISA registers never extend the cycle. For the registers that can extend the cycle (including the 46 registers indexed by IGIDXR), the following categories exist:

[0241] Buffered I/O writes are important because they allow the CPU to continue without having to wait. However, if not handled properly, they can be the source of problems resulting from mixing up the order in which the I/O cycles are handled. For example, if there were a buffered I/O write to local memory immediately followed by a write to the local memory I/O address registers, then the write to local memory may be sent to the wrong address. This kind of problem is handled by forcing any subsequent accesses to the circuit C to be extended while there is a buffered I/O write in progress. Referring now to FIG. 16, IIOR#, IIOW#, and IBIOWIP# are internal signals. IIOR# and IIOW# become active after the previous buffered write has completed, signaled by IBIOWIP# (buffered I/O Write) becoming inactive. Note that IIOR# and IIOW# are not gated by IBIOWIP# during DMA cycles.

[0242] The registers that allow buffered I/O writes—called buffered registers—are the synthesizer voice-specific registers, IGIDXR=00h-0Dh and 10h-18h, the Local Memory Control (LMC) 16-bit access register, IGIDXR=51h, and the LMC Byte Data Register, LMBDR. An I/O write to any of these registers automatically causes IBIOWIP# to become active so that IOCHRDY will become inactive during the next I/O access to the circuit C. An I/O read to any of the buffered registers causes the logic to (1) force IOCHRDY inactive (regardless as to whether IBIOWIP# is active), (2) if IBIOWIP# is active, wait until it becomes inactive and keep IOCHRDY inactive, (3) wait for the read-data to become available to the ISA bus, and (4) allow IOCHRDY to become active; at this point the cycle is finished off like a zero-wait-state cycle.

[0243] Control IGIDXR

[0244] If IGIDXR is in auto-increment mode (SVSR), then it will increment on the trailing edge of either an 8-bit I/O write to P3XR+5 or a 16-bit write to P3XR+(4-5); if the write was to a buffered port, then IGIDXR is incremented after the trailing edge of IBIOWIP#.

[0245] 4. Existing Game Card Compatibility

[0246] The system control module 2 includes logic and registers needed for compatibility with existing game-card software. The circuit C is compatible with software written for native mode Ultrasound, MPU-401, Sound Blaster and AdLib. Logic circuits and timers for compatibility are designated generally as block 152 in FIG. 12. These include the following functions: (i) registers described in the register description part of this document; and (ii) two 8-bit timers, one having an 80 microsecond resolution and the other a 320 microsecond resolution; (iii) two general purpose registers; (iv) MPU-401 status emulation flags and control registers.

[0247] a. AdLib Timer 1 and Adlib Timer 2

[0248] AdLib Timer 1 is an 8-bit preloadable counter that increments to 0FFh before generating an interrupt. It is clocked by an 80 microsecond clock. AdLib Timer 2 is the same, except that it is clocked by a 320 microsecond clock. On the next clock after they reach OFFH, the interrupt becomes active and they are re-loaded with their programmed value (UAT1I and UAT2I). The interrupts are cleared and enabled by UASBCI[3:2]. Both timers can be changed to run off the 1 MHz clock by UASBCI[4]. These timers are also enabled by UADR[STRT1] and UADR[STRT2].

[0249] b. Auto-Timer Mode

[0250] It is possible to place the circuit C into auto-timer mode by writing to UASBCI[0]. This mode is used to emulate AdLib hardware. When in auto-timer mode, reads of UASRR provide the state of various flags instead of UASWR. When in auto-timer mode and UACWR has been set to 04h, the following changes take place: (1) write to UADR no longer cause interrupts; (2) writes to UADR are no longer latched in the simple register that is readable from that same address; and (3) writes to UADR are instead latched in a register that drives out various flags related to the control of the AdLib timers.

[0251] c. General Purpose Registers

[0252] Logic block 152 also includes two 8-bit general purpose registers that are used for MPU-401 emulation and to support other emulation software. The general purpose registers, referred to as UGP1I and UGP2I, can be located anywhere in the ISA 10-bit I/O address space via UGPA1I, UGPA2, and ICMPTI[3:0]. Each register actually represents two registers: one that is read out to the application and one that is written in by the application. When the registers are written (by the application) at the emulation address, they may be enabled to generate an interrupt; they are subsequently read (by the emulation software that received the interrupt) via a back-door access location in the GUS Hidden Register Data Port (UHRDP). Writing to those same back-door locations, updates the general purpose registers associated with the read operation. This emulation protocol is schematically illustrated in FIG. 16a.

[0253] d. MPU-401 Emulation

[0254] Several controls have been added to the general purpose registers in support of MPU-401 emulation; the assumption is that there is an MPU-401 emulation TSR running concurrently with the application (typically game software). To match the MPU-401 card, the emulation address (UGPA1I, UGPA2I, and ICMPTI[3:0]) may be set to match the MIDI UART address. The two UART addresses can be swapped so that the receive/transmit data is accessed via P3XOR+0 and the control/status data is accessed via IVERI[M401]. Application writes to the general purpose registers cause interrupts (potentially NMIs). Emulation software captures the interrupts, reads the data in the emulation registers via the back door (UHRDP), and uses it to determine how to control the synthesizer. The MIDI commands may also be sent to the UART so that the application can be driven by the same interrupts and observe the same status as the MPU-401 card.

[0255] FIG. 16bis a schematic block diagram showing the access possibilities for the application and the emulation TSR. The switch symbols are enables that are controlled by the IEMUAI and IEMUBI emulation control registers.

[0256] MPU-401 Status Emulation

[0257] Two MPU-401 status bits are generated, DRR# (Data Receive Ready, bit 6) and DSR# (Data Send Ready, bit 7), which are readable via UBPA1I. The intended meaning of these bits is as follows: DRR# becomes active (low) when the host (CPU) is free to send a new command or data byte to the UART; DSR# becomes active (low) when there is data available in the UART's receive data register. Note that the names of these bits are derived from the perspective of the MPU-401 hardware rather than the CPU. Selection between reading these bits and the actual data written to the emulation register comes from IEMUBI[5:4].

[0258] DRR# is set inactive (high) by the hardware whenever there is a write to either of the emulation registers via the emulation address (ICMPTI[3:0], UGPA1I, UGPA2I), if a write to that register is enabled. Writes to UGP1I[6] via the back door (UHRDP) also updates the state of this flag. This bit defaults to high at reset.

[0259] DSR# is set inactive (high) by the hardware when there is a read of UGP2I via the emulation address (ICMPTI[3:2], UGPA2I). Writes to UGP1I[7] via the back door (UHRDP) also update the state of this flag. This bit defaults to low at reset.

[0260] 5. Plug-n-Play Logic

[0261] The system control module 2 includes registers and logic needed to implement the Plug and Play ISA (PNP) specification from Microsoft. There are several state machines within the PNP block of the circuit C (see discussion below); some of these utilize a clock that is derived from the 16.9 MHz. oscillator (C59N).

[0262] The circuit C includes two PNP-compliant logical devices. The AUDIO-functions logical device consists of most of the circuit C including the synthesizer, the codec, the ports, etc. The external function or CD-ROM logical device is associated with only the external functions.

[0263] a. PNP I/O Ports and Registers

[0264] In support of PNP, the circuit C provides a number of specialized registers. These are indexed via PIDXR and accessed via the read and write ports PNPRDP and PNPWRP.

[0265] b. Power-Up PNP Mode Selection

[0266] The reset signal latches the state of the output pin 76 (PNPCS, FIG. 6) at power-up to determine the PNP mode. If it is latched low, then the circuit C is assumed to be on a PNP-compliant card that contains a serial EEPROM 78 (PNP card mode). If it is latched high, then the circuit C is assumed to be on a system board that does not contain a serial EEPROM 78 (PNP-system mode).

[0267] In PNP-system mode, the Card Select Number (CSN) is assigned via a different method than that of the PNP standard (see PCSNBR). This is so the system board implementation can exist without the external serial EEPROM. If external decoding is selected (see the PIN SUMMARY section of the general description), then all PNP registers are accessible regardless of the PNP mode. Thus, in this mode, it is not necessary to assign a CSN or incorporate any of the PNP protocol into the software to obtain access to the PNP registers.

[0268] c. PNP State Machine

[0269] Referring now to FIG. 17, PNP interface can be in one of four possible states: wait-for-key, isolation, configuration, and sleep. In FIG. 17, wake is the wake command, X is the data value associated with the command, and CSN is the current card select number, all as explained in the Plug And Play ISA specification. The output of the PNP state machine is PNPSM[1:0], as shown in the diagram.

[0270] Wait For Key

[0271] In this state, the PNP logic waits for a key of 32 specific bytes to be written to PIDXR. No PNP registers are available when in this state (except PIDXR for the key).

[0272] Isolation

[0273] In this state, PNP software executes a specific algorithm of IOR cycles to PISOCI to isolate each PNP card and assign it a distinct CSN. If the circuit C is in PNP-system mode, then reads of PISOCI always cause the part to “lose” the isolation and go into sleep mode.

[0274] Configuration

[0275] From this state PNP software can read all resource data from the PNP EEPROM 78, assigns the resources (I/O address space, IRQ numbers, and DMA numbers), and send specific PNP commands (such as “activate”).

[0276] Sleep

[0277] In this mode, the PNP hardware is dormant.

[0278] d. Interface to the Serial EEPROM

[0279] When the audio logical devices is not activated (PUACTI[0]), then it is possible to access the PNP serial EEPROM 78. There are two modes of access—PNP-initialization and PNP-control-selected by PSEENI[0]. In PNP-initialization mode, data is automatically read out of the EEPROM based on the state of PNPSM[1:0] as follows:

[0280] Referring now to FIG. 18, timing for reading the serial-EEPROM data is provided. Note that the data is required to enter the circuit C in the reverse order from what is standard for a serial EEPROM. Also, bits[7:0] represent the even byte (the first byte read via PRESDI) and bits[15:8] represent the odd byte. SK, the serial clock, is ICLK1M (see the CLOCKS description below), which is a frequency of 996 KHz.

[0281] In PNP-control mode the EEPROM pins are controlled directly via bits in PSECI.

[0282] e. Initiation Key and Linear Feedback Shift Register

[0283] Access to PNP registers is preceded by a hardware/software unlock mechanism that requires the implementation of a linear feedback shift register (LFSR). Implementation of the LFSR 230 is illustrated in FIG. 20. The unlock is complete after the software writes the following 32 values to PIDXR: 6A, B5, DA, ED, F6, FB, 7D, BE, DF, 6F, 37, 1B, OD, 86, C3, 61, B0, 58, 2C, 16, 8B, 45, A, D1, E8, 74, 3A, 9D, CE, E7, 73, 39. These values are internally calculated with LFSR 230. LFSR 230 is reset to 6Ah anytime the value written to PIDXR does not match the LFSR. If all 32 proper bytes are written to PIDXR, then the PNP state machine changes from Wait-For-Key mode to Sleep mode (See FIG. 17).

[0284] f. Isolation Mode

[0285] When in Isolation mode, the data contained at the beginning of the serial EEPROM 78 is shifted in, one bit at a time, and used in the algorithm shown in FIG. 21.

[0286] The PNP specification allows for the last eight bits of the serial identifier, the checksum, to either be calculated or simply transferred from the serial EEPROM 78. These values are not calculated by the circuit C; they are transferred directly from the serial EEPROM 78. The algorithm of FIG. 21 enables transition from isolation mode to either configuration mode or sleep mode.

[0287] g. Card Select Number Register

[0288] The Plug-n-Play specification requires that a card select number (CSN) be assigned to all devices on the system bus, and that such number be accessible. In the circuit C, there is an 8-bit register, designated card select number back door (PCSNBR) where the card select number (CSN) is stored. The CSN is writeable when the PNP state machine is in Isolation mode. It can be read when the PNP state machine is in Configuration mode.

[0289] It is possible to write to the CSN without going through the normal PNP protocol by using the following procedure:

[0290] 1. Place a pull-up resistor on PNPCS to place the card in PNP system mode at power-up.

[0291] 2. While the AUDIO logical device is not active (PUACTI[0]=0), place the PNP state machine into Isolation mode.

[0292] 3. Write the CSN to the Game Control Register, 201h.

[0293] h. Plug-n-Play Resource Requirement Map

[0294] An example of resources required for programming the PNP serial EEPROM 78 is provided in FIG. 22.

[0295] 6. Interrupts and IRQ Channel Selection

[0296] There are several groups of signals associated with interrupts. They are:

[0297] These are combined into the three IRQ channel selection possibilities for the circuit C as follows:

[0298] The following equation shows how the above three equations are mapped to the IRQ pins (see FIGS. 3A and 3B), where “x” in IRQx specifies the IRQ number. The notation “(UICI[2:0]==IRQx)” should read “UICI[2:0] specifies IRQx”.

[0299] The Non-Maskable Interrupt (NMI) function is controlled as follows (between being driven low and being high-impedance):

[0300] In the above equations and those that follow, note that a “/” preceding a variable or signal signifies logic not. The * signifies the AND function, + signifies the OR function and the “/”, “*”, and “+” are prioritized as first, second and third, respectively. The programmable bit fields and signals associated with the above equations are:

[0301] Interrupt Events

[0302] The table in FIG. 23 provides data on all interrupt-causing events in the circuit C. Note that when the circuit C is in auto-timer mode and the UACWR has been written to a 04h, then the write to the UADR does not generate an interrupt.

[0303] 7. DMA Channel Selection

[0304] The following are the signals used in the circuit C which are associated with DMA data transfer requests:

[0305] These are combined into the three DRQ channel selection possibilities for the circuit C as follows:

[0306] The following equation shows how the above three equations are mapped to the DRQ pins (see FIGS. 3A and 3B), where “x” in DRQx specifies the DRQ number. The notation “(UDCI[2:0]==DRQx)” should read “UDCI[2:0] specifies DRQx”.

[0307] Enabling DRQs from High-Impedance. Here are the equations for the signals that enable the DRQ lines from high-impedance:

[0308] Driving the Data Bus During DMA

[0309] DMA reads of the circuit C will cause the system data bus to be driven only if the circuit C has set the DMA request signal; also, the circuit C will ignore all DMA writes if the acknowledge occurred without a DMA request.

[0310] DMA Rates

[0311] For DMA transfers between local and system memory, the rate of transfer is controlled by LDMACI[4:3]. The fastest rate for all DMA transfers allows about one-half to 1 microseconds from the end of the last DAK signal to the beginning of the next DRQ signal. This is incorporated by counting two edges of the ICLK2M, the 2 MHz clock.

[0312] 8. Clocks

[0313] The circuit C has numerous internal clock requirements. This section of the description refers to all internal clocks which are generated from external crystals 16 and 18 (FIG. 1). Referring now to FIG. 24, all of the clocks that are generated by this block off of crystal 16 are guaranteed to be steady (held high) when either oscillator is not valid and to start toggling again after the oscillator is stable. The logic is designed such that there is no possibility of glitching on these clocks while the oscillators are stabilizing. This is the purpose of the oscillator stabilization logic 232 in FIG. 24. It is used: (1) to exit suspend mode; (2) to exit shut-down mode; and (3) to stabilize the oscillators following a software reset (PCCCI) in which the IC is in the shut-down mode. It is bypassed when the RESET pin becomes active.

[0314] In FIG. 24, the IOSC16M signal is the input clock signal from the 16.9344 MHz clock 16. This clock signal is provided as an input clock signal to oscillator stabilization logic 232 via a control or gate signal on line 233. Gating logic 242 also generates an enable signal on line 235 to control the on/off state of clock 16.

[0315] As explained below, gating logic 242 provides an output ICLK16M signal via a buffer 237 which is used as the basic system clock for the circuit C, and a 16.9344 MHz output via buffer 239 which is utilized by logic block 241 to generate various clock signals of different frequencies for specific subcircuits or functions. Note that similar stabilization logic could be provided for crystal 18 if desired. In the present embodiment, crystal 18 provides a buffered 24 MHz output on line 234 in response to activation signal PPWRI(PWR24).

[0316] Oscillator Stabilization Logic

[0317] Referring now to FIG. 25, the oscillator stabilization logic 232 consists of a 16-bit counter 238 that is clocked by oscillator 16, and a flip-flop 240 that controls the counter 238. The result is a gate to the gating logic 242 (FIG. 24) that either allows the clock to pass or disables it glitch-free. The signal STOP_CLK for the 16.9 MHz. clock 16 clears counter 238 during suspend and shut-down modes. In the preferred embodiment, a software reset (PCCCI) requires that system reset PCARST# be held active for either 256 states or 64K states of clock 16 depending on whether the circuit C is in a shut-down mode (see discussion below). Logic counters within the stabilization logic 232 also provide control signals to implement the required delay. The signal GO_CLK sets control flip-flop 240 while the RESET pin is active. Once the circuit C exits suspend and shut-down mode, STOP_CLK becomes inactive, counter 238 clocks out 64K states, and the CLOCK_ENABLE output of the circuit 238 becomes active. STOP_CLK, GO_CLK signals are internally generated from logic circuits responsive to the status of power control registers and reset signals as described elsewhere herein.

[0318] Referring now to FIGS. 24A-1 and 24A-2, further details of the clock generation, control and stabilization circuitry are described. It should be noted that the logic and counters shown in FIGS. 24A-1 and 24A-2 are intended to be an example of how the logic described could be implemented. Those of ordinary skill in the art will realize there are numerous variations which might be used without deviating from the functional specification.

[0319] System reset signal 430 is an external ISA bus signal. System reset 430 is asserted for at least ten milliseconds (thereby enabling PCARST#) to allow enough time for oscillators 16 and 18 to stabilize before signal PCARST# on line 431 goes inactive (high). Signal PCARST# forces most memory functions (registers, latches, flip-flops, bits in RAM) into the default state, causes all ISA-bus activity to be ignored and halts local memory cycles. System reset is provided as a GO-CLK asynchronous set signal 435 to flip-flop 240, which forces the Q-output high on line 233 to immediately enable gating logic 242, thereby enabling the 16 MHz clock signal. The 24 MHz clock is also enabled by reset since it is controlled by the PWR24 bit of register PPWRI which in turn is set high as its default state in response to the PCARST# signal.

[0320] Still referring to FIGS. 24A-1 and 24A-2, the PCCCI signal is an I/O mapped command from the PNP logic (software reset) controlled by the status of the PCCCI register. Assertion of PCCCI is provided on line 434 as an alternative source of signal PCARST#.

[0321] Still referring to FIGS. 24A-1 and 24A-2, suspend mode is entered in response to an active input from the Suspend# pin. For ease of reference, the suspend mode logic is shown in active-positive mode in FIGS. 24A-1 and 24A-2. An active input suspend signal is provided on line 446 and input to ORGATE 448 and ANDGATE 450. In response, ISUSPRQ becomes active at line 452 which activates modular signals I2LSUSPRQ and I2SSUSPRQ via gates 454 and 456, respectively. The suspend input on line 446 is also provided to a 2-bit delay counter 458 which provides an 80 μ second delayed output to ORGATE 448 and ANDGATE 450. Delay circuit 458 is clocked by the ICLK12K internally generated clock signal provided on line 460. Consequently, after 80 μ seconds ANDGATE 450 is enabled and generates suspend-in-progress signal ISUSPIP on line 462. This signal is provided to generate modular suspend-in-progress signals, as desired. For example, ISUSPIP is provided as an input to ORGATE 464 to generate a modular I2LSUSPIP signal for the local memory module of the circuit C, which is used to disable the 16.9 MHz clock signal used by the local memory module during normal operations.

[0322] ISUSPIP is also provided via ORGATE 467 and ORGATE 466 to ground oscillator 16 approximately 80 μ seconds after ISUSPRQ has been asserted, and as a STOP_CLK input on line 436 to clear counter 238. Clearing counter 238 requires the oscillator 16 to stabilize after being enabled when the suspend signal is deactivated. Similarly, ISUSPIP is provided as an input to ANDGATE 468 via ORGATE 470 to disable the 24 MHz oscillator 18.

[0323] Various Clocks

[0324] The clock circuit of the system control module 2 provides various clocks for functions throughout the circuit C. Here is a summary:

[0325] ICLK1M is implemented with a duty cycle of 9 clocks high and 8 clocks low to comply with the requirements of PNP serial EEPROM 78. All other clocks are implemented such that their duty cycle is a close to 50—50 as possible.

[0326] Test-Mode Requirements

[0327] When the chip is in test mode, the circuit for many of these clocks is bypassed (see register description below). Additionally, the 16.9 and 24.5 MHz clocks are directly controlled without the intervening logic or 64K state counters.

[0328] 9. Power Consumption Modes

[0329] The circuit C has the ability to disable various blocks of logic from consuming very much current. It also can be in shut-down mode, wherein both oscillators are disabled, and in suspend mode, wherein both oscillators are disabled and most of the pins become inaccessible. Control for disabling various blocks and placing the circuit C in shut-down mode comes from programmable register PPWRI; suspend mode is controlled by the SUSPEND# pin (see FIG. 6). Suspend mode causes the I/O pins to change behavior as shown in the table:

[0330] The pins SA[11:6], SBHE#, DAK[7:5,3], and SD[15:8] have weak internal pull-up resistors; however, the power to these resistors can be disabled via IVERI[PUPWR] so that they do not drive voltage onlo the ISA bus during suspend mode. For those pins forced to a high-impedance state to prevent current consumption, a controlled buffer is provided internal to the pin. In suspend mode, this buffer is disabled and its output (the input to the circuit C) is grounded.

[0331] a. Register Controlled Low-Power Modes

[0332] Register PPWRI is a 7-bit register used to reduce the power being consumed by various blocks of logic within the circuit C and place it into shut-down mode. The table set forth in FIG. 26 describes what happens when various bits in register PPWRI are cleared or set. Each of the bits in PPWRI are defined such that they are low when in low-power mode.

[0333] The 100 microsecond timers referenced in FIG. 26 consist of two conventional timer circuits within logic block 158 (FIG. 12), each driven by ICLK100K (divide by 10). One of the timers is used to count out the going-to-low-power-state time and the other is used to count out the coming-out-of-low-power-state time. These same timers may be used for suspend mode as well.

[0334] Referring now to FIGS. 24A-1 and 24A-2, register PPWRI is schematically illustrated as register 472. Shut-down mode is activated in response to each bit of register 472 being cleared to a logic low state. The status of each of the bits from register 472 is provided as an inverted input to ANDGATE 474, which provides an output to timer 476 when all bits are low. After the appropriate 100 μ second delay an output is provided at line 478 which disables (grounds) oscillator 16 via ANDGATE 480, provided that none of the bits from register 472 have changed state to a logic high in the interim delay. This status check is provided via ORGATE 482 which provides a second, enabling input to ANDGATE 480. The output of timer 476 is also provided as a STOP_CLK input to clear counter 238 of stabilization circuit 232 to provide an appropriate delay when exiting shut-down mode.

[0335] As noted elsewhere, the status of the PWR24 bit controls power to oscillator 18 via gate 468. Modular power modes are implemented in response to the status of individual bits within register 472 (PPWRI). For example, the status of bit 4 (PWRS) is provided as an input to counter circuit 484, ORGATE 486 and ANDGATE 488. These circuit elements provide a synthesizer suspend request signal 490 followed by a delayed synthesizer suspend in progress signal 492 which is also used to disable the synthesizer clock signals via gate 493. A similar delay and logic circuit 494 is provided for the local memory module. The remaining bits of register 472 control the status of various modules and portions of modules within the circuit C, as described elsewhere in this specification. Logic implementation of these functions is schematically illustrated in FIG. 24a.

[0336] FIGS. 24B-1 and 24B-2 is a flow chart schematically representing the response of circuit C to suspend mode activation and deactivation. FIGS. 24C-1, 24C-2 and 24C-3 is a flow chart illustrating the register-controlled low-power modes.

[0337] b. Suspend Mode

[0338] When the SUSPEND# pin becomes active, the circuit C behaves similarly to when it is placed into shut-down mode. The timing diagram in FIG. 27 shows how the oscillators, clocks, and signals respond to the SUSPEND# pin. Note that in FIG. 27 the ICLK24M signal is illustrated as being stabilized, which is optional but not required. ISUSPRQ is logically ORed into I2LSUSPRQ and I2SSUSPRQ from the shut-down logic. ISUSPIP is logically ORed into I2LSUSPIP (see FIG. 26) If the circuit C is already in shut-down mode when SUSPEND# is asserted, then: (i) the I/O pins are changed to match the requirements of suspend mode shown above; and (ii) the codec analog circuitry is placed into low-power mode if it is not already in that mode. The CODEC analog circuitry is placed in low-power mode whenever SUSPEND# is active by providing the ISUSPIP signal on line 461 to ANDGATE via invertor 465.

[0339] After the ISUSPRQ# is asserted, the logic waits for greater than 80 microseconds before stopping the clocks to the rest of the circuit C and disabling the oscillators. Clock signals ICLK16M and ICLK24M from oscillators 16 and 18, respectively, are disabled (as well as re-enabled) such that there are no distortions or glitches; after they go into one of their high phases, they never go back low. After SUSPEND# is deactivated, the oscillators are re-enabled, but clock signal ICLK16M does not toggle again until oscillator 16 has stabilized, 4 to 8 milliseconds later; this occurs after the oscillator 16 has successfully clocked 64K times. After ICLK16 has been toggling for at least 80 microseconds, the ISUSPRQ# signal is de-asserted to allow the logic in the rest of the circuit C to operate. All of the ISA bus pins, and many of the other pins, are disabled while ISUSPRQ# is active. It is not possible to access the circuit C via the ISA bus while ISUSPRQ# is active; therefore, software must delay for about 10 milliseconds after SUSPEND# is released before attempting to access the circuit C. ISUSPIP (suspend in progress) is active during the time when the internal clocks are not valid; it is used to change the behavior of the I/O pins in the Local Memory Control module per the suspend requirements (suspend-mode refresh).

[0340] 10. Reset

[0341] There are two main sources of reset: (1) assertion of the RESET pin and (2) the I/O mapped command for reset from the PNP logic (PCCCI). Both generate long pulses over the PCARST# signal. There is also a reset of the synthesizer module 6 and Gravis Ultrasound functions, caused by a write to Reset Register (URSTI). There is also a reset for the MIDI interface controlled by bits in GMCR.

[0342] PCARST#

[0343] PCARST# is an internally generated signal which forces most memory functions in the circuit C-registers, latches, flip-flops, bits of RAM—into their default state. While it is active, all ISA-bus activity is ignored and no local memory cycles take place. PCARST# is generated as a logical OR of the reset from the RESET pin and the software reset (PCCCI) described below. The RESET pin is required to be asserted for at least 10 milliseconds, which provides enough time for the oscillators to stabilize before PCARST# becomes inactive. If the software reset occurs when the IC is in shut-down mode, PCARST# becomes active and the oscillator stabilization logic counts through 64K states before releasing PCARST#. If the software reset occurs when the IC is not in shut-down mode, then PCARST# becomes active for 256 16.9 MHz clocks (about 15 microseconds). While PCARST# is active, all the 16.9 MHz and 24.5 MHz clocks are passed onto the other blocks in the IC; however, the various divide-down clocks shown in the CLOCKS section above do not toggle because the divide-down circuitry used to generate them is also reset.

[0344] RESET-Pin-Only Functions

[0345] The following items are affected by the RESET pin, but not by PCARST#: the state of the I/O pins that are latched at the trailing edge of reset, the PCSNI, PSRPAI, and PNPSM[1:0] registers and state machine which have there own specific reset requirements, the test control register (ITCI), and control for the oscillator stabilization logic (which is used to count out software resets). All other functions are reset into their default state.

[0346] The Software Reset, PCCCI

[0347] The software reset holds PCARST# active while the 16.9 MHz oscillator is forced to clock through either 256 states (if not shut-down is in progress or if ITCI[BPOSC] is active) or 64K states.

[0348] Synthesizer RAM block

[0349] After PCARST# becomes inactive, the synthesizer logic (see discussion below) will sequence through all 32 voice-RAM blocks to clear them out. This will take about 22 microseconds.

[0350] External Function Interface

[0351] When PCARST# is active, the pins RAS# and ROMCS# both become active (RAS#=ROMCS#=0). This is the only way that this situation can occur. When it does occur, it can be decoded by the external function (e.g., CD-ROM) to determine that reset is active.

[0352] B. System Control PIN Summary

[0353] The pins set forth in FIGS. 28A and 28B are associated with the system bus interface.

[0354] C. System Control Rester Overview

[0355] In the following register definitions, RES or RESERVED specifies reserved bits. All such fields must be written with zeros; reads return indeterminate values; a read-modify-write operation can write back the value read.

[0356] 1. P2XR Direct Registers

[0357] a. Mix Control Register (UMCR)

[0358] Address: P2XR+0 read, write

[0359] Default: 03h

[0360] See IVERI[HRLEN#] for a description of how this register controls access to the hidden registers.

[0361] CRS

[0362] Control Register Select. If URCR[2:0] is set to 0, then this bit selects between indexing the Interrupt Control Register (UICI) and the DMA Control Register (UDCI). 1=UICI; 0=UDCI.

[0363] MLOOP

[0364] MIDI Loop Back. A logical 1 causes MIDITX to loop into MIDIRX. This does not block the transfer of data out of the MIDITX line; it does, however, block data reception via MIDIRX.

[0365] GF122

[0366] Channel Synthesizer Interrupts. A logical 1 causes (1) the ORing of all the synthesizer and CODEC interrupts into the selected channel 2 IRQ pin and (2) the masking of synthesizer interrupts to the selected channel 1 IRQ pin.

[0367] IQDMA

[0368] IRQ and DMA Enable. A logical 1 enables the IRQ and DRQ pins (for audio functions only; does not affect the selected IRQ and DRQ lines for the external device controlled by the EX_IRQ and EX_DRQ pins. A logical 0 forces all IRQ and DRQ pins into the high-impedance mode (for audio functions only).

[0369] ENMIC

[0370] Enable Mono and Stereo Microphone Input. A logical 0 causes both the mono and stereo microphone inputs to the part the be disabled (no sound).

[0371] ELOUT

[0372] Enable Line Out. A logical 1 causes the stereo line-out outputs be disabled (no sound). This switch is after all enables and attenuators in the codec module.

[0373] ENLIN

[0374] Enable Line In. A logical 1 causes the stereo line-in inputs to be disabled (no sound). This switch is before all enables and attenuators in the codec module.

[0375] b. Sound Blaster 2X6 Register (U2X6R)

[0376] Address: P2XR+6 write

[0377] A write to this address sets the 2X6IRQ bit in the AdLib Status Register (UASRR). No data is transferred or latched at this address.

[0378] C. IRQ Status Register (UISR)

[0379] Address: P2XR+6 read

[0380] Default: 00h (after initialization)

[0381] This register specifies the cause of various interrupts.

[0382] DMATC

[0383] DMA Terminal Count IRQ. A high indicates that the ISA-bus terminal count signal, TC, has become active as a result of DMA activity between system and local memory. The flip-flop that drives this bit is cleared by a read of LDMACI. It is ORed into the interrupt associated with the synthesizer. If TC interrupt is not enabled (LDMACI[5]), then this will be read as inactive, even if the interrupt's flip-flop has been set.

[0384] VOLIRQ

[0385] Volume Loop IRQ. A logical 1 indicates that the volume ramp for one of the voices reached an end point. This bit will be cleared after the General Index Register (IGIDXR) is written with 8Fh, the value to access the synthesizer voice interrupt request register, SVII.

[0386] LOOIRQ

[0387] Address Loop IRQ. A logical 1 indicates that the local memory address of one of the voices has reached an end point. This bit will be cleared after the General Index Register (IGIDXR) is written with 8Fh, the value to access SVII. This bit is enabled (but not cleared) by URSTI[2].

[0388] ADIRQ

[0389] AdLib-Sound Blaster Register IRQ. This is the OR of the write-to-UADR interrupt bit (set high by a write to UADR), the write-to-U2X6R interrupt bit (set by a write to U2X6R), and the write-to-UI2XCR interrupt bit (set by a write to UI2XCR). The flip-flop that drives the UADR interrupt is enabled when UASBCI[1] is high and asynchronously cleared when UASBCI[1] is low; the other two bits are enabled when UASBCI[5] is high and asynchronously cleard when UASBCI[5] is low. ADIRQ is ORed into the IRQ associated with AdLib-Sound Blaster.

[0390] ADT2

[0391] AdLib Timer 2. This bit is set high when AdLib Timer 2 rolls from FF to the preload value, UAT2I. It is cleared and disabled by UASBCI[3]. The flip-flop that drives this bit is ORed into the interrupt associated with AdLib-Sound Blaster and is also readable in UASRR[1].

[0392] ADT1

[0393] AdLib Timer 1. This bit is set high when Adliv Timer 1 rolls from FF to the preload value, UAT1I. It is cleared and disabled by UASBCI[2]. The flip-flop that drives this bit is ORed into the interrupt associated with AdLib-Sound Blaster and is also readable in UASRR[2].

[0394] MIDIRX

[0395] MIDI Receive IRQ. A logical 1 indicates the MIDI Receive Data Register contains data. It is cleared by reading GMRDR.

[0396] MIDITX

[0397] MIDI Transmit IRQ. A logical 1 indicates the MIDI Transmit Data Register is empty. It is cleared by writing to GMTDR.

[0398] d. AdLib Command Read and Write Register (UACRR, UACWR)

[0399] Address: P2XR+0 Ah read (UACRR); P2XR+08h and 388h write (UACWR)

[0400] Default: 00h

[0401] This register is used to emulate AdLib operation. This register is written by AdLib application software and is read by AdLib emulation software in order to program the internal synthesizer to duplicate the AdLib sound.

[0402] e. AdLib Status Read and Write Register (UASRR, UASWR)

[0403] Address: P2XR+08h and 388h read (UASRR); P2XR+0 Ah write (UASWR)

[0404] Default: 00h

[0405] When not in auto-timer mode, this is a read-write register with different values for the read and write addresses. In auto-timer mode (UASBCI[0]=0), writes to this register are latched but not readable; reads provide the following status information:

[0406] OR56

[0407] OR of bits 5 and 6. This bit represents the logical OR of bits 5 and 6 of this register.

[0408] T1M

[0409] Timer 1, Maskable. This bit is set high when AdLib Timer 1 rolls from FF to the preload value, UAT1I. This bit is cleared by writing to UADR[AIRST]. This bit will not become active if the AdLib Timer 1 Mask is set (UADR[MT1]).

[0410] T2M

[0411] Timer 2, Maskable. This bit is set high when AdLib Timer 2 rolls from FF to the preload value, UAT2I. This bit is cleared by writing to UADR[AIRST]. This bit will not become active if the AdLib Timer 2 Mask is set (UADR[MT2]).

[0412] 2XCIRQ

[0413] Write to 2XC Interrupt. This is the write to UI2XCR interrupt bit, set high by a write to UI2XCR. The flip-flop driving this bit is enabled when UASBCI[5] is high and asynchronously cleared when UASBCI[5] is low.

[0414] 2X6IRQ

[0415] Write to 2×6 Interrupt. This is the write to U2X6R interrupt bit, set high by a write to UI2XCR. The flip-flop driving this bit is enabled when UASBCI[5] is high and asynchronously cleared when UASBCI[5] is low.

[0416] T1NM

[0417] Timer 1, Non-Maskable. This bit is set high when AdLib Timer 1 rolls from FF to the preload value, UAT1I. It is cleared and disabled by UASBCI[2]. The flip-flop that drives this bit is ORed into the interrupt associated with AdLib-Sound Blaster and is also readable in UISR[2].

[0418] T2NM

[0419] Timer 2, Non-Maskable. This bit is set high when AdLib Timer rolls from FF to the preload value, UAT2I. It is cleared and disabled by UASBCI[3]. The flip-flop that drives this bit is ORed into the interrupt associated with AdLib-Sound Blaster and is also readable in UISR[3].

[0420] DIRQ

[0421] Data IRQ. This is the write-to-UADR interrupt bit, set high by a write to UADR. The flip-flop that drives this bit is enabled when UASBCI[1] is high and asynchronously cleared when UASBCI[1] is low. It is ORed into the interrupt associated with AdLib-Sound Blaster and is also readable in UISR[4].

[0422] f. AdLib Data Register (UADR)

[0423] Address: P2XR+9 and 389h read, write

[0424] Default: 00h

[0425] This register performs AdLib-compatibility functions based on the state of various bits as follows:

[0426] For case 2, the following AdLib timer emulation bits are written. All of these bits also default to low after reset. Note that when the MSB is set high, the other bits do not change. When IVERI[RRMD] is active, the following bits are readable from this address, regardless of the state of UASBCI[0] or UACWR.

[0427] AIRST

[0428] AdLib IRQ reset. When set to a logical 1, the flip-flops driving UASRR[T1M] and UASRR[T2M] will be cleared; this bit is automatically cleared after UASRR[T1M] and UASRR[T2M] are cleared. Also, when this bit is written high, the other four bits of this register are not altered; when this bit is written as low, the other bits of this register are latched.

[0429] MT1

[0430] Mask Timer 1. When high, the flip-flop that drives UASRR[T1M] is disabled from becoming active.

[0431] MT2

[0432] Mask Timer 2. When high, the flip-flop that drives UASRR[T2M] is disabled from becoming active.

[0433] STRT2 Start Timer 2. When low, value found in UAT2I is loaded into AdLib timer 2 with every 320 microsecond rising clock edge. When high, the timer increments with every 320 microsecond rising clock edge; on the next clock edge after the timer reaches FFh, UAT2I is again loaded into the timer.

[0434] STRT1

[0435] Start Timer 1. When low, value found in UAT1I is loaded into AdLib timer 1 with every 80 microsecond rising clock edge. When high, the timer increments with every 80 microsecond rising clock edge; on the next clock edge after the timer reaches FFh, UAT1I is again loaded into the timer.

[0436] g. GUS Hidden Register Data Port (UHRDP)

[0437] Address: P2XR+0Bh write;

[0438] This is the port through which the hidden registers are accessed. Note: see IVERI[HRLEN#] for a description of how access to the hidden registers may be restricted.

[0439] h. Sound Blaster Interrupt 2XC Register (UI2XCR)

[0440] Address: P2XR+0Ch read, write

[0441] Default: 00h

[0442] Writes to this simple read-write register cause an interrupt. This register can also be written to via U2XCR, through which no interrupt is generated. The interrupt is cleared by writing UASBCI[5]=0.

[0443] i. Sound Blaster 2XC Register (U2XCR)

[0444] Address: P2XR+0Dh write

[0445] Default: 00h (after initialization)

[0446] This provides access to the Sound Blaster Interrupt 2xC Register (UI2XCR) without generating an interrupt.

[0447] j. Sound Blaster Register 2XE (U2XER)

[0448] Address: P2XR+0Eh read, write

[0449] Default: 00h

[0450] This is a simple read-write register used for Sound Blaster emulation. I/O reads of this register cause interrupts (if enabled).

[0451] k. Register Control Register (URCR)

[0452] Address: P2XR+0Fh write, read (if IVERI[RRMD] is active)

[0453] Default: 0000 0000

[0454] Note: When IVERI[RRMD] is active, this register becomes readable; if IVERI[RRMD] is not active, then reads from this address provide the data in USRR.

[0455] IQ2XE

[0456] Enable interrupts caused by reads of U2XER. A logical 1 causes interrupts to be generated by reads of U2XER. These are logically ORed with the Sound Blaster-AdLib interrupts.

[0457] EGPRA

[0458] Enable General Purpose Register Access. A logical 1 enables accesses to the general purpose registers through the addresses specified by ICMPTI[3:0], UGPA1I, and UGPA2I.

[0459] TG2XC

[0460] Toggle bit 7 of 2XC. A logical 1 causes UI2XCR[7] to toggle with each I/O read of that register.

[0461] GP2IRQ

[0462] General-purpose register 2 interrupt. A logical 1 enables the interrupt caused by either a read or write to General-purpose register 2 via the address specified by ICMPTI[3:2] and UGPA2I. The interrupt is logically ORed with the Sound Blaster/AdLib interrupt. Accesses to this register via UHRDP, the back doorr, do not cause an interrupt.

[0463] GP1IRQ

[0464] General-purpose register 1 interrupt. A logical 1 enables the interrupt caused by either a read or write to General-purpose register 1 via the address specified by ICMPTI[1:0] and UGPA1I. The interrupt is logically ORed with the Sound Blaster/AdLib interrupt. Accesses to this register via UHRDP, the back doorr, do not cause an interrupt.

[0465] RS[2:0]

[0466] Register selector. This field selects which register will be accessed via writes to the Hidden Register Data Port (UHRDP).

[0467] 0=DMA and Interrupt Control Registers (UDCI and UICI).

[0468] 1=General Purpose Register 1 Back Door (UGP1I).

[0469] 2=General Purpose Register 2 Back Door (UGP2I).

[0470] 3=General Purpose Register 1 Address [7:0] (UGPA1I).

[0471] 4=General Purpose Register 2 Address [7:0] (UGPA1I).

[0472] 5=Clear IRQs (UCLR2I).

[0473] 6=Jumper register (UJMPI).

[0474] l. Status Read Register (USRR)

[0475] Address: P2XR+0Fh read

[0476] Default: 01h

[0477] This register provides the state of various interrupts. These are all cleared by a write to the UCLRII even if multiple bits are active at the same time. Note: When IVERI[RRMD] is active, the data in this register is not accessible.

[0478] IQ2XE

[0479] 2xE Interrupt. A logical 1 indicates that a read of the U2XER caused an interrupt.

[0480] IQGP2R

[0481] General Purpose Register 2 Read Interrupt. A logical 1 indicates that a read of General Purpose Register 2 via the address specified by UCMPTI[3:2] and UGPA2I caused an interrupt.

[0482] IQGP2W

[0483] General Purpose Register 2 Write Interrupt. A logical 1 indicates that a write of General Purpose Register 2 via the address specified by UCMPTI[3:2] and UGPA2I caused an interrupt.

[0484] IQGP1R

[0485] General Purpose Register 1 Read Interrupt. A logical 1 indicates that a read of General Purpose Register 1 via the address specified by UCMPTI[1:0] and UGPA1I caused an interrupt.

[0486] IQGP1W

[0487] General Purpose Register 1 Write Interrupt. A logical 1 indicates that a write of General Purpose Register 1 via the address specified by UCMPTI[1:0] and UGPA1I caused an interrupt.

[0488] PURES

[0489] Always reads as low. Is not writeable.

[0490] IQDMA

[0491] Contains the status of the IRQ/DMA enable bit, UMCR[3].

[0492] ENJMP

[0493] Always reads as high. Is not writeable.

[0494] 2. URCR[2:0], UHRDP Indexed Registers

[0495] a. DMA Channel Control Register (UDCI)

[0496] Address: P2XR+0 Bh read, write; indexes UMCR[6]=0 and URCR[2:0]=0; also writes to PUD1SI modify the DMA1[2:0] field and writes to PUD2SI modify the DMA2[2:0] field. The ability to alter bits [5:0] through this register can be disabled via ICMPTI[4]. Note: see IVERI[HRLEN#] for a description of how access to this register is restricted.

[0497] Default: 00h

[0498] EXINT

[0499] Extra Interrupt. When both interrupt sources are combined via UICI[6], setting this bit high drives the IRQ line selected by the channel 2 interrupt selection bits UICI[5:3].

[0500] CMBN

[0501] Combine DMA channels. A logical 1 combines both DMA channels using the channel selected DMA1[2:0].

[0502] DMA2[2:0]

[0503] DMA select channel 2 (codec play):

[0504] 0=no DMA

[0505] 1=DRQ/DAK1

[0506] 2=DRQ/DAK3

[0507] 3=DRQ/DAK5

[0508] 4=DRQ/DAK6

[0509] 5=DRQ/DAK7

[0510] 6=DRQ/DAK0

[0511] DMA1[2:0] DMA select channel 1 (system memory to local memory and codec record):

[0512] 0=no DMA

[0513] 1=DRQ/DAK1 (8-bit)

[0514] 2=DRQ/DAK3 (8-bit)

[0515] 3=DRQ/DAK5 (16-bit)

[0516] 4=DRQ/DAK6 (16-bit)

[0517] 5=DRQ/DAK7 (16-bit)

[0518] 6=DRQ/DAK0 (8-bit)

[0519] b. Interrupt Control Register (UICI)

[0520] Address: P2XR+0 Bh read, write; indexes UMCR[6]=1 and URCR[2:0]=0; also writes to PUI1SI modify the IRQ1[2:0] field and writes to PUI2SI modify the IRQ2[2:0] field. The ability to alter bits [5:0] through this register can be disabled via ICMPTI[4].

[0521] Default: 07h

[0522] Note: see IVERI[HRLEN#] for a description of how access to this register is restricted.

[0523] ALSB

[0524] AdLib/Sound Blaster to NMI. A logical 1 causes IOCHK# (NMI) to be selected for Sound Blaster and AdLib “iaalsb” from the disables iaalsb from going to the IRQ selected by the Channel 1 selection bits (UICI[2:0]).

[0525] CMBN

[0526] Combine interrupt channels. A logical 1 combines both interrupt sources to the IRQ selected by IRQ1[2:0]

[0527] IRQ2[2:0]

[0528] Channel 2 (MIDI) IRQ selection:

[0529] 0=No Interrupt

[0530] 1=IRQ2

[0531] 2=IRQ5

[0532] 3=IRQ3

[0533] 4=IRQ7

[0534] 5=IRQ11

[0535] 6=IRQ12

[0536] 7=IRQ15

[0537] IRQ1[2:0]

[0538] Channel 1 (codec, synthesizer, Sound Blaster, and AdLib) IRQ selection:

[0539] 0=IOCHK#

[0540] 1=IRQ2

[0541] 2=IRQ5

[0542] 3=IRQ3

[0543] 4=IRQ7

[0544] 5=IRQ11

[0545] 6=IRQ12

[0546] 7=IRQ15

[0547] c. General Purpose Register 1 (UGP1I)

[0548] Address: P2XR+0Bh read/write; index URCR[2:0]=1

[0549] Default: 00h

[0550] General purpose register 1 consists of two 8-bit registers, UGP1I IN and UGP1I OUT, used for AdLib, Sound Blaster, and MPU-401 compatibility; it does not control any signals of the circuit C. They are accessed by a combination of this address (UHRDP) and the address specified by UCMPTI[1:0] and UGPA1I (the emulation address). UGP1I IN is written via the emulation address and read via UHRDP. UGP1I OUT is read via the emulation address and written via UHRDP. Accesses to these registers via the emulation address result in interrupts (if enabled). Note: see IVERI[HRLEN#] for a description of how access to this register is restricted.

[0551] d. General Purpose Register 2 (UGP2I)

[0552] Address: P2XR+0 Bh read/write; index URCR[2:0]=2

[0553] Default: 00h

[0554] General purpose register 2 consists of two 8-bit registers, UGP2I IN and UGP2I OUT, used for AdLib, Sound Blaster, and MPU-401 compatibility; it does not control any signals of the circuit C. They are accessed by a combination of this address (UHRDP) and the address specified by UCMPTI[3:2] and UGPA2I (the emulation address). UGP2I IN is written via the emulation address and read via UHRDP. UGP2I OUT is read via the emulation address and written via UHRDP. Accesses to these registers via the emulation address result in interrupts (if enabled). Note: see IVERI[HRLEN#] for a description of how access to this register is restricted.

[0555] e. General Purpose Register 1 Address (UGPA1I)

[0556] Address: P2XR+0 Bh write; index URCR[2:0]=3

[0557] Default: 00 h

[0558] This register controls the address through which general-purpose register 1 is accessed. The 8 bits written become bits [7:0] of the emulation address for UGP1I; emulation address bits [9:8] are specified by ICMPTI[1:0]. Note: see IVERI[HRLEN#] for a description of how access to this register is restricted.

[0559] f. General Purpose Register 2 Address (UGPA2I)

[0560] Address: P2R+0Bh read, write; index URCR[2:0]=4

[0561] Default: 00h

[0562] This register controls the emulation address through which general-purpose register 2 is accessed The 8 bits written become bits [7:0] of the emulation address for UGP2I; emulation address bits [9:8] are specified by ICMPTI[3:2]. Note: see IVERI[HRLEN#] for a description of how access to this register is restricted.

[0563] g. Clear Interrupt Register (UCLRII)

[0564] Address: P2XR+0Bh write; index URCR[2:0]=5

[0565] Writing to this register causes all the interrupts described in the USRR to be cleared. Note:see IVERI[HRLEN#] for a description of how access to this register is restricted.

[0566] h. Jumper Register (UJMPI).

[0567] Address: P2XR+0Bh read, write; index URCR[2:0]=6

[0568] Default: 06h

[0569] Note: see IVERI[HRLEN#] for a description of how access to this register is restricted.

[0570] ENJOY

[0571] Enable joystick. A logical 1 enables the game port address decode located at 201h.

[0572] ENMID Enable MIDI.

[0573] A logical 1 enables the MIDI address decodes located at P3XR+0 and P3XR+1.

[0574] 3. P3XR Direct Registers

[0575] a. General Index Register (IGIDXR).

[0576] Address: P3XR+3 read, write

[0577] Default: 00h

[0578] This register specifies the indexed address to a variety of registers within the circuit C. The data ports associated with this index are I8DP and I16DP. When in auto-increment mode (SVSR[7]), the value in this register is incremented by one after every I/O write to either I8DP or I16DP (but not 8-bit writes to the low byte of I16DP).

[0579] b. General 8/16-Bit Data Port (I8DP, I16DP).

[0580] Address: P3XR+5 for I8DP, P3XR+4-5h for I16DP, read, write

[0581] These are the data ports that are used to access a variety of registers within the circuit C. 8-bit I/O accesses to P3XR+5 are used to transfer 8-bit data. 16-bit I/O accesses to P3XR+4 are used to transfer 16-bit data. It is also possible to transfer 16-bit data by using an 8-bit I/O access to P3XR+4 followed by an 8-bit access to P3XR+5. The index associated with these ports is IGIDXR. When in auto-increment mode (SVSR[7]), the value in IGIDXR is incremented by one after every I/O write to either I8DP or I16DP (but not 8-bit writes to the low byte of I16DP, P3XR+4).

[0582] 4. IGIXR, I8DP-I16DP Indexed Registers.

[0583] a. AdLib, Sound Blaster Control (UASBCI)

[0584] Address: P3XR+5 read, write; index IGIDXR=45h

[0585] Default: 00h

[0586] This register is used to control the AdLib and Sound Blaster compatibility hardware.

[0587] SBIEN

[0588] Sound Blaster Interrupts Enable. Enables interrupts for writes to U2X6R and UI2XCR. When set to logical 1, the interrupts are enabled. When set to logical 0 the interrupts are disabled and asynchronously cleared.

[0589] ETTST

[0590] Enable Timer Test. A logical 1 enables a high-speed clock to operate AdLib Timer 1 and 2. A logical 0 allows normal clocks to operate these timers. The high-speed clock is 16.9344 MHz divided by 17, or 0.99614 MHz.

[0591] EIRQT2

[0592] Enable Interrupt For Timer 2. A logical 1 enables the interrupt associated with AdLib Timer 2. A logical 0 disables and asynchronously clears the interrupt.

[0593] EIRQT1

[0594] Enable Interrupt For Timer 1. A logical 1 enables the interrupt associated with AdLib Timer 1. A logical 0 disables and asynchronously clears the interrupt.

[0595] EDIRQ

[0596] Enable Data Interrupt. A logical 1 enables the interrupt that results from a write to the AdLib Data Register (UADR). A logical 0 disables and asynchronously clears the interrupt.

[0597] ATOFF

[0598] Disable Auto-Timer Mode. This bit low places the circuit C into auto-timer mode. This bit high disables auto-timer mode. See AUTO-TIMER MODE in the system control module and the register descriptions for UASRR, UASWR, and UADR for an explanation of auto-timer mode.

[0599] b. AdLib Timer 1 (UAT1I)

[0600] Address: P3XR+5 read, write; index IGIDXR=46h

[0601] Default: 00h

[0602] Timer 1 Load Value. This is the value that will be loaded into AdLib timer 1 whenever: (1) UADR[STRT1] is high and this timer increments past 0FFh; or (2) UADR[STRT1] is low and there is a rising clock edge of this timer's 80 microsecond clock (16.9344 MHz divided by 1344). Reads of this register provide the preload values, not the actual state of the timer.

[0603] c. AdLib Timer 2 (UAT2I)

[0604] Address: P3XR+5 read, write; index IGIDXR=47h

[0605] Default: 00h

[0606] Timer 2 Load Value. This is the value that will be loaded into AdLib timer 2 whenever: (1) UADR[STRT2] is high and this timer increments past 0FFh; or (2) UADR[STRT2] is low and there is a rising clock edge of this timer's 320 microsecond clock (timer 1's clock divided by 4). Reads of this register provide the preload values, not the actual state of the timer.

[0607] d. GF-1 Reset Register (URSTI)

[0608] Address: P3XR+5 read, write; index IGIDXR=4Ch

[0609] Default: XXX X000

[0610] DMIE

[0611] Synthesizer Interrupt Enable. This bit high enables the synthesizer's loop and volume interrupts (UISR[6:5]). Disabling these interrupts with this bit does not clear the interrupts.

[0612] DACEN

[0613] Digital to Analog Converter Enable. This bit high enables the synthesizer DAC. This bit low mutes the output of the synthesizer DAC.

[0614] RGF1

[0615] Reset GF-1. This bit low resets several of the MIDI, synthesizer, and GUS-compatibility registers. These items are reset by this bit: interrupt associated with write to U2X6R, interrupt associated with write to UI2XCR, any DMA or I/O read-write activity to local memory (including IOCHRDY), LDMACI, LMCI[1:0], LMFSI, LDICI, SGMI[ENH], the TC interrupt flip-flop (IDMATC), URSTI[2:1], UASBCI, interrupt associated with write to UADR, UADR[AIRST, MT1, MT2, STRT2, STRT1], the flip-flops that drive UASRR[T1M, T2M, 2XCIRQ, 2X6IRQ, T1NM, T2NM, DIRQ], and all the memory elements in the MIDI UART and its associated logic. Also, while this bit is low, the synthesizer IRQs are all cleared away and the synthesizer's state machines are all prevented from operating, they stay frozen and no sound is generated. This bit is fully controlled by software. Note: this bit must remain low for at least 22 microseconds after hardware and software resets have completed in order for the synthesizer register array to be properly initialized.

[0616] e. Compatibility Register (ICMPTI)

[0617] Address: P3XR+5 read, write; index IGIDXR=59h

[0618] Default: 0001 1111

[0619] STM[2:0]

[0620] Serial Transfer Mode. These specify the mode of the serial transfer block of the codec module. This block is fully specified in the codec module. When STM[2] is high, the four external function (CD-ROM) pins are switched to become the external serial port pins. The possible modes are:

[0621] CPEN

[0622] Compatibility Enable. When high, this specifies that writes to UDCI[5:0] and UICI[5:0] are allowed. When low they are not allowed. Those bits can also be altered by writes to PUD1SI, PUD2SI, PUI1SI, and PUI2SI, regardless of the state of CPEN.

[0623] GPR2A[9:8]

[0624] General Purpose Register 2 Address[9:8]. This specifies ISA-address bits[9:8] of the relocateable register UGPA2I.

[0625] GPR1A[9:8]

[0626] General Purpose Register 1 Address[9:8]. This specifies ISA-address bits[9:8] of the relocateable register UGPA1I.

[0627] f. Decode Control Register (IDECI)

[0628] Address: P3XR+5 read, write; index IGIDXR=5Ah

[0629] Default: 7Fh

[0630] This register enables and disables the docodes for various address spaces.

[0631] IAC22

[0632] Interrupt Associated With Codec To Channel 2. When high, the interrupt associated with the codec comes out on the channel 2 IRQ pin and not on the channel 1 IRQ pin. When low, this interrupt comes out on channel 1.

[0633] EICH1

[0634] Enable Interrupts on Channel 1. When high, channel 1 interrupts are enabled. When low, the selected channel 1 IRQ output becomes high-impedance.

[0635] EICH2

[0636] Enable Interrupts on Channel 2. When high, channel 2 interrupts are enabled. When low, the selected channel 2 IRQ output becomes high-impedance.

[0637] EINMI

[0638] Enable NMI Interrupts. When high, IOCHK# interrupts are enabled. When low, IOCHK# becomes high-impedance.

[0639] ECOD

[0640] Enable Decode of Codec. When high, I/O reads and writes to the codec address space, the four bytes of PCODAR, are enabled. When low, the decodes of these addresses are disabled.

[0641] E3889

[0642] Enable Decodes of 388h and 389h. When high, decodes of the AdLib Command-Status and Data registers—fixed addresses 388 and 389—are enabled. When low, the decodes of these addresses are disabled.

[0643] EEDC

[0644] Enable Decodes of 2×E, 2×D, and 2×C. When high, reads and writes to P2XR+Eh, P2XR+Dh, and P2XR+Ch are enabled. When low, the decodes of these addresses are disabled.

[0645] EA98

[0646] Enable Decodes of 2×A, 2×9, and 2×8. When high, reads and writes to P2XR+Ah, P2XR+9h, and P2XR+8h are enabled. When low, the decodes of these addresses are disabled.

[0647] g. Version Number Register (IVERI)

[0648] Address: P3XR+5 read, write; index IGIDXR=5Bh

[0649] Default: 0000 0100

[0650] VER

[0651] Version Number. This contains the version number of the die. Here are the possibilities: 0h=rev A silicon. This field is read only.

[0652] RRMD

[0653] Register Read Mode. When high, this bit specifies that reads of three of the circuit Cs normally-unreadable registers will return the data written to those registers. Reads of UADR (P2Xr+9, 389h) will return the bits [AIRST, MT1, MT2, 0, 0, 0, STRT2, STRT1], regardless of the state of UASBCI[0] or UACWR; reads of URCR (P2XR+Fh) return the data last written to that address instead of USRR and reads of GMCR (P3XR+0) return the data last written to that address instead of GMSR.

[0654] PUPWR

[0655] Pull-Up Power. This bit low disables the power to the internal pull up resistors on the signals IOCS16#, IRQ[15, 12, 11, 7, 5], SA[11:6], SBHE#, DRQ[7:5, 3], DAK[7:5,3]#, and SD[15:8] so that these signals do not drive voltages onto the ISA bus during suspend mode, or, in general, add current load. This bit high enables the pull-up resistors on those signals. Normally, this bit will be left high for 120-pin parts and set low for 160-pin parts.

[0656] M401

[0657] MPU-401 Emulation mode. This bit high enables the following: (1) the MIDI transmit-receive registers (GMTDR, GMRDR) are moved from P3XR+1 to P3XR+0 and (2) the MIDI control-status registers (GMCR, GMSR) are moved from P3XR+0 to P3XR+1.

[0658] HRLEN#

[0659] Hidden Register Lock Enable. When high (inactive), accesses to the registers located at UHRDP are always enabled. When low (active), access to the registers located at UHRDP must conform to a protocol. The protocol is initiated by a write to UMCR which enables the next subsequent I/O access to the hidden registers at UHRDP. An I/O read or write (while AEN is low) to any address except P2XR+0 (UMCR) or P2XR+0Bh (UHRDP) will lockout further I/O accesses to the hidden registers.

[0660] h. MPU-401 Emulation Control A (IEMUAI)

[0661] Address: P3XR+5 read, write; index IGIDXR=5Ch

[0662] Default: 00h

[0663] The emulation address described in the following bit definitions is the address specified by UGPA1I, UGPA2I, and ICMPTI[3:0].

[0664] URRE#

[0665] UART Receive Buffer Read Enable. When low, reads of the UART's receive data buffer, GMRDR, are allowed. When high, reads of that buffer are ignored internally (although, the ISA data bus will still be driven).

[0666] USRE#

[0667] UART Status Read Enable. When low, reads of the UART's status register, GMSR, allowed. When high, reads of that register are ignored internally (although, the ISA data bus will still be driven).

[0668] E2RE#

[0669] Emulation Register 2 Read Enable. When low, reads of emulation register 2, UGP2I, via the emulation address are allowed. When high, reads of UGP2I via the emulation address are ignored internally (although, the ISA data bus will still be driven).

[0670] E1RE#

[0671] Emulation Register 1 Read Enable. When low, reads of emulation register 1, UGP1I, via the emulation address are allowed. When high, reads of UGP1I via the emulation address are ignored internally (although, the ISA data bus will still be driven).

[0672] UTWE#

[0673] UART Transmit Buffer Write Enable. When low, writes to the MIDI UART's transmit buffer, GMTDR, are allowed. When high, writes to that buffer are ignored by the UART.

[0674] UCWE#

[0675] UART Command Buffer Write Enable. When low, writes to the MIDI UART's command register, GMCR, are allowed. When high, writes to that register are ignored by the UART.

[0676] E2WE#

[0677] Emulation Register 2 Write Enable. When low, writes to emulation register 2, UGP2I, via the emulation address are allowed. When high, UGP2I does not change during writes to the emulation address.

[0678] E1WE#

[0679] Emulation Register 1 Write Enable. When low, writes to emulation register 1, UGP1I via the emulation address are allowed. When high, UGP1I does not change during writes to the emulation address.

[0680] i. MPU-401 Emulation Control B (IEMUBI)

[0681] Address: P3XR+5 read, write; index IGIDXR=5Dh

[0682] Default: 30h

[0683] MRXE#

[0684] MIDI Receive Data Enable. When low, MIDI receive data from MIDIRX pin is allowed to pass into the UART. When high, the data is disabled from coming into the UART.

[0685] MTXE#

[0686] MIDI Transmit Data Enable. When low, MIDI transmit data from the UART is allowed to pass to the MIDITX pin. When high, the data is disabled from coming out of the pin.

[0687] SLSE7

[0688] Select Status Emulation Register 1, Bit[7] for I/O Reads. This bit high causes the circuit C to enable UGP1IOUT[7] onto the data bus during reads of UGP1IOUT via the emulation address (ICMPTI[1:0] and UGPA1I[7:0]. This bit low causes the circuit C to enable the DSR# onto bit[7] of the data bus during those reads; DSR# is set inactive (high) by the hardware when there is a read of UGP2IOUT via the emulation address (ICMPTI[3:2], UGPA2I), if reads of UGP2IOUT are enabled (IEMUAI[5]); this flag is also controlled by writes to UGP1IOUT[7] via the back door (UHRDP).

[0689] SLSE6

[0690] Select Status Emulation Register 1, Bit[6] for I/O Reads. This bit high causes the circuit C to enable UGP1IOUT[6] onto the data bus during reads of UGP1IOUT via the emulation address (ICMPTI[1:0] and UGPA1I[7:0]). This bit low causes the circuit C to enable DRR# onto bit[6] of the data bus during those reads; DRR# is set inactive (high) by the hardware whenever there is a write to either of UGP1IIN or UGP2IIN via the emulation address (ICMPTI[3:0), UGPA1I, UGPA2I), if a write to that register is enabled (IEMUAI[1:0]); it is also controlled by writes to UGP1IOUT[6] via the back door (UHRDP).

[0691] E2WIE#

[0692] Emulation Register 2 Write Interrupt Enable. When low, writes to the address selected by ICMPTI[3:2] and UGPA2I[7:0] for UGP2I cause interrupts. When high, writes to UGP2I do not cause interrupts.

[0693] E1WIE#

[0694] Emulation Register 1 Write Interrupt Enable. When low, writes to the address selected by ICMPTI[1:0] and UGPA1I[7:0] for UGP1I cause interrupts. When high, writes to UGP1I do not cause interrupts.

[0695] E2RIE#

[0696] Emulation Register 2 Read Interrupt Enable. When low, reads of the address selected by ICMPTI[3:2] and UGPA2I[7:0] for UGP2I cause interrupts. When high, reads of UGP2I do not cause interrupts.

[0697] E1RIE#

[0698] Emulation Register 1 Read Interrupt Enable. When low, reads of the address selected by ICMPTI[1:0] and UGPA1I[7:0] for UGP1I cause interrupts. When high, reads of UGP1I do not cause interrupts.

[0699] j. Test Control Register (ITCI)

[0700] Address: P3XR+5 read, write; index IGIDXR=5Fh; also, in external decoding mode, this register is directly readable (see REGISTER SUMMARY for a discussion of external decoding mode).

[0701] Default: 000 0000b; see TE below for the default description of bit[7].

[0702] Access to this register can be disabled by the state of MIDITX at the trailing edge of reset. See the PIN SUMMARY section for details. Also, none of the bits in this register are reset by the software reset, PCCCI; they are only reset by activation of the RESET pin.

[0703] TE

[0704] Test Enable. This bit high indicates that the device is in test mode. When it is low, the device is in normal or functional mode. The default state of this bit is latched at the trailing edge of reset by the state of the MWE# pin. If MWE# is low, TE will be high; if MWE# is high, TE will be low. This bit is not reset by the software reset (PCCCI).

[0705] BPOSC

[0706] Bypass Oscillator Stabilization Circuit. When high, the oscillator stabilization circuit—which is responsible for counting out oscillator clocks to guarantee that the 16.9 MHz oscillator is stable—only counts 256 states. When it is low, the oscillator stabilization logic counts out 64K states. This bit is reset by the RESET pin, but only by the software reset (PCCCI).

[0707] TMS[5:0]

[0708] Test Mode Select. These bits are available to provide selection of various circuit test modes. These are reset by the RESET pin, but not by the software reset (PCCCI).

[0709] 5. PNP Direct Registers

[0710] a. Card Select Number Back Door (PCSNBR)

[0711] Address: 0201h write

[0712] Default: 00h

[0713] If the circuit C is in PNP system mode (latched by the state of the PNPCS pin at the end of reset), the AUDIO logical device has not been activated (PUACTI[0]=0), and the PNP state machine is in isolation mode, then it is possible to write a card select number (CSN) to the circuit C via this I/O port.

[0714] b. PNP Index Address Rester (PIDXR)

[0715] Address: 0279h write

[0716] Default: 00h

[0717] This is the 8-bit index address register which points to standard Plug and Play registers.

[0718] c. PNP Data Write Port (PNPWRP)

[0719] Address: 0A79h write

[0720] This is the port used to write to Plug and Play ISA registers, indexed by PIDXR.

[0721] d. PNP Data Read Port (PNPRDP)

[0722] Address: Address is relocatable between 003h and 3FFh, read only. Address is set by (1) setting the PIDXR register to 00h, and (2) writing the byte that represents bits 9 through 2 to PNPWRP; bits 0 and 1 are both always assumed to be high (1 1).

[0723] This is the port used to read from Plug and Play ISA registers, indexed by PIDXR.

[0724] 6. PER PNPWRP-PNPRDP PNP Indexed Registers

[0725] These PNP registers are indexed with PIDXR and accessed via PNPRDP and PNWRP. Many of the registers—PIDXR=30h and greater—are further indexed by the Logical Device Number Register (PLDNI); all such registers can only be accessed when the PNP state machine is in the configuration state.

[0726] a. PNP Set Read Data Port Address Register (PSRPAI)

[0727] Address: 0A79h write; index PIDXR=0

[0728] Default: 00h

[0729] Writes to this register set up SA[9:2] of the address of the PNP Read Data Port (PNPRDP). SA[1:0] are both assumed to be high. Writes to this register are only allowed when the PNP state machine is in the isolation state.

[0730] b. PNP Isolate Command Register (PISOCI)

[0731] Address: PNPRDP read; index PIDXR=1

[0732] Reading this register will cause the circuit C to drive a specific value—based on data read out of the PNP serial EEPROM 78—onto the ISA bus 156 and observe the data back into the circuit C to see if there is a difference. This can result in a “lose-isolation” condition and cause the PNP state machine to go into sleep mode. If the circuit C is in PNP-system mode (see the POWER-UP PNP MODE SELECTION section), then it is assumed that there is no serial EEPROM 78 and no data will ever be driven on the bus for reads from this register; in PNP-system mode, reads of PISOCI always cause the circuit C to “lose” the isolation and go into sleep mode. Reads from this register are only allowed when the PNP state machine is in the isolation state.

[0733] c. PNP Confirguration Control Command Register (PCCCI)

[0734] Address: 0A79h write; index PIDXR=2

[0735] RCSN

[0736] Reset CSN. If the PNP state machine is in either sleep, isolate or configuration mode, then a high on this bit causes the CSN to be set to zero. This command is ignored if the PNP state machine is in the wait-for-key mode, but it is valid for the other three modes.

[0737] WFK

[0738] Wait For Key. A high on this bit causes the PNP state machine to enter the wait-for-key mode. This command is ignored if the PNP state machine is in the wait-for-key mode, but it is valid for the other three modes.

[0739] RESET

[0740] A high on this pin causes the circuit C to be reset. This will result in 3 to 10 millisecond pulse over the general reset line to the entire circuit C. The only devices that will not be reset by this command are PSRPAI (PNP Set Read Data Port), PCSNI (PNP Card Select Number), and the PNP state machine. This command is ignored if the PNP state machine is in the wait-for-key mode, but it is valid for the other three modes.

[0741] d. PNP WAKE[CSN] Command Register (PWAKEI)

[0742] Address: 0A79h write; index PIDXR=3

[0743] Writes to this register affect the PNP state machine based on the state of the CSN register and the data written. If the data is 00h and the CSN is 00h, then the PNP state machine will enter the isolation state. If the data is not 00h and the CSN matches the data, then the PNP state machine will enter the configuration state. If the data does not match the CSN, then the PNP state machine will enter the sleep state. This command also resets the serial EEPROM 78 control logic that contains the address to that part. This command is ignored if the PNP state machine is in the wait-for-key mode, but it is valid for the other three modes.

[0744] e. PNP Resource Data Register (PRESDI)

[0745] Address: PNPRDP read; index PIDXR=4

[0746] Default: 00h

[0747] This register provides the data from the local memory control module 8 (LMC) that has been read out of the PNP serial EPROM 78. Note: if the serial EEPROM 78 has been placed into direct control mode (PSEENI[0]), then the wake command must be executed before access via PRESDI is possible. This command is only valid when the PNP state machine is in the configuration state.

[0748] f. PNP Resource Status Register (PRESSI)

[0749] Address: PNPRDP read; index PIDXR=5

[0750] Default: 00h

[0751] A high on bit 0 of this register indicates that the next byte of PNP resource data is available to be read; all other bits are reserved. After the PRESDI is read, this bit becomes cleared until the next byte is available. This command is only valid when the PNP state machine is in the configuration state.

[0752] g. PNP Card Select Number Register (PCSNI)

[0753] Address: 0A79h write, PNPRDP read; index PIDXR=6

[0754] Default: 00h

[0755] Writes to this register while the PNP state machine is in the isolation state set up the CSN for the circuit C and send the PNP state machine into configuration mode. When the PNP state machine is in configuration mode, this register is readable, but not writeable.

[0756] h. PNP Logical Device Number Register (PLDNI)

[0757] Address: 0A79h write, PNPRDP read; index PIDXR=7

[0758] Default: 00h

[0759] This register further indexes the PNP address space into logical devices. The circuit C has two logical device numbers (LDN): 00h=all AUDIO functions, synthesizer, codec, and ports; 01h=the external (CD-ROM) interface. This register can only be accessed when the PNP state machine is in the configuration state.

[0760] i. PNP Audio Activate Register (PUACTI)

[0761] Address: 0A79h write, PNPRDP read; indexes PIDXR=30h and PLDNI=0

[0762] Default: 00h

[0763] A high on bit 0 of this register activates all the AUDIO functions; all other bits are reserved. When low, none of the AUDIO-function address spaces are decoded and the interrupt and DMA channels are not enabled.

[0764] j. PNP Audio I/O Range Check Register (PURCI)

[0765] Address: 0A79h write, PNPRDP read; indexes PIDXR=31h and PLDNI=0

[0766] Default: 00h

[0767] RCEN

[0768] Check Enable. This bit high causes reads of all AUDIO logical device address spaces to drive either 55 or AA based on the state of H5LA. This only functions when the PUACTI[0] is not set (the Audio device is not activited).

[0769] H5LA

[0770] High 55-Low AA. When RCEN is active, this bit selects the data value that is driven back onto the ISA data bus 156 during a read. A high specifies that 55h be driven and a low specifies AAh. Note: this register is not available when iin external decoding mode.

[0771] k. PNP Address Control Registers

[0772] The following table shows all the various PNP registers that control the address of blocks of I/O space within the circuit C.

[0773] Notes: There are three indexes that identically control P2XR. This is in support of the non-contiguous addresses in the P2XR block. Only the first of these, P2X0[H,L]I, are used for P2XR.

[0774] All unused bits in the above PNP address control registers are reserved. All of the above PNP address control registers are written via 0A79h and read via PNPRDP. The unspecified LSBs of P2XR, P3XR, PCODAR, and PCDRAR are all assumed to be zero. See the General Description section for a description of the functions controlled by the various address blocks.

[0775] 1. PNP Audio IRQ Channel 1 Select Register (PUI1SI)

[0776] Address: 0A79h write, PNPRDP read; indexes PIDXR=70h and PLDNI=0

[0777] Default: 00h

[0778] Bits[3:0] select the IRQ number for channel 1 interrupts as follows:

[0779] Bits[7:4] are reserved. Writes to this register appropriately affect UICI[2:0].

[0780] m. PNP Audio IRQ Channel 1 Type Register (PUI1TI)

[0781] Address: PNPRDP read; indexes PIDXR=71h and PLDNI=0

[0782] Default: 02h

[0783] The registers provides data back to standard PNP software concerning the type of interrupts supported by the circuit C. It will always be read back as 02h to indicate edge-triggered, active-high interrupts.

[0784] n. PNP Audio IRQ Channel 2 Select Register (PUI2SI)

[0785] Address: 0A79h write, PNPRDP read; indexes PIDXR=72h and PLDNI=0

[0786] Default: 00h

[0787] Bits[3:0] select the IRQ number for channel 2 interrupts as follows:

[0788] Bits[7:4] are reserved. Writes to this register appropriately affect UICI[5:3].

[0789] o. PNP Audio IRQ Channel 2 Type Register (PUI2TI)

[0790] Address: PNPRDP read; indexes PIDXR=73h and PLDNI=0

[0791] Default: 02h

[0792] The registers provides data back to standard PNP software concerning the type of interrupts supported by the circuit C. It will always be read back as 02h to indicate edge-triggered, active-high interrupts.

[0793] p. PNP Audio DMA Channel Select Registers (PUD1SI, PUD2SI)

[0794] Address: 0A79h write, PNPRDP read; indexes PIDXR=74h (PUD1SI), PIDXR=75h (PUD2SI), and PLDNI=0

[0795] Default: 04h

[0796] Bits[2:0] of these registers select the DMA request number for channels 1 and 2 as follows:

[0797] Bits[7:3] are reserved. Writes to these registers appropriately affect UDCI[5:0].

[0798] q. PNP Serial EEPROM Enable (PSEENI)

[0799] Address: 0A79h write, PNPRDP read; index PIDXR=F0h and PLDNI=0

[0800] Default: 00h

[0801] This register is only accessible when the PNP state machine is in the configuration state.

[0802] ISADR

[0803] ISA-Data-Bus Drive. This specifies the output-low drive capability, Iol, of the ISA data bus, SD[15:0], IOCHROY, IOCS16# AND IOCHK#. At 5 volts: 00=24 mA, 01=12 mA, 10=3 mA, 11=reserved. At 3.3 volts, the drive is at least 3 mA for ISADR=00, 01, and 10.

[0804] SEM

[0805] Serial EEPROM Mode. A low specifies that the serial EEPPROM interface circuitry is in initialization mode whereby the data transfer is controlled by the PNP state machine. A high specifies the control mode whereby the serial EEPROM 78 is controlled directly by PSECI.

[0806] r. PNP Serial EEPROM Control (PSECI)

[0807] Address: 0A79h write, PNPRDP read; index PIDXR=F1h and PLDNI=0

[0808] Default: XXXX 000X

[0809] When in control mode (PSEENI[0]), if PUACTI is inactive, then bits[3:0] are used to directly control the serial EEPROM 78. Bits[7:4] are read-only status bits that show the state of various control signals that are latched at the trailing edge of RESET (see the PIN SUMMARY section in the general description above for details). This register is only accessible when the PNP state machine is in the configuration state.

[0810] SUS32

[0811] SUSPEND-C32KHZ Select. Provides the state of the internal signal IPSUS32 which is latched off the RA[21] pin at the trailing edge of RESET.

[0812] XDEC

[0813] External Decode Select. Provides the state of the internal signal IPEXDEC which is latched off the RA[20] pin at the trailing edge of RESET.

[0814] PSYS

[0815] PNP System Board Select. Provides the state of the internal signal IPPNPSYS which is latched off the PNPCS pin at the trailing edge of RESET.

[0816] VCC

[0817] VCC is 5 Volts. Provides the state of the internal 5-volt-3.3-volt detect circuitry. It is high for 5 volts and low for 3.3 volts.

[0818] SECS

[0819] Serial EEPROM Chip Select. Writes to this bit are reflected on the PNPCS pin. Reads provide the latched value.

[0820] SESK

[0821] Serial EEPROM Serial Clock. Writes to this bit are reflected on the MD[2] pin. Reads provide the latched value.

[0822] SEDI

[0823] Serial EEPROM Data In. Writes to this bit are reflected on the MD[1] pin. Reads provide the latched value.

[0824] SEDO

[0825] Serial EEPROM Data Out. Writes to this bit are ignored; reads provide the state of the MD[0] pin.

[0826] S. PNP Power Mode (PPWRI)

[0827] Address: 0A79h write, PNPRDP read; index PIDXR=F2h and PLDNI=0

[0828] Default: X111 1111

[0829] This register is used to disable clocks and enable low-power modes for major sections of the circuit C. Writes to this register are accomplished differently than most. The MSB of the data, ENAB, is used to specify whether ones or zeros are to be written; for bits[6:0], a high indicates that ENAB is to be written into the bit and a low indicates that the bit is to be left unmodified. Thus, when there is a need to modify a subset of bits[6:0], it is not necessary for software to read the register ahead of time to determine the state of bits that are not to change. Examples are: to set bit[0] high, a write of 81h is needed; to clear bit[4] to a low, a write of 10h is needed.

[0830] If a single command comes to clear bits[6:1] to the low state (I/O write of 0111 111X, binary), then the circuit C enters shut-down mode and the 16.9 MHz. oscillator 16 becomes disabled. When, subsequently, one or more of bits[6:1] are set high, the 16.9 MHz oscillator 16 is re-enabled. After being re-enabled, the 16.9 MHz clock will require 4 to 8 milliseconds before becoming stable.

[0831] This register is only accessible when the PNP state machine is in the configuration state.

[0832] ENAB

[0833] Enable. Used to specify the value that is to be written to bits [6:0] of the register (see above). In all seven cases, a high specifies that the block is functional and a low indicates that it is in low-power mode.

[0834] PWR24

[0835] 24.576 MHz. Oscillator Enable. This bit low causes the 24.576 MHz. oscillator 18 to stop. It is not recommended that this oscillator be disabled if either CPDFI[0] or CRDFI[0] are low. However, it is legal to set this bit low as part of the shut-down command, despite the state of CPDFI[0] and CRDFI[0].

[0836] PWRL

[0837] Local Memory Control Enable. This bit low disables the 16.9 MHz. clock to the local memory control module 8 and allows slow refresh cycles to local DRAM 110 using C32KHZ input 72.

[0838] PWRS

[0839] Synthesizer Enable. This bit low disables the 16.9 MHz. clock to the synthesizer module 6 and the clocks to the synthesizer DAC input to the codec mixer (see discussion in synthesizer and CODEC section of this application).

[0840] PWRG

[0841] Game-MIDI Ports Enable. This bit low disables all clocks to the ports module 10 and disables internal and external resistors from consuming current.

[0842] PWRCP

[0843] Codec Playback Path Enable. This bit low disables clocks to the codec playback path including the playback FIFO, format conversion, filtering, and DAC.

[0844] PWRCR

[0845] Codec Record Path Enable. This bit low disables clocks to the codec record path including the record FIFO, format conversion, filtering, and ADC.

[0846] PWRCA

[0847] Codec Analog Circuitry Enable. This bit low disables all the codec analog circuitry and places it in a low-power mode. When low, all the analog pins—MIC[L,R], AUX1[L,R], AUX2[L,R], LINEIN[L,R], MONOIN, LINEOUT[L,R], MONOOUT, CFILT, IREF—are placed into the high-impedance state.

[0848] t. PNP CD-ROM Activate Register (PRACTI)

[0849] Address: 0A79h write, PNPRDP read; indexes PIDXR=30h and PLDNI=1

[0850] Default: 00h

[0851] A high on bit 0 of this register activates the external interface (e.g., CD-ROM) function; all other bits are reserved. When low, the external function (CD-ROM) address space is not decoded; the external function (e.g., CD-ROM) interrupt and DMA channels are not enabled.

[0852] u. PNP CD-ROM I/O Range Check Register (PRRCI)

[0853] Address: 0A79h write, PNPRDP read; indexes PIDXR=31h and PLDNI=1

[0854] Default: 00h

[0855] RCEN

[0856] Range Check Enable. This bit high causes reads of all external function address space to drive either 55 or AA based on the state of H5LA. This only functions when the PRACTI[0] is not set (the external device is not activated).

[0857] H5LA

[0858] High 55-Low AA. When RCEN is active, this bit selects the data value that is read back. A high specifies that 55h be driven and a low specifies AAh.

[0859] v. PNP CD-ROM High, Low Address Register (PRAHI, PRALI)

[0860] See the PNP address control registers above.

[0861] w. PNP CD-ROM IRQ Select Register (PRISI)

[0862] Address: 0A79h write, PNPRDP read; indexes PIDXR=70h and PLDNI=1

[0863] Default: 00h

[0864] Bits[3:0] select the IRQ number for external function (CD-ROM) interrupts as follows:

[0865] Bits[7:4] are reserved.

[0866] x. PNP CD-ROM IRQ Type Regster (PRITI)

[0867] Address: PNPRDP read; indexes PIDXR=71h and PLDNI=1

[0868] Default: 02h

[0869] The registers provides data back to standard PNP software concerning the type of interrupts supported by the circuit C. It will always be read back as 02h to indicate edge-triggered, active-high interrupts.

[0870] y. PNP CD-ROM DMA Select Register (PRDSI)

[0871] Address: 0A79h write, PNPRDP read; indexes PIDXR=74h and PLDNI=1

[0872] Default: 04h

[0873] Bits[2:0] of these registers select the DMA request number for the external function (CD-ROM) as follows:

[0874] Bits[7:3] are reserved.

[0875] FIG. 44 depicts, in block diagram format, the various features and functions included within the CODEC module device 505. The CODEC device 505 includes on-chip memory, which is preferably configured as 16-sample, 32-bit wide, record and playback FIFOs, 538, 532, with selectable thresholds capable of generating DMA and I/O interrupts for data read and write operations. The Mixing and Analog Functions block 510 includes left and right channel analog mixing, muxing and loopback functions. Left channel and right channel stereo, and single channel mono, analog audio signals are summed in Mixing and Analog Functions block 510. These mono and stereo audio signals are output from the CODEC 505 for external use, on analog output pins 522. Inputs to the Mixing and Analog Functions block 510 are provided from: external Analog Input Pins 520, analog output from a Synthesizer Digital-to-Analog Converter block 512, which is external to CODEC 505 or may be a processing block within CODEC 505, and from the Playback Digital-to-Analog Converter block 514. Analog audio output from Mixing Analog Functions block 510 is provided to record Analog-to-Digital Converter 516 block. Synthesizer Digital-to-Analog Converter block 512 receives Digital data from a synthesizer 524. Throughout this description, it should be understood that synthesizer 524 is an external device, or may be integrated onto the same monolithic integrated circuit as the CODEC device 505.

[0876] The record path for the CODEC 505 is illustrated in FIG. 44, with analog audio data being output from Mixing and Analog Functions block 510 and provided to record Analog-to-Digital Converter (ADC) 516 block to be converted to 16-bit signed data. The selected sample rate for record ADC 516 affects the sound quality such that the higher the sample rate for record ADC 516, the better the recorded digital audio signal approaches the original audio signal in quality. The function and operation of a fourth order cascaded delta-sigma modulator, preferably implemented in record ADC 516 block, is described in application Ser. No. 08/071,091, filed Dec. 21, 1993, entitled “Fourth Order Cascaded Sigma-Delta Modulator,” assigned to the common assignee of the present invention. The converted digital audio data is then sent to format conversion block 536 which converts the 16-bit digital audio data to a preselected data format. The formatted digital data is then sent to 32-bit wide record FIFO 538 as 16-bit left and 16-bit right channel data for further submission to register data bus 526 for output to external system memory (not shown) or to off-chip local memory record FIFO 530 (LMRF).

[0877] The playback path for CODEC 505 includes digital data, in a preselected data format, being sent to 32-bit wide playback FIFO 532 from the off-chip local memory playback FIFO (LMPF) 528 or from external system memory (not shown), via the register data bus 526. It should be understood throughout this application that LMRF 530 and LMPF 528 may be discreet off-chip FIFOs, or may be dedicated address space within off-chip local memory 110 configured as FIFOs. The formatted data is then input to format conversion clock 534, where it is converted to 16-bit signed data. The data is then sent to the CODEC playback DAC 514, where it is converted to an analog audio signal and output to the input of Mixing and Analog functions block 510.

[0878] A Serial Transfer Control block 540 provides serial-to-parallel and parallel-to-serial conversion functions, and loop back capability between the output of 32-bit wide record FIFO 538 and the input of 32-bit wide playback FIFO 532. Also, synthesizer serial input data port 542 (FIG. 44), which receives serial data from synthesizer 524, communicates with serial Transfer Control block 540. Serial Transfer Control block 540 is connected to record FIFO 538, playback FIFO 532, off-chip local memory 110 (or, LMRF 530 and LMPF 528) via local memory control 790, synth serial input data port 542, and to External Serial Interface. Bi-directional serial data communication over External Serial Interface 544, which includes an external serial port, is provided to Serial Transfer Control block 540 (also see FIG. 49). External serial interface 544 may be a UART, or other device that provides either synchronous or asynchronous controlled serial data transfers. External Serial Interface 544 (FIG. 44) can be connected to communicate serially with an external digital signal processor (DSP) for off-chip generation of special audio effects, or with any other device capable of bidirectional serial data communication. External serial interface 544 can also connect to and provide a serial data path from external synthesizer serial input port 542. Bi-directional data transfer is also accomplished via data path 550 between serial transfer control 540 and local memory control 790.

[0879] The various loop back and data conversion functions associated with Serial Transfer Control block 540 are shown in more detail in FIGS. 49 and 49a.

[0880] The CODEC 505 includes A/D conversion functions in the record path and D/A conversion functions in the playback path. These conversion functions are capable of operating independently of each other at different sample rates so A/D and D/A operations may be performed simultaneously, each having a different sample rate and data format. Loop access circuitry (in mixing block 606) provides a capability to sample an audio signal and perform an A/D operation at one rate, digitize the signal, and then playback the digitized sample back through the playback D/A at a different sample rate.

[0881] The block designated Counters, Timers and Miscellaneous digital functions 518 includes circuitry which controls: the A/D and D/A conversions in CODEC 505, format conversion blocks 532, 536, and data transfer functions. CODEC 505 operation allows the following data formats: 8-bit unsigned linear; 8-bit μ-law; 8-bit A-law, 16-bit signed little endian; 16-bit signed big endian; or 4-bit 4:1 IMA ADPCM format.

[0882] Referring to FIG. 45, the left channel of CODEC analog mixer 606 of Mixing and Analog functions block 510 is depicted. The layout of the right channel of mixer 606 is identical to the left channel, but is not shown in FIG. 45. Except for minor signal name modifications, all descriptions of left channel signals and functions are applicable to the right channel.

[0883] The CODEC analog mixer 606 has more programmable features and more functions than prior CODEC audio devices. Each of the five input lines to the analog mixer 606 in FIG. 45 (LINEINL 682, MICL 684, AUXIL 686, AUX2688 and MONOIN 690) includes a programmable attenuation/gain control circuit 608, 610, 612, 614 and 696, respectively. All inputs and outputs to and from analog mixer 606, are stereo signals, except for input MONOIN 690 and output MONOOUT 668, which are mono signals. The choice of mono or stereo audio signal inputs or outputs is also selectable.

[0884] Each of the triangle blocks depicted in FIG. 45 represents a programmable attenuation/gain control circuit. The registers that control the respective attenuation/gain control circuits and the attenuation/gain range for that circuit are identified in FIG. 45 next to the respective triangle block, and are located in the Registers block 566 in FIG. 50. The description and address of each of these registers is described below. Individual bits in these registers are capable of being modified as described in application Ser. No. 08/171,313, entitled Method and Apparatus for Modifying the Contents of a Register via a Command Bit, which describes a single-bit manipulation technique that obviates the need to address an entire register, and is assigned to the common assignee of the present invention and incorporated herein for all purposes.

[0885] The range of attenuation values for these registers are shown in FIGS. 45A-1 and 45A-2. The value stored in each attenuation/gain control register is used to provide the selected gain or attenuation value to CODEC control logic in the Counters, Timers and Misc. Digital Functions block 518, and Gain/attenuation Block 734 (FIG. 47) explained below. The amplitude of the analog audio signal input to the respective attenuation/gain circuit is controlled by the value stored in the respective attenuation/gain control register.

[0886] The overview of the registers used in CODEC 505 Registers block 566, including their preferred functions, are as follows:

[0887] The CODEC 505 is designed to be generally register-compatible with the CS4231 (Modes 1 and 2), with the AD1848 and other prior art. An indirect addressing mechanism is used for accessing most of the CODEC registers. In Mode 1 (discussed below), there are 16 indirect registers; in Mode 2 (discussed below), there are 28 indirect registers; and in Mode 3 (discussed below), there are 32 indirect registers.

[0888] In the following register definitions, RES or RESERVED specifies reserved bits. All such fields must be written with zeros; reads return indeterminate values; a read-modify-write operation can write back the value read.

[0889] CODEC DIRECT REGISTERS

[0890] CODEC INDEX ADDRESS REGISTER (CIDXR)

[0891] Address: PCODAR+0 read, write

[0892] Default: 0100 0000

[0893] Modes: bits[7:5,3:0] modes 1, 2, and 3; bit[4] modes 2 and 3

[0894] Initialization.

[0895] This read-only bit will be read as high if the CODEC is in an initialization phase and unable to respond to I/O activity. This bit is set only by software resets and cleared once the 16 MHz oscillator is stable and the CODEC 505 has initialized.

[0896] MCE

[0897] Mode Change Enable. This bit protects the CPDFI, CRDFI, and CFIG1I from being written (except CFIG1I[1:0]; these can be changed at any time). When high, the protected registers can be modified; also, the DAC outputs (CLDACI and CRDACI) are forced to mute. When low, the protected registers cannot be modified.

[0898] DTD

[0899] DMA Transfer Disable. This bit high causes DMA transfers to be suspended when either of the sample counter interrupts of CSR3R becomes active.

[0900] Mode 1:

[0901] DMA is suspended (whether it be playback or record) and the sample counter stops after the sample counter causes an interrupt; also, the active FIFO is disabled from transferring more data to CODEC 505. DMA transfers, FIFO transfers and the sample counter resume when GINT is cleared or DTD is cleared.

[0902] Modes 2 and 3:

[0903] Record DMA, the record FIFO and the record sample counter stop when the record sample counter causes an interrupt; playback DMA, the playback FIFO and the playback sample counter stop when the playback sample counter causes an interrupt. The pertinent DMA transfers and sample counter resume when the appropriate interrupt bit in CSR3I is cleared or DTD is cleared.

[0904] In mode 3, this bit also works to discontinue the transfer of data between the CODEC FIFOs and the LMRF and the LMPF.

[0905] IA[4:0]

[0906] Indirect Address Pointer. These bits are used to point to registers in the indirect address space. In mode 1, a 16-register space is defined; IA[4] is reserved. In modes 2 and 3, a 32-register space is defined.

[0907] CODEC INDEXED DATA PORT (CDATAP)

[0908] Address: PCODAR+1 read, write

[0909] Modes: 1, 2, and 3

[0910] This is the access port through which all CODEC indexed registers—pointed to by the CODEC Indexed Address Register (CIDXR[4:0])—are written or read.

[0911] CODEC STATUS 1 REGISTER (CSR1R)

[0912] Address: PCODAR+2 read, (also, a write to this address clears GINT)

[0913] Default: 11001110

[0914] Modes: 1, 2, and 3

[0915] This register reports the interrupt status and various playback and record FIFO conditions. Reading this register also clears CSR2I[7:6] and CSR3I[3:0], if any are set. Writing to this register will clear all CODEC interrupts.

[0916] RULB

[0917] Record Channel Upper/Lower Byte Indication. When high, this bit indicates that a read of the record FIFO will return the upper byte of a 16-bit sample (bits[15:8]) or that the record data is 8-or-less bits wide. When low, this bit indicates that a read of the record FIFO will return the lower byte of a 16-bit sample (bits[7:0]). After the last byte of the last received sample has been read from the record FIFO, this bit does not change from its state during that byte until the next sample is received.

[0918] RLR

[0919] Record Channel Left/Right Sample Indication. When high, this bit indicates that a read of the record FIFO will return the left sample or that the record path is in either mono or ADPCM mode (or both). When low, a read will return the right sample. After the last byte of the last received sample has been read from the record FIFO, this bit does not change from is state during that byte until the next sample is received.

[0920] RDA

[0921] Record Channel Data Available. When high, there is valid data to be read from the record FIFO. When low, the FIFO is empty.

[0922] SE

[0923] Sample Error. This bit is high whenever data has been lost because of either a record FIFO overrun or a playback FIFO underrun (it is a logical OR of CSR2I[7:6]). If both record and playback channels are enabled, the specific channel that set this bit can be determined by reading CSR2I or CSR3I.

[0924] PULB

[0925] Playback Channel Upper/Lower Byte Indication. When high, this bit indicates that the next write to the playback FIFO should be the upper byte of a 16-bit sample (bits[15:8]) or that playback data is 8-or-less bits wide. When low, this bit indicates that next write to the playback FIFO should be the lower byte (bits[7:0]) of a 16-bit sample. After the playback FIFO becomes full, this stays in the state of the last byte written until a space becomes available in the FIFO.

[0926] PLR

[0927] Playback Channel Left/Right Sample Indication. When high, this bit indicates that the next write to the playback FIFO should be the left sample or that the playback path is in either mono or ADPCM mode. When low, the right sample is expected. After the playback FIFO becomes full, this stays in the state of the last byte written until a space becomes available in the FIFO.

[0928] PBA

[0929] Playback Channel Buffer Available. When high, there is room in the playback FIFO for additional data. When low, the FIFO is full.

[0930] GINT

[0931] Global Interrupt Status. This bit is high whenever there is an active condition that can request an interrupt. It is implemented by ORing together all the sources of interrupts in the CODEC: CSR3I[6:4].

[0932] PLAYBACK and RECORD DATA REGISTERS (CPDR, CRDR)

[0933] Address: PCODAR+3 read (record FIFO), write (playback FIFO)

[0934] Modes: 1, 2, and 3

[0935] Data written to this address is loaded into the playback FIFO. Data read from this address is removed from the record FIFO. Bits in Status Register 1 indicate whether the data is the left or right channel, and, for 16-bit samples, the upper or lower portion of the sample. Writes to this address when either the playback FIFO is in DMA mode or the playback path is not enabled (CFIG1I) are ignored; reads from this address when either the record FIFO is in DMA mode or the record path is not enabled (CFIG1I) are ignored.

[0936] CODEC CIDXR, CDATAP INDEXED REGISTERS LEFT, RIGHT A/D INPUT CONTROL (CLICI, CRICI)

[0937] Address: PCODAR+1 read, write; left index CIDXR[4:0]=0, right index CIDXR[4:0]=1

[0938] Default: 000X 0000 (for both)

[0939] Modes: 1, 2, and 3

[0940] This pair of registers is used to select the input source to the A/D converters, and to specify the amount of gain to be applied to each signal path. The registers are identical, one controls the left channel and the other controls the right channel.

[0941] LSS[1:0]

[0942] Left, Right ADC Source Select. These bits select which input source will

[0943] RSS[1:0]

[0944] be fed to the analog to digital converter.

[0945] RWB

[0946] Read/Write Bit. This bit does not control anything. Whatever is written to it will be read back.

[0947] LADIG[3:0]

[0948] Left, Right A/D Input Gain Select. The selected input source is fed to the

[0949] RADIG[3:0]

[0950] A/D converter via a gain stage. These four bits specify the amount of gain applied to the signal The values vary from 0h=0 dB to 0Fh=+22.5 dB with 1.5 dB per step (see FIGS. 45A-1 and 45A-2).

[0951] LEFT, RIGHT AUX 1/SYNTH INPUT CONTROL (CLAX1I, CRAX1I

[0952] Address: PCODAR+1 read, write; left index CIDXR[4:0]=2, right index CIDXR[4:0]=3

[0953] Default: 1XX0 1000 (for both)

[0954] Modes: 1, 2, and 3

[0955] This register pair controls the left and right AUX1 or Synth (multiplexed by CFIG3I[1]) inputs to the mixer. The registers are identical, one controls the left channel and the other controls the right channel.

[0956] LA1ME, Left, Right AUX1/Synth Mute Enable. When high, the selected input is

[0957] RA1ME

[0958] muted. When low, the input operates normally.

[0959] LA1G[4:0],

[0960] Left, Right AUX1/Synth Gain Select. This specifies the amount of gain.

[0961] RA1G[4:0]

[0962] applied to the selected—AUX1 or synth—input signal. The values vary from 00h+12 dB to 1Fh=−34.5 dB with 1.5 dB per step (see FIGS. 45A-1 and 45A-2).

[0963] LEFT, RIGHT AUXILIARY INPUT CONTROL (CLAX2I, CRAX2I)

[0964] Address: PCODAR+1 read, write; left index CIDXR[4:0]=4, right index CIDXR[4:0]=5

[0965] Default: 1XX0 1000 (for both)

[0966] Modes: 1, 2, and 3

[0967] This register pair controls the left and right AUX2 inputs to the mixer. The registers are identical, one controls the left channel and the other controls the right channel.

[0968] LA2ME, Left, Right AUX2 Mute Enable. When high, the AUX2 input is muted.

[0969] RA2ME

[0970] When low, the input operates normally.

[0971] LA2G[4:0],

[0972] Left, Right AUX2 Gain Select. This specifies the amount of gain applied.

[0973] RA2G[4:0]

[0974] to the AUX2 input signal. The values vary from 00h=+12 dB to 1Fh=−34.5 dB with 1.5 dB per step (see FIGS. 45A-1 and 45A-2).

[0975] LEFT, RIGHT PLAYBACK DAC CONTROL (CLDACI, CRDACI)

[0976] Address: PCODAR+1 read, write; left index CIDXR[4:0]=6, right index CIDXR[4:0]=7

[0977] Default: 1X00 0000 (for both)

[0978] Modes: 1, 2, and 3

[0979] This register pair controls the left and right DAC analog outputs as they are input to the mixer. The registers are identical, one controls the left channel and the other controls the right channel.

[0980] LDME,

[0981] Left, Right Mute Enable. When high, the DAC input to the mixer is

[0982] RDME

[0983] muted. When low, the input operates normally.

[0984] LA[5:0]

[0985] Left, Right D/A Attenuation Select. This specifies the amount of

[0986] RA[5:0]

[0987] attenuation applied to the DAC input signal. The values vary from 00h=0 dB to 3Fh=−94.5 dB with 1.5 dB per step (see FIGS. 45A-1 and 45A-2).

[0988] PLAYBACK DATA FORMAT REGISTER (CPDFI)

[0989] Address: PCODAR+1 read, write; index CIDXR[4:0]=8

[0990] Default: 0000 0000

[0991] Modes: The definition of this register varies based on the mode

[0992] This register specifies the sample rate (selects which of the two oscillator is to be used and the divide factor for that oscillator), stereo or mono operation, linear or companded data, and 8 or 16 bit data. It can only be changed when the mode change enable bit (CIDXR[6]) is active.

[0993] In mode 1, this register controls both the playback and record paths.

[0994] In mode 2, bits[3:0] of this register controls both the record and playback sample rate (i.e., they must be the same) and bits[7:4] specify the state of the playback-path data format.

[0995] In mode 3, this register controls only the playback path; the record sample rate is controlled by CRDFI.

[0996] PDF[2:0]

[0997] Playback Data Format Selection. These three bits specify the play-back data format for the CODEC. * Modes 2 and 3 only. In Mode 1, PDF[2] is treated as a low regardless of the value written by the user.

[0998] PSM

[0999] Playback Stereo/Mono Select. When high, stereo operation is selected; samples will alternate left then right. When low, mono mode is selected; playback samples are fed to both left and right FIFOs. Record samples (in mode 1) come only from the left ADC.

[1000] PCD[2:0]

[1001] Playback Clock Divider Select. These three bits specify the playback clock rate in mode 3, and the record and playback rate in modes 1 and 2. *These divide-downs are provided, to function when XTAL1 is less than 18.5 MHz.

[1002] PCS

[1003] Playback Crystal Select. When high, the 16.9344 MHz crystal oscillator (XTAL2) is used for the playback sample frequency. When low, the 24.576 MHz crystal oscillator (XTAL1) is used.

[1004] CONFIGURATION REGISTER 1 (CFIG1I)

[1005] Address: PCODAR+1 read, write; index CIDXR[4:0]=9

[1006] Default: 00XX 1000

[1007] Modes: 1, 2, and 3

[1008] This register specifies whether I/O cycles or DMA are used to service the CODEC FIFOs, one or two channel DMA operation, and enables/disables the record and playback paths. Bits[7:2] are protected; to write to protected bits, CIDXR[MCE] must be set.

[1009] RFIOS

[1010] Record FIFO I/O Select. When high, the record FIFO can only be serviced via I/O cycles. When low, DMA operation is supported.

[1011] PFIOS

[1012] Playback FIFO I/O Select. When high, the playback FIFO can only be serviced via I/O cycles. When low, DMA operation is supported.

[1013] CALEM

[1014] Calibration Emulation. This is a readable-writable bit. When high, it affects CSR2I[5].

[1015] DS1/2

[1016] 1 or 2 Channel DMA Operation Select. When high, single channel DMA operation is selected; only record or playback operation is allowed, not both; when both record and playback DMA are enabled in this mode, only the playback transfers will be serviced. When low, two-channel DMA operation is allowed.

[1017] RE

[1018] Record Enable. When high, the record CODEC path is enabled. When low, the record path is turned off and the record data available status bit (Status Register 1) is held inactive (low).

[1019] PE

[1020] Playback Enable. When high, the playback CODEC path is enabled. When low, the playback path is turned off and the playback buffer available status bit (Status Register 1) is held inactive (low).

[1021] EXTERNAL CONTROL REGISTER (CEXTI)

[1022] Address: PCODAR+1 read, write; index CIDXR[4:0]=Ah

[1023] Default: 00XX 0X0X

[1024] Modes: 1, 2, and 3

[1025] This register contains the global interrupt enable control as well as control bits for the two general purpose external output pins.

[1026] GPOUT[1:0]

[1027] General Purpose Output Flags. The state of these bits are reflected on the GPOUT[1:0] pins.

[1028] RWB

[1029] Read Write Bit. This bit is writable and readable; it does not control anything within the Device.

[1030] GIE

[1031] Global Interrupt Enable. When high, CODEC interrupts are enabled. When low, CODEC interrupts will not be passed on to the selected IRQ pin. The status bits are not affected by the state of this bit.

[1032] STATUS REGISTER 2 (CSR2I)

[1033] Address: PCODAR+1 read; index CIDXR[4:0]=Bh

[1034] Default: 0000 0000

[1035] Modes: 1, 2, and 3

[1036] This register reports certain FIFO errors, the state of the record and playback data request bits, and allows testing the A/D paths for clipping.

[1037] RFO

[1038] Record FIFO Overrun. This bit is set high whenever the record FIFO is full and the CODEC needs to load another sample (the sample is discarded). This bit is cleared to low by either a read of CSR1R or when CIDXR[MCE] goes from 1 to 0.

[1039] PFU

[1040] Playback FIFO Underrun. This bit is set high whenever the playback FIFO is empty and the CODEC needs another sample. This bit is cleared to low by either a read of CSR1R or when CIDXR[MCE] goes from 1 to 0. (In mode 1, the previous sample is reused. In modes 2 and 3, either the previous sample is reused or the data is forced to all zeros depending on the programming of CFIG2I[0].)

[1041] CACT

[1042] Calibration Active Emulation. If CFIG1I[3] is high, this bit goes high as a result of the mode change enable bit (CIDXR[6]) going inactive; it goes back low after the trailing edge of the first subsequent read of CSR2I.

[1043] DRPS

[1044] DMA Request Pin Status. This bit is high anytime that either the record or playback DMA request pins are active.

[1045] RADO[1:0],

[1046] Right and Left Overrange Detect. These two pairs of bits are updated on

[1047] LADO[1:0]

[1048] a sample by sample basis to reflect whether the signal into the DAC is causing clipping.

[1049] MODE SELECT, ID REGISTER (CMODEI)

[1050] Address: PCODAR+1 read, write; index CIDXR[4:0]=Ch

[1051] Default: 100X 1010

[1052] Modes: 1, 2, and 3

[1053] This register specifies the operating mode of the CODEC and reports the revision number of the circuit C.

[1054] ID[4],

[1055] Revision ID Number. These five bits specify the revision number of the

[1056] ID[3:0]

[1057] present invention CODEC circuit C, which is initially 1,1010. These bits are read-only and cannot be changed.

[1058] MODE[1:0]

[1059] Mode Select. (0,0)=mode 1; (1,0)=mode 2; (0,1)=reserved; (1,1)=mode 3. In order to enter mode 3, a write of 6Ch must be made to this port; i.e., bit[5] will be forced low for writes of any other value.

[1060] LOOPBACK CONTROL REGISTER (CLCI)

[1061] Address: PCODAR+1 read, write; index CIDXR[4:0]=Dh

[1062] Default: 0000 00X0

[1063] Modes: 1, 2, and 3

[1064] This register enables and specifies the attenuation of the analog path between the output of the ADC path gain stage (at the input to the ADC) and the input of the DAC-loopback sum. This register affects both the left and right channels.

[1065] LBA[5:0]

[1066] Loopback Attenuation. This specifies the amount of attenuation applied to the loopback signals before being summed with the DAC outputs. The values vary from 00h=0 dB to 3Fh=−94.5 dB with 1.5 dB per step (see FIGS. 45A-1 and 45A-2).

[1067] LBE

[1068] Loopback Enable. When high, the loopback path is enabled to be mixed with the DAC outputs. When cleared, the path is disabled and the signal is muted.

[1069] UPPER, LOWER PLAYBACK COUNT REGISTERS (CUPCTI, CLPCTI)

[1070] Address: PCODAR+1 read, write; upper index CIDXR[4:0]=Eh, lower index CIDXR[4:0]=Fh

[1071] Default: 0000 0000 (for both)

[1072] Modes: definition of these registers vary based on the mode

[1073] These registers collectively provide the 16-bit preload value used by the playback sample counters. CUPCTI provides the upper preload bits [15:8] and CLPCTI provides the lower preload bits [7:0]. All 16 bits are loaded into the counter during the write of the upper byte; therefore, the lower byte should be written first; however, if only the low byte is written and the counter underflows, the new value will be placed into the timer. Reads of these registers return the value written into them, not the current state of the counter. In mode 1, this register is used for both playback and capture; in modes 2 and 3 it is used for playback only.

[1074] CONFIGURATION REGISTER 2 (CFIG2I)

[1075] Address: PCODAR+1 read, write; index CIDXR[4:0]=10h

[1076] Default: 0000 XXX0

[1077] Modes: 2 and 3

[1078] OFVS

[1079] Output Full Scale Voltage Select. When high, the full scale Output is 2.9V for Vcc=5V and 1.34 for Vcc=3.3V. When low, the full scale output is 2.0V for Vcc=5V and 1.00 for Vcc=3.3V. This bit affects the left and right signals that exit the mixers, prior to entering CLOAI and CROAI; so it also changes the input to the record multiplexer.

[1080] TE

[1081] Timer Enable. When high, the timer and its associated interrupt are enabled. When low, the timer is disabled. The timer count is specified in CLTIMI and CUTIMI.

[1082] RSCD

[1083] Record Sample Counter Disable. When high, this bit disables the record sample counter from counting. This bit is mode 3 accessible only and only affect the sample counter in mode 3.

[1084] PSCD

[1085] Playback Sample Counter Disable. When high, this bit disables the playback sample counter from counting. This bit is mode accessible only and only affect the sample counter in mode 3.

[1086] DAOF

[1087] D/A Output Force Enable. When high, the output of the D/A converters are forced to the center of the scale whenever a playback FIFO underrun error occurs. When cleared, the last valid sample will be output in the event of an underrun.

[1088] CONFIGURATION REGISTER 3 (CFIG3I)

[1089] Address: PCODAR+1 read, write; index CIDXR[4:0]=11h

[1090] Default: 0000 X000

[1091] Modes: bits[7:1] mode 3; bit[0] modes 2 and 3

[1092] In mode 3 this register provides for the programming of FIFO thresholds and the generation of I/O-mode FIFO service interrupts.

[1093] RPIE

[1094] Record FIFO Service Request Interrupt Enable. When the record path is enabled and I/O operation is selected (CFIG1I), setting this bit high enables the generation of an interrupt request whenever the record FIFO/DMA interrupt bit in Status Register 3 becomes set. This bit is mode 3 accessible only.

[1095] PPIE

[1096] Playback FIFO Service Request Interrupt Enable. When the playback path is enabled and I/O operation is selected (CFIG1I), setting this bit high enables the generation of an interrupt request whenever the playback FIFO/DMA interrupt bit in Status Register 3 becomes set. This bit is mode 3 accessible only.

[1097] FT[1:0]

[1098] FIFO Threshold Select. These two bits specify the record and playback FIFO thresholds for when DMA or interrupt requests become active. These bits are mode 3 accessible only and do not have an effect in modes 1 and 2.

[1099] PVFM

[1100] Playback Variable Frequency Mode. This bit high selects playback-variable-frequency mode. In this mode, the sample rate is selected by a combination of CPDFI[0] and CPVFI to allow variable frequencies between 3.5 KHz and 32 KHz. The sound quality may be reduced when in this mode. This bit is mode 3 accessible only.

[1101] SYNA

[1102] AUX1/Synth Signal Select. This bit selects the source of the signals that enter the CLAX1I and CRAX1I attenuators before entering the left and right mixers. This bit low selects the AUX1[L,R] input pins. This bit high selects the output of the synth DACs. This bit is mode 3 accessible only.

[1103] RWB

[1104] Read Write Bit. This bit is writable and readable; it does not control anything within the device. This is mode 2 and mode 3 accessible.

[1105] LEFT, RIGHT LINE INPUT CONTROL REGISTERS (CLLICI, CRLICI)

[1106] Address: PCODAR+1 read, write; left index CIDXR[4:0]=12h, right index CIDXR[4:0]=13h

[1107] Default: 1XX0 1000 (for both)

[1108] Modes: 2 and 3

[1109] This register pair controls the gain/attenuation applied to the LINEIN inputs to the mixer. The registers are identical, one controls the left channel and the other controls the right channel.

[1110] LLIME,

[1111] Left, Right LINE Input Mute Enable. When high, the LINEIN input is

[1112] RLIME

[1113] muted. When low, the input operates normally.

[1114] LLIG[4:0],

[1115] Left, Right LINE Input Gain Select. This specifies the amount gain

[1116] RLIG[4:0]

[1117] applied to the LINEIN[L,R] input signals. The values vary from 0=+12 dB to 1Fh=−34.5 dB with 1.5 dB per step (see FIGS. 4A-1 and 45A-2).

[1118] LOWER, UPPER TIMER REGISTERS (CLTIMI, CUTIMI)

[1119] Address: PCODAR+1 read, write; low index CIDXR[4:0]=14h, upper index CIDXR[4:0]=15h

[1120] Default: 0000 0000 (for both)

[1121] Modes: 2 and 3

[1122] These registers collectively provide the 16-bit preload value used by the general purpose timer. Each count represents 10 microseconds (total of 650 milliseconds). CUTIMII provides the upper preload bits [15:8] and CLTIMI provides the lower preload bits [7:0]. Writing to CLTIMI causes all 16 bits to be loaded into the general purpose timer. Reads of these registers return the value written into them, not the current state of the counter.

[1123] LEFT, RIGHT MIC INPUT CONTROL REGISTERS (CLMICI, CRMICI)

[1124] Address: PCODAR+1 read, write; left index CIDXR[4:0]=16h, right index CIDXR[4:0]=17h

[1125] Default: 1X0 1000 (for both)

[1126] Modes: 3

[1127] This register pair controls the left and right MIC inputs to the mixer. The registers are identical, one controls the left channel and the other controls the right channel.

[1128] LMME,

[1129] Left, Right MIC Mute Enable. When high, the MIC input is muted.

[1130] RMME

[1131] When low, the input operates normally.

[1132] LMG[4:0],

[1133] Left, Right MIC Gain Select. This specifies the amount of gain applied to

[1134] RMG[4:0]

[1135] the MIC [L,R] input signals. The values vary from 0=+12 dB 1Fh=−34.5 dB with 1.5 dB per step (see FIGS. 45A-1 and 45A-2).

[1136] STATUS REGISTER 3 (CSR3I)

[1137] Address: PCODAR+1 read, write (to clear specific bits); index CIDXR[4:0]=18h

[1138] Default: X000 0000

[1139] Modes: 2 and 3; definition of bits[5:4] vary based on the mode

[1140] This register provides additional status information on the FIFOs as well as reporting the cause of various interrupt requests. Each of the TIR, RFDI, and PFDI bits are cleared by writing a 0 to the active bit; writing a 1 to a bit is ignored; these bits can also be cleared by a write of any value to CSR1R. Bits[3:0], the overrun-underrun bits, are cleared to a low by reading CSR1R; these bits are also cleared when the mode change enable bit in CIDXR goes from high to low.

[1141] TIR

[1142] Timer Interrupt Request. This bit high indicates an interrupt request from the timer. It is cleared by a writing a zero to this bit or by writing any value to CSR1R.

[1143] RFDI

[1144] Record FIFO Interrupt Request. This bit high indicates a record path interrupt. It is cleared by a writing a zero to this bit or by writing any value to CSR1R. Mode 2: this bit indicates an interrupt request from the record sample counter. Mode 3 and CFIG1I[7]=0 (DMA): this bit indicates an interrupt request from the record sample counter. Mode 3 and CFIG1I[7]=1 (I/O): this bit indicates that the record FIFO threshold (CFIG3I) has been reached.

[1145] PFDI

[1146] Playback FIFO Interrupt Request. This bit high indicates a playback path interrupt. It is cleared by a writing a zero to this bit or by writing any value to CSR1R. Mode 2: this bit indicates an interrupt request from the playback sample counter. Mode 3 and CFIG1I[6]=0 (DMA): this bit indicates an interrupt request from the playback sample counter. Mode 3 and CFIG1I[6]=1 (I/O): this bit indicates that the playback FIFO threshold (CFIG3I) has been reached.

[1147] RFU

[1148] Record FIFO Underrun (Modes 2, 3). This bit is set high if there is an attempt to read from an empty record FIFO.

[1149] RFO

[1150] Record FIFO Overrun (Modes 2, 3). This bit is set high if the ADC needs to load a sample into a full record FIFO. It is identical to CSR2I[RFO].

[1151] PFO

[1152] Playback FIFO Overrun (Modes 2, 3). This bit is set high if there is an attempt to write to a full playback FIFO.

[1153] PFU

[1154] Playback FIFO Underrun (Modes 2, 3). This bit is set high if the DAC needs a sample from an empty playback FIFO. It is identical to CSR2I[PFU].

[1155] LEFT, RIGHT OUTPUT ATTENUATION REGISTER (CLOAI, CROAI)

[1156] Address: PCODAR+1 read, write; left index CIDXR[4:0]=19h, right index CIDXR[4:0]=1Bh

[1157] Default: 1XX0 0000 (for both);

[1158] Modes: 3 only; in mode 2 CLOAI is a read-only register that drives an 80h when read.

[1159] This register pair controls the left and right MONO and LINE output levels. The Line output mute control bit is also located in this register pair.

[1160] LLOME,

[1161] Line Output Mute Enable. When high, the LINE output is muted. When

[1162] RLOME

[1163] low, the output operates normally.

[1164] LLOA[4:0],

[1165] Fine Output Attenuation Select. This specifies the amount of attenuation

[1166] RLOA[4:0]

[1167] applied to the both the MONO and LINE output signals. The values vary from 00h=0 dB to 1Fh=−46.5 dB with 1.5 dB per step (see FIGS. 45A-1 and 45A-2).

[1168] MONO I/O CONTROL REGISTER (CMONOI)

[1169] Address: PCODAR+1 read, write; index CIDXR[4:0]=1Ah

[1170] Default: 110X 0000

[1171] Modes: bits [7:6,4:0] modes 2 and 3; bit [5] mode 3 This register specifies the amount of attenuation applied to the mono input path. The mute controls for the mono input and output are also located here.

[1172] MIME

[1173] Mono Input Mute Enable. When high, the mono input is muted. When low, the input is active.

[1174] MOME

[1175] Mono Output Mute Enable. When high, the mono output is muted. When low, the output operates normally.

[1176] AR3S

[1177] AREF to high impedance. When high, the AREF pin is placed into high impedance mode. When low, AREF operates normally. this bit is mode 3 accessible only.

[1178] MIA[3:0]

[1179] Mono Input Attenuation. This specifies the amount of attenuation to be applied to the mono input path. The values vary from 0=0 dB to 0Fh=−45 dB with 3.0 dB per step (see FIGS. 45A-1 and 45A-2).

[1180] RECORD DATA FORMAT REGISTER (CRDFI)

[1181] Address: PCODAR+1 read, write; index CIDXR[4:0]=1Ch

[1182] Default: 0000 0000

[1183] Modes: 2 and 3; definition of register varies based on the mode

[1184] This register specifies the sample rate (selects which of the two oscillator is to be used and the divide factor for that oscillator), stereo or mono operation, linear or companded data, and 8 or 16 bit data. It can only be changed when the mode change enable bit (CIDXR[6]) is active.

[1185] In mode 2, bits[3:0] are not used (the record-path sample rate is specified in CPDFI) and bits[7:4] specify the record-path data format.

[1186] In mode 3, all of this register controls record path attributes; the playback attributes are controlled by CPDFI.

[1187] RDF[2:0]

[1188] Record Data Format Selection. These three bits specify the record data format for the CODEC. These bits are accessible in Modes 2 and 3 only.

[1189] RSM

[1190] Record Stereo/Mono Select. When high, stereo operation is selected; samples will alternate left then right. When low, mono mode is selected; record samples come only from the left ADC. This bit is accessible in modes 2 and 3 only.

[1191] RCD[2:0]

[1192] Record Clock Divider Select. These three bits specify the record clock rate. These bits are accessible from mode 3 only; in mode 2, these bits are reserved. *These divide-downs are provided to function when XTAL1 is less than 18.5 MHz.

[1193] RCS

[1194] Record Crystal Select. When high, the 16.9344 MHz crystal oscillator is used. When low, the 24.576 MHz crystal oscillator is used. This bit is accessible from mode 3 only; in mode 2, this bit is reserved.

[1195] UPPER, LOWER RECORD COUNT REGISTERS (CURCTI, CLRCTI)

[1196] Address: PCODAR+1 rd, wr; upper index CIDXR[4:0]=1Eh, lower index CIDXR[4:0]=1Fh

[1197] Default: 0000 0000 (for both)

[1198] Modes: 2 and 3; in mode 1, function is moved to CUPCTI and CLPCTI These registers collectively provide the 16-bit preload value used by the record sample counters. CURCTI provides the upper preload bits [15:8] and CLRCTI provides the lower preload bits [7:0]. All 16 bits are loaded into the counter during the write of the upper byte; therefore, the lower byte should be written first; however, if only the low byte is written and the counter underflows, the new value will be placed in the timer. Reads of these registers return the value written into them, not the current state of the counter.

[1199] PLAYBACK VARIABLE FREQUENCY REGISTER (CPVFI)

[1200] Address: PCODAR+1 read, write; index CIDXR[4:0]=1Dh

[1201] Default: 0000 0000

[1202] Modes: 3 only

[1203] This 8-bit register specifies the playback frequency when variable-frequency-playback mode has been enabled via CFIG3I[2]. The playback frequency will be PCS/(16*(48+CPVFI)), where PCS is the frequency of the oscillator selected by CPDFI[0]. The 16.9 MHz oscillator provides a range from about 3.5 KHz to 22.05 KHz; the 24.5 MHz oscillator provides a range from about 5.0 KHz to 32 KHz. It is not necessary to set CIDXR[MCE] when altering the value of this register.

[1204] Referring to FIG. 45, in mixer 606, for the record path of CODEC 505, the status of control register CLICI 604 controls multiplexer (MUX) 602 such that only one of four analog audio signals pass through MUX 602 and attenuation/gain control circuit 664. If not muted by attenuation/gain control circuit 664, the selected signal is then provided to either left record ADC 666, or looped back through attenuation/gain control circuit 606 to be summed in playback mixer 678 with the output of left playback DAC 680. This loop back is accomplished over loop back path 676, which provides a loop back path for system test and dub-over capability so that in playback mode, MICL 684, LINEINL 682, AUX1L 686, or left synthesizer DAC 692 output signals may be superimposed over audio signals coming from the output of left playback DAC 680. This provides a Karioke-type capability with stored audio signals coming from left playback DAC 680.

[1205] The contents of control register CFIG3I[SYNA] 607 is used to control left synth DAC MUX 694 to select between analog inputs AUX1L 686 and left synthesizer DAC 692. The selected analog audio signal then passes to the input of MUX 602 and to attenuation/gain control circuit 612. The output of attenuation/gain control circuit 612 is then input to main mixer 698 to be summed with all other non-muted analog audio input signals available at the input to main mixer 698.

[1206] Main mixer loopback path 677 provides the output of main mixer 698 to the input of MUX 602. Main mixer 698 output is also provided to attenuation/gain control circuit 674 for further submission to mono mixer 672, as LEFTOUT, where it is summed with analog output RIGHTOUT 616 from the right channel mixer (not shown). Signals LEFTOUT and RIGHTOUT are summed in mono mixer 672 and then sent through mute control 604 to be available as analog output signal MONOOUT 668. Signal LEFTOUT is also input to attenuation/gain control circuit 602. If not muted, LEFTOUT is available as an analog output left channel stereos signal LINEOUTL 670.

[1207] The analog audio input signal MONOIN 690 passes through attenuation/gain control circuit 696 and is available to main mixer 698 as an input signal, and as an analog mono input signal 618 to the right channel main mixer (not shown).

[1208] As shown in FIG. 47, the CODEC 505 includes circuitry to ensure that the amplitude of each respective analog audio signal in analog mixer 606 is maintained until the signal attains a nominal value. This is accomplished by zero detect circuit 715. Updated attenuation/gain control information is not loaded into the respective attenuation/gain control register until the analog audio signal that is to be acted on with the new attenuation/gain control value either crosses zero volts 714 (FIG. 46) with respect to a reference voltage, or until a time-out count is reached by 25 millisecond timer 718 which will result in a default condition causing the respective attenuation/gain control register in Registers block 566 (FIG. 50) to be loaded with the new gain/attenuation control value.

[1209] The attenuation/gain control circuit 710, shown within dotted line in FIG. 47, is provided for each attenuation/gain control register in Registers block 566 of FIG. 44. In the preferred embodiment, there are sixteen attenuation/gain control registers (CLCI, CLICI, CRICI, CLAX1I, CRAX1I, CLAX2I, CRAX2I, CLDACI, CRDACI, CLLICI, CRLICI, CLMICI, CRMICI, CLOAI, CROAI and CMONOI) which may be written to change the gain or attenuation control values stored therein, which value is in turn is used to change the amplitude of the analog audio signal being processed by the particular attenuation/gain control register being written to. In other applications, more or less attenuation/gain control registers may be implemented.

[1210] In operation, whenever one of the attenuation/gain control registers is written to, Register Select Decode block 716 latches the new attenuation/gain control value into gain latch 730. After decoding the write to one of the attenuation/gain control registers, Register Select Record block 716 sends an enable to 25 millisecond timer block 718 and 100 To 300 Microsecond block 720 to initiate a power-up. Power is then provided for 100 to 300 microseconds to each of the Near Zero Detect blocks 732, by Comparator Power-On Control block 738, enabled by 100 to 300 microsecond block 720.

[1211] The 25 millisecond timer block 718 utilizes ICLK3K, the 3.15 KHz clock, to count to 80. The timing in 100 to 300 Microsecond timer block 720 is accomplished by the logic therein waiting for two edges of 3.15 KHz clock, ICLK3K. Once powered, the Near Zero detect block 732 generates a strobe when the audio input signal 740 approaches nominal voltage. The zero detect logic in each Near Zero Detect block 732 may be implemented with comparators, or other circuits capable of providing an output signal whenever the input audio signal 740 is equal to a predetermined reference voltage. The zero detect strobe is used to latch the new attenuation/gain value into latch 726. The zero detect circuitry 732 will remain powered until the fixed 25 millisecond timer 718 completes its count.

[1212] An analog reference voltage (AREF) is used such that when VCC is 5 volts, the value of AREF is 0.376 times VCC, nominal. When VCC is 3.3 volts, the value of AREF is 0.303 times VCC, nominal. AREF is capable of driving up to 250 microamps without degradation and can be placed into high-impedance mode, controlled by CMONOI[AR3S].

[1213] If input signal 740 has not reached nominal voltage before the 25 millisecond timer 718 completes its count, the new attenuation/gain control value is nevertheless loaded into the respective attenuation/gain control register, as a default condition. If a write to any of the attenuation/gain control registers in Register block 566 (FIG. 50) occurs before the 25 millisecond timeout is reached, the 25 millisecond timer 718 is reset, regardless of its count status.

[1214] The zero detect circuit 715 minimizes “zipper” noise or other audible discontinuities when input signal 740 is to be increased or decreased in amplitude. By powering up the near zero detect circuits 732 only when an attenuation/gain register is written to, unnecessary noise, from comparators or other voltage detect circuits in Near Zero Detect block 732 switching every time a zero crossing is sensed is eliminated.

[1215] Referring to FIG. 46, by increasing the gain at input signal zero crossing 714, signal discontinuity 710 is eliminated. By using zero detect block 732, input signal 740 changes amplitude at zero crossing 714 is output from zero detect circuit 715 as output signal 736 (FIG. 47), and continues with its new amplitude along curve 712 (FIG. 46).

[1216] All programmable attenuation/gain control circuits in CODEC 505 (triangles in analog mixer 606) include zero crossing detect circuitry 715. Zero crossing circuit 715 performs identically for each attenuation/gain control register in Registers block 566 (FIG. 50).

[1217] An additional noise management feature of CODEC 505 is used to suppress noise on power-up. Audible glitches from audio outputs LINEOUT 670 and MONOOUT 668 (FIG. 45) are suppressed when power is being applied or removed from CODEC 505, or when low-power mode is entered or exited. During all power-up and power-down phases, CODEC 505 output amplifiers in mute circuits 602 and 604 (FIG. 45) are muted.

[1218] To enhance the performance of the CODEC, digital operations occur on the rising edge of the 16.9 MHz system clock, and analog operations are performed on the falling edge of the system clock, or at some other time prior a to the next rising edge of the system clock. Generally, digital operations inherently produce noise which must be attenuated as much as possible before analog operations are performed. Using different edges of the system clock, in addition to delaying the clocks generated from the system clock that are used by the analog circuitry with respect to the clocks used by the digital circuitry, will produce the desired result. Inherently noisy digital operations include, RAM reads, precharging a bus and performing an addition. Analog functions require a quiet supply and ground. For example, a comparator requires a low level noise background to be able to detect a one millivolt level to achieve a proper compare.

[1219] The record and playback paths of CODEC 505 are independently programmable to provide a different sample rate for playback and record. A continuously variable rate playback mode is provided for playback DAC 514 (FIG. 44), which includes a choice of two ranges of sample clock rates ranging from 3.5 to 22.05 KHz or from 5.0 to 32.00 KHz. Each sample rate range contains 256 incremental clock rates. By enabling this variable playback mode by modifying the status of control register CFIG3I[2], the playback frequency for playback DAC 514 can be continuously varied over 256 steps, resulting in smooth transitions between audio sample rates which produces high quality sounds. Previously, with only fourteen different sample rates being used, the data sample rate had to be increased and interpolated, then the rate increased again and the signal interpolated again to achieve the desired sound and transition between sample rates. This required excessive processor intervention.

[1220] Utilizing the feedback loops within CODEC analog mixer 606 (FIG. 45), and the independent programmability of the sample rates of record ADC 516 and playback DAC 514, an analog audio signal may be sampled and converted to digital by record ADC 516 at one rate, then played back through playback DAC 514 at another rate. This feature provides a translator capability between an audio signal recorded and played at different sample rates. For example, the direct recording of compact disc (CD) audio, or digital audio tape data (DAT) onto formatted tapes without significant degradation of signal quality is implemented by CD audio data being converted to analog through playback DAC 514 at 44.1 KHz, then being processed through record ADC 516 circuitry and made available as serial or parallel digital audio data that can be recorded by external audio equipment on DAT at 48 KHz.

[1221] In the present invention, the continuously variable playback frequency mode can be selected to incrementally increase the playback sample rate in CODEC 505 without external processor intervention for up-sampling and interpolation. The frequency range is preferably selected by control register CPDFI[0] in the Registers block 566 (FIG. 50), which is programmable to be able to select, at any time, the playback frequency to be used, and thus, which clock is to be used. See FIG. 48. This requires some external processor intervention to load the frequency select instruction, but not as much overhead as previous audio systems. For software compatibility with existing systems, however, the playback-variable frequency mode is different than the 14 sample rate mode operation of playback DAC 514 and record ADC 516.

[1222] Oscillators with external crystals 560 (FIG. 50) are used to generate the range of frequencies for the playback variable frequency mode. Preferably, two external crystals in conjunction with on-chip circuitry are used to produce two clocks, one being at 24.576 MHz and one being at 16.9344 MHz. Selecting the 16.9 MHz clock with select logic circuit 762 will provide a 256 step frequency range from between 3.5 KHz to 22.05 KHz. Selecting the 24.5 MHz crystal will provide a 256 step frequency range of 5.0 to 32.00 KHz.

[1223] To provide each of the 256 steps over a selected frequency range, the chosen crystal oscillator is divided by three or more to create an X256 clock (sample rate times 256). The X256 clock is then divided by four to create the X64 clock (sample rate times 64). The X64 clock repeats an 8-cycle, aperiodic pattern which produces the frequencies within the selected range. The various clocks, generated by the divide-down logic in FIG. 48, are used to change the sample rate (pitch) during playback through the playback DAC 514 (FIG. 44), such that the higher the sample rate, the higher the pitch and the lower the sample rate, the lower the pitch. This capability of continuously variable playback sample rates can be used with any DAC, and is not limited to the Σ-Δ playback DAC 514 described herein.

[1224] Table C1 describes the formulas preferably used to select the sample frequency for each range.

[1225] Table C2 illustrates how the first ten clock frequencies in one range are generated using the 16.9 MHz external crystal oscillator.

[1226] Table C3 illustrates the preferred way of using the X256 clock to create the wave forms illustrated Table C2. The 4-cycle pattern illustrated in Table C3, given by the status of register SMX64[1:0], is used to ensure that the X64 clock maintains a 50 percent duty cycle, which is preferred.

[1227] FIG. 48 illustrates the clock select circuitry which provides the independently selectable sample rates for the record and playback paths of CODEC 505, and the continuously variable playback sample rates for playback DAC 514. Playback DAC 514 and record ADC 516 (FIG. 44) are each capable of operating at one of 14 different sample rates ranging from 5.5 to 48.0 KHz. These sample rates are preferably derived from the two external crystal oscillators 560 (FIG. 50). Select logic circuitry 762 in CODEC 505 controls each 2:1 MUX 766 to select the output of either the 16 MHz or 24 MHz oscillator, depending on which sample rate is selected.

[1228] Gate logic block 752 in the record path, and 764 in the playback path, provide the selected clock signal to divide-down logic blocks 754, 756, and blocks 760, 757, respectively, to provide a selected slower clock. As shown in FIG. 48, the status of control registers CPDFI[0], CPDFI[3:1], CRDFI[0], CRDFI[3:10], CFIG3I[2] and CPVFI[7:0] controls the divide-down logic to be used to generate a selected clock signal. Clock CP256X is used to control operations in the playback DAC 514. Clock CP64X is used to control operations in the semi-digital filter 804 (FIG. 51).

[1229] Referring to FIGS. 49 and 49a,CODEC 505 includes logic and control for transfers of serial digital audio data, including parallel-to-serial (PTS) conversion blocks 788, 789 and serial-to-parallel (STP) conversion logic 782. A record multiplexer (MUX) 784 is controlled by control register ICMPTI[8:6]. If bits [8:6] equal zero, MUX 784 selects parallel digital audio data from record ADC 516. If equal to one, MUX 784 selects the output of STP conversion logic 782. In the record path, the output of record MUX 784 is provided to the CODEC record FIFO 538. Referring to FIG. 44, the output of record FIFO 538 is available on register data bus 526; at local memory control 790 (which may transfer the data to off-chip local memory 110, FIG. 44, for storage as a record FIFO) via parallel to serial converter 789, serial transfer control 540 and data path 550; and at the input of PTS block 789 whereby the data is then provided, via Serial Transfer Control block 540, to: record FIFO 538, playback FIFO 532 (via serial to parallel converter 782), or to External Serial Interface 544.

[1230] As shown in FIG. 49a, in the CODEC playback path, a playback MUX 794 is controlled by control registers ICMPTI[8:6] and LMFSI[PE]. If ICMPTI[8:6] is not equal to one, or if LMFSI[PE] equals one, then audio data from STP block 782 is available at the input to playback FIFO 532. Otherwise, data from register data bus 526 is available at playback FIFO 532. As shown if FIG. 49, data from local memory control 790 (which may obtain data from local memory 110, FIG. 44) is provided to playback FIFO 532 via playback MUX 794. Audio data from synth DSP 796 or record FIFO 538 may also be available at the input of playback MUX 794. As illustrated in FIG. 49a, the value of ICMPTI[8:6] determines the operation of serial transfer control MUXES 554 and 548. Serial transfer control MUX 546 operation is controlled by the status of LMFSI[PE].

[1231] As shown in FIG. 44, audio data from synthesizer DSP 796 is also available at the input of synthesizer DAC 512. The output of synth DAC 512 is provided as an analog input to left synth DAC MUX 649 (and right synth DAC MUX, not shown) in CODEC analog mixer 606 (FIG. 45). Synthesizer DSP 796 may be an external device, or may be included in a synthesizer module on the same monolithic integrated circuit as the CODEC device 505 to increase the flexibility and speed of operation between the CODEC 505 and the synthesizer.

[1232] With the arrangement of STP and PTS converter logic blocks 782 and 789, respectively, and Serial Transfer Control block 540, a digital loop back capability between record and playback paths of CODEC 505 exists. This provides greater flexibility for testing and for data transfer of audio data from external sources to or from record FIFO 538 or playback FIFO 532, or to off-chip local memory 110, FIG. 44, via local memory control 790, or to external system memory (not shown). A digital data path (FIGS. 44, 49), via PTS and STP blocks 789 and 782 is depicted between the record FIFO 538 output and the playback FIFO 532 input. The loop between the playback DAC 514 output and the record ADC 516 input is analog and resides in Mixer 606, FIG. 45, and is illustrated with left playback DAC 680 looping to left record ADC 666.

[1233] External serial interface 544 may be connected to a synthesizer DSP having a serial input and output (not shown) whereby that synthesizer DSP could receive serial data from, via Serial Transfer Control block 540, record FIFO 538, and could send serial data to, via Serial Transfer Control block 540, playback FIFO 532.

[1234] The record and playback MUXES 784 and 794, in the serial data transfer logic of CODEC 505 are preferably bit-stream multiplexers. Preferably, state machines are used to generate and/or operate on the control signals and clocks necessary to accomplish the transfers. See the description of control signals during serial data transfers, above. Most transfers in Serial Transfer Control block 540, operate off a 2.1 MHz, 50 percent duty cycle clock, derived by dividing the 16.9344 MHz crystal oscillator by eight. Transfers from the synth DSP 796 to an external device utilize 32 clocks per frame, based on the synth DSP frame rate. The STP logic blocks 782 are 16-bit slaves to the bit streams that drive them. A pulse, STSYNC, generated by serial transfer control block 540, is followed by 16 bits of data, MSB first. As with the PTS blocks 788, 789 the data configuration and order is the same as for 16-bit DMA transfers. STSYNC toggles after the LSB of each 16-bit left or right data sample is transferred.

[1235] Each PTS converter blocks 788, 789 transfer operation brings in 16-bits of data to be shifted out serially. The number of transfers, the data configuration, and the order of the data varies based on the transfer mode selected, discussed below. The PTS blocks 788, 789 behave the same as that of 16-bit DMA transfers to the FIFOs, described below and depicted in Table C4 (e.g., if in 8-bit mono mode, there is one serial transfer for every two data samples, with the first sample being the LSBs and the second being the MSBs or, if in 16-bit stereo mode, there are two transfers for every sample received.)

[1236] The PTS blocks 788, 789 indicates that there is data ready to be transferred out by setting a flag. The serial transfer control block 540 responds by generating a pulse, STSYNC (serial transfer sync) that is intended to initiate the flow of serial data, MSB first. After 16 bits are transferred, a clear pulse is sent to PTS blocks 788, 789 from the serial transfer control block 540 so new data can be loaded into the respective PTS block 788 or 789.

[1237] Preferably, there are three sources and three destinations for all digital audio data multiplexed through the serial transfer control block 540. Various operating modes can be selected by modifying the contents of a control register, ICPMTI in Registers block 566 (FIG. 50), to the selected mode of operation shown in Table C5.

[1238] In general, if record or playback FIFO 538, 532 is the data destination, the format and sample rate of that path must conform to that shown in Table C5, otherwise, indeterminate data transfers will result. For example, with STM=2, the playback path sample rate and format must be the same as the synth DSP 796 (16-bit stereo, 44.1 KHz). With STM=3, the playback path sample rate and format must match the record path. In mode 4, the sample rate is 44.1 KHz or less. The modes where synth DSP 796 specifies that the sample rate can be lower than 44.1 KHz is where the value in synthesizer global mode register SGMI[ENH] is low and the register indicating the number of active synthesizer voices, SAVI[AV], is set to greater than 14. That is, if more than 14 audio voices, or signals, are being processed, the sample rate in these modes can be lower than 44.1 KHz. Otherwise, the first fourteen signals are processed at 44.1 KHz. For modes STM=1 and STM=2, CODEC 505 only supports a sample rate of 44.1 KHz. In these two modes, if synth DSP 796 operates at other than 44.1 KHz, proper operation will not occur.

[1239] As shown in FIG. 50, during playback mode, digital audio data, from external system memory (not shown), which may be formatted in one of several selectable formats, is provided, via DMA or I/O transfers, to external bus 562, through control logic and external bus interface block 568, and on to register data bus 526 as left and right channel 16-bit stereo data, for ultimate submission to 32-bit wide playback FIFO 532, or LMPF 528 (FIG. 44). The LMPF 528 (FIG. 44) may down-load prerecorded left and right channel 16-bit wide digital stereo audio data signals directly over register data bus 526 to the playback FIFO 532, whereby prior I/O or DMA transfers would have been made between the external system memory and the LMPF 528, which reduces the number of DMA transfers necessary between external system memory and CODEC playback FIFO 532. During playback, the most common mode of data transfer is DMA transfers between the external system memory and the CODEC playback FIFO 532.

[1240] In either case, the audio data is then output from playback FIFO 532, formatted (decompressed) to 16-bit signed data, as described in discussion of Format Conversion block 534 in FIG. 44, and then input to the playback DAC 514 as 16-bit signed data. The data is then sent to the Mixing Analog Functions block 510, which contains left and right analog mixers, discussed previously regarding description of FIG. 45.

[1241] In the record path, external analog audio signals that are input through the CODEC analog input pins 520 are sent through Mixing and Analog Functions block 510, and are provided as left and right channel stereo 16-bit signed digital signals to record ADC 516. The 16-bit left and right channel stereo data from record ADC 516 is then formatted to a pre-selected format and sent to 32-bit wide record FIFO 538 for further submission to register data bus 526, then to external bus 562, then to external system memory (not shown) via DMA or I/O data transfers or to LMRF 530 (FIG. 44). In record mode, DMA data transfers occur between either the LMRF 530 (where LMRF 530 has been loaded with audio data from on-chip record FIFO 538) and the external system memory via external bus 562 or, directly between the on-chip record FIFO 538 and the external system memory.

[1242] CODEC 505 is capable of performing I/O between the external system memory and the CODEC on-chip record and playback FIFOs 538, 532, and also between the system memory and the off-chip LMPF 528 and LMRF 530, for improved system flexibility.

[1243] Referring to FIG. 50, when the playback path of CODEC 505 is in mono mode, with control register CPDFI[4] being active low, both the left and right channel stereo DACs in playback DAC 514 block are provided with the same audio data from playback FIFO 532. When the record path is in mono mode, with control register CRDFI[4] being active low, preferably only data from the left stereo ADC in record ADC 516 block (data from right stereo ADC ignored) is processed and provided to the record FIFO 538. In an alternative embodiment in mono mode, only data from the right stereo ADC is provided to record FIFO 538.

[1244] Aliasing problems arise in the record ADC 516 when audio signal frequencies are processed at greater than the Nyquist rate, i.e. greater than 0.5 f_s(one-half the sample rate). Stop band and reject circuitry is used to eliminate signal reflections at multiples of f_s, plus and minus the signal frequency. The stop band rejection at 0.6 F_sfor 22 KHz is preferably greater than 75 dB. Stop band rejection is used in combination with analog filtering to eliminate high frequency images (reflections) during D/A conversions in playback DAC 514.

[1245] Oversampling in record ADC 516 is performed at 64 times the sample rate at a lower bit resolution. The signal is then down-sampled and filtered in record ADC 516 until the desired resolution and sample rate, for instance, 44.1 KHz at 16 bits, is achieved. The detailed description of the functions and operation of record ADC 516 circuitry is discussed below.

[1246] Table C4, above, provides information regarding the number of audio data samples transformed per DMA transfer, and the number of cycles per DMA transfer for each 8-bit or 16-bit DMA transfer, depending on the type of DMA transfer selected. For example, in 8-bit DMA transfer mode, audio data formatted as 4-bit ADPCM mono audio data will transfer two 4-bit samples during one DMA cycle. In 16-bit DMA transfer mode, four 4-bit ADPCM mono samples will be transferred during one DMA cycle. During 16-bit DMA cycles, the first byte to playback FIFO 532 is assigned to bits [7:0] and the second byte bits [15:8]. Simultaneous record and playback (read and write) operation is provided.

[1247] During I/O operations, the external system processor (not shown) reads the CODEC 505 status registers to determine if an I/O operation is needed and addresses CODEC 505 via Control Logic and External Bus Interface 568 to determine which area within CODEC 505 has requested data. The external system control (not shown) can perform an I/O operation for data transfer to the playback or record FIFOs (532, 538), asynchronously. Error conditions for record FIFO 538 and playback FIFO 532 are shown in Table C6.

[1248] With the 16-sample, 32-bit record and playback FIFOs, 538, 532, preferably configured with 16-bits dedicated to left channel data and 16-bits to the right channel data, thresholds, or taps, on the record and playback FIFOs 538, 532 at the 0, 7, and 15 sample address, correspond to “empty,” “half-full” and “full.” These addresses are monitored by control logic block 568 so a I/O interrupt request (IRQ) or DMA request (DRQ) can be generated (Mode 3 only, explained below) depending on the state of CODEC 505's record or playback FIFOs 538, 532. This operation is explained in greater detail, below.

[1249] Separate DRQ signals are capable of being generated for the record and playback FIFOs 538, 532. In external systems that can spare only a single DMA channel for CODEC 505, a mode is provided that allows the playback DRQ to be shared so it can function as either the record or playback DMA request channel. Systems lacking DMA capability may use I/O transfers instead. The DMA transfer mode is specified in configuration control register CFIG1I of Registers block 566 (FIG. 50). If the record or playback paths are disabled (via CFIG1I [1:0]), after the associated DRQ request signal has become active, the audio data sample will continue to be transferred, while waiting for the acknowledge, as if the path were still enabled. After the final audio sample is transferred, no other DMA requests will be serviced.

[1250] When the record path is disabled, via CFIG1I[1], or when the record and playback paths both are being enabled for DMA transfers but single channel DMA operation is selected with CFIG1I[2:0]=[1,1,1], then all data remaining in record FIFO 538 is cleared so that when record FIFO 538 is re-activated, no old data will be available for processing. Before being disabled, however, the record path prior to record FIFO 538, including format conversion block 536 (FIG. 44), and filtering functions in record ADC 516, is not cleared for four sample periods.

[1251] When the playback path is disabled, via CFIG1I[0], the playback audio is immediately muted and all samples remaining in playback FIFO 532 are allowed to shift out of FIFO 532 at the sample rate. Four sample periods after playback FIFO 532 is empty, with zeros driven through the post-FIFO playback path, the playback path is disabled to minimize power consumption.

[1252] Off-chip local memory 110 (FIG. 44) is preferably used in conjunction with the on-chip playback and record FIFOs 532, 538. Preferably, local memory 110 is figured as a large record and a large playback FIFO, each with approximately 16-megabits of 8-bit addresses. A 19-bit counter in CODEC Counters, Timers block 518 is programmed to select the size of the area in DRAM to form the respective LMPF 528 and LMRF 530, which can be configured to hold up to 512K samples. More or less audio sample memory for the LMPF 528 and LMRF 530, or local memory 110, may be configured depending on design and/or application requirements. It is preferable to use DRAM instead of SRAM due to lower cost and power requirements.

[1253] CODEC 505 includes a mode for performing interleaved DMA transfers of data between external system memory and the LMPF 528, and vice versa. In interleaved data mode, external digital audio data samples, which are stored sequentially in external system memory as L1, R1, L2, R2, . . . are transferred over external bus 562, to local memory control 790 (FIG. 49), in Control Logic block 568 (FIG. 50), which reformats the data prior to storing it in the LMPF 528 such that the left channel data samples are stored in one area of off-chip local memory 110 as L1, L2, L3, . . . block and the right channel data samples are stored in another area of local memory 110 data as R1, R2, R3, . . . block. In mono mode, the same data is stored in both blocks of local memory 110. For record mode in CODEC 505, the samples would be sent from LMRF 530 to external system memory, using the same method in reverse.

[1254] Two 16-sample counters in Counters, Etc. block 518 (FIG. 44) are provided, one for playback FIFO 532 and one for record FIFO 538. The sample counters count the number of samples that go into or come out of each respective FIFO. Each counter decrements by one every sample period, except in ADPCM mode. After the counter reaches zero, an interrupt is generated, if not masked, and the counter is reloaded with the next value the -counter is to decrement from. The count value of the counters are programmed by way of record and playback count registers (CURCTI, CORCTI, CUPCTI and CLPCTI) in Registers block 566 (FIG. 50). Status of the counters is reported via control register CSR3I in Registers block 566. In mode 3, explained below, the CODEC playback counter can be made to decrement when a DMA transfer is made from external system memory to off-chip local memory 110, as well as when DMA transfers are made from external system memory to the on-chip record or playback FIFOs 538, 532.

[1255] Table C7 shows the relationship between the data format and the events causing the sample counters to decrement.

[1256] Table C8 identifies the events causing the sample counters to decrement, and the variables used in the preferable Boolean equations, below, which are used to generate the count enable inputs to the counters.

[1257] Table C9 shows the format by which audio data is provided to and received from the record and playback FIFOs 538, 532 of CODEC 505 from the prospective of an external system or microprocessor (not shown). The letter “S” in Table C6 refers to “sample” and the number following the letter “S” refers to the sample number. The letter “R” or “L” after the sample number refers to right or left channel stereo audio data.

[1258] The CODEC timers, located in Counters and Timers block 518 (FIG. 44), are used to time certain external system functions, such as length of time to play an audio signal, etc. An interrupt is generated when the timer count is complete. CODEC 505 preferably does not utilize a timer in this block for its functions, but having this capability for industry compatibility and expandability purposes is necessary.

[1259] The CODEC 505 can operate in one of three modes during playback or record. The CODEC 505 is generally register compatible with present audio systems, by operating in modes 1 and 2. An indirect addressing mechanism is used for accessing most of the CODEC registers, contained in Registers block 566 FIG. 50. In mode 1, there are preferably 16 indirect registers. In mode 2, there are preferably 28 indirect registers. In mode 3, which is unique to CODEC 505, there are preferably 32 indirect registers. These modes operate as follows:

[1260] MODE 1

[1261] The playback sample counter in Counters, etc. block 518, FIG. 44, decrements when the playback path is enabled (CFIG1I[0]) or the record path is enabled (CFIG1I[1]). When both paths are enabled, only the playback path affects the counter and the record sample counter is not available. If register CODEC index address register, CIDXR[DTD], is set and the active path generates an interrupt (CSR1R[GINT]), then the sample counter stops counting. The counter starts counting again once the interrupt or CIDXR[DTD] is cleared. The DMA or I/O cycle control bits, CFIG1I[7:6], do not affect the sample counter's behavior.

[1262] MODE 2

[1263] The playback sample counter decrements when the playback path is enabled (CFIG1I[0]). The record sample counter decrements when the record path is enabled (CFIG1I[1]), unless CFIG1I[2] and CFIG1I[0] are also enabled. If CODEC index address register, CIDXR[DTD], is set and the active path generates an interrupt (CSR3R[5:4]), then the respective path that requested the interrupt stops operating. That data path begins operation and the counter starts counting again once the interrupt or CIDXR[DTD] is cleared. The DMA or I/O cycle control bits, CFIG1I[7:6], do not affect the sample counter's behavior.

[1264] MODE 3

[1265] Same as mode 2 operation, except the sample counters do not count when in I/O mode (CFIG1I[7:6]). Also, an enable is provided for each sample counter from configuration register, CFIG2I[5:4]. This is an enhanced mode, with independent record and playback path sample rates, record and playback programmable FIFO thresholds, additional analog mixer input enabled for synthesizer DAC audio signals, attenuation/gain controls for mixer 606 (FIG. 45) LINE/MONO outputs, and continuously variable programmable sample frequency mode (256 steps) in playback path.

[1266] A programmable 16-bit timer is provided in modes 2 and 3. This timer has approximately a 10 microsecond resolution and uses a 100 KHz clock, CLK100K. The timer is enabled by CFIG2I[6].

[1267] A programmable register pair in CODEC 505 specifies the 16-bit counter preset (CUTIMI and CLTIMI). The counter decrements every 10 microseconds until it reaches zero. At this point, the timer interrupt bit in Status Register 3 is set, the interrupt bit in Status Register 1 is set, and an interrupt is generated, if enabled via CEXTI[1]. The counter is reloaded with CUTIMI and CLTIMI values on the next timer clock.

[1268] The record and playback FIFOs 538, 532 include programmable thresholds, or taps, for signaling an IRQ or DRQ from or to the respective FIFO from external system memory. Threshold operation is as follows: a pointer tree at record and playback FIFOs, 538, 532, indicates, if equal to zero, that the address is empty of data, and if equal to one, that data is present. The transition of the index pointer tree from a one (full) to a zero (empty) for a particular address in either FIFO will trigger an IRQ or DRQ interrupt for an external system to fill the playback FIFO 532 above the preselected threshold level (playback), or to empty the record FIFO 538 to an external system so it is below the preselected level (record).

[1269] The CODEC Logic Control block 568 (FIG. 50) is connected to each tap on either FIFO. The threshold select in configuration register CFIG3I[4, 5]) in Registers block 566 (FIG. 50) determines whether the empty, full or mid-level threshold is selected. The Logic Control block 568 continuously monitors the taps and automatically generates and performs whatever functions it is designed to perform (e.g., DMA or I/O interrupt generation). When the tap signals that the threshold address is empty (playback) or full (record), depending on whether the tap is located at the position of full, empty or mid-range in the FIFO, an interrupt request is generated. DMA counters in Counters, Timers, Etc. block 518 (FIG. 44) are set for a certain number of data samples to be transferred to or from CODEC 505. Whenever a counter has completed its count, an interrupt request is generated.

[1270] The value in the index pointer of the record and playback FIFO 538, 532 is provided to the CODEC control block 568. When the index pointer has reached the FIFO threshold, a bit will be changed in a status register, in Registers block 566. This status bit can be read by the external system processing to perform a write and read operation to or from that FIFO. The status register in Registers block 566 is changed in real-time based on the threshold (taps) in the FIFOs changing from a one to a zero. When that occurs, a bit toggles in a status register, and when the status register is checked by the external system processor, the processor will determine which device is requesting the interrupt. The CODEC registers in Register block 566 are addressed with a direct address over Register Data Bus 526, or via indirect addressing by way of an index register in Registers block 566.

[1271] In the CODEC 505, the following interrupts can be generated: (1) playback and record FIFO I/O threshold reached; (2) playback and record sample counters have decremented to zero; and (3) CODEC timer has decremented to zero. The result of the CODEC interrupt logic located in Control Logic block 568 (FIG. 50) is combined into one interrupt signal, IACODEC, which is passed to interrupt selection logic in Control Logic block 568. The interrupt may be masked by a global enable, CEXTI[1]. The state of the interrupts are displayed in the global status register, CSR1R[0] located in Registers block 566 (FIG. 50).

[1272] The following interrupt equations describe the states required to set (CSET) and clear (CCLR) the logic in Control Logic block 568 associated with CODEC 505 interrupts. There is one latch in Control Logic block 568 to drive each of the three interrupt status bits in CSR2I. Referring now to Table C10, the definitions of the variables in the following interrupt equations are given.

[1273] Two general purpose control signals are provided from Control Logic block 568, referenced, GPOUT [1:0]. The state of these digital outputs reflects the state of the corresponding control bit located in the External Control Register (CEXTI) in Registers block 566 (FIG. 50).

[1274] The CODEC includes a low-power mode. Three programmable bits, selecting the low-power shut-down status of CODEC 505, power control register, PPWRI[2:0], located in Registers block 566 (FIG. 50) can disable the record path, the playback path or the analog circuitry of CODEC 505. In other embodiments, more or less bits may be used. In the shut-down mode, both external crystal oscillators 560 (FIG. 50) are disabled but all registers in Registers block 566FIG. 44 are readable. In suspend mode, selected by the external computer system or processor, CODEC 505 performs as if all 3-bits in the power control register, PPWRI, are selecting low-power states, both oscillators 560 are disabled and most of the CODEC I/O pins (not shown) become inaccessible. A dedicated suspend mode control pin, SUSPEND# (active low), causes the CODEC I/O pins to be forced high, forced low, or be set into a digital or analog high-impedance mode. See Table C11, which describes the state of the I/O pins in suspend mode. A technique for reducing power consumed by clock driven circuits is described in application Ser. No. 07/918,622, entitled “Clock Generator Capable of Shut-Down Mode and Clock Generation Method,” assigned to the common assignee of the present invention and incorporated herein for all purposes.

[1275] Table C12 describes what the PPWRI[2:0] bits cause to happen to CODEC 505 circuitry in power shut-down mode.

[1276] When the SUSPEND# pin becomes active (goes low), the CODEC behaves similarly to when it is placed into shut-down mode. Signal ISUSPRQ is logically ORed into I2LSUSPRQ and I2SSUSPRQ from the shut-down logic. ISUSPIP is logically ORed into I2LSUSPIP. If CODEC 505 is already in shut-down mode when SUSPEND# is asserted, then: (1) the I/O pins are changed to match the requirements of suspend mode described above; and (2) CODEC 505 analog circuitry in playback DAC 514, record ADC 516 and synth DAC 512 (if synth DAC 512 is embodied as a processing block within CODEC 505) is placed into low-power mode, if it is not already in that mode.

[1277] After the ISUSPRQ# is asserted, the logic in Control Logic block 568 waits for more than 100 microseconds before stopping the clocks of CODEC 505 and before disabling the oscillators. The 16 MHz clock ICLK16M and the 24 MHz clock ICLX24M are disabled (and later re-enabled) such that there are no distortions or glitches. After the clocks go into one of their high phases, they are held there until suspend mode is deactivated.

[1278] After SUSPEND# is deactivated, the external oscillators 560 are re-enabled, but ICLK16M and ICLK24M do not toggle again until the oscillators 560 have stabilized, 4 to 8 milliseconds later. This occurs after both oscillators 560 have successfully clocked 64K times. After the output clocks have been toggling for at least 100 microseconds, the ISUSPRQ# signal is de-asserted to allow the logic in the rest of CODEC 505 to operate. Signal ISUSPIP (suspend in progress) is active while the clocks are not valid. It is used to change the status of the I/O pins per the suspend requirements in Table C11.

[1279] The CODEC 505 can operate at either VCC=+3.3 or 5 volts. A voltage detect circuit in Control Logic block 568 (FIG. 50) determines whether the CODEC is in the 5 volt or 3.3 volt operating mode. The operating status is determined by the output of the voltage detect circuit register AVCCIS5. The operating voltage detect circuitry is utilized so the external computer system, or processor, can be informed that a signal cannot be generated greater than the operating VCC. For example, during 3.3 volt operation, a 4 volt signal cannot be generated. It also is used to set the analog full scale reference voltage and the range of drive capability of the digital I/O pins.

[1280] The CODEC 505 is capable of interacting with an external CD-ROM interface 568 (FIG. 50). Signals including chip select, DMA request, DMA acknowledge and interrupt request from the CD-ROM interface are supported by the CODEC 505.

[1281] An external serial EPROM or EEPROM 570 (FIG. 50) may be utilized by CODEC 505 to make the CODEC 505 Plug-n-Play (PNP) compatible with ISA, EISA or other industry standard buses or devices. Commercially available PNP software may be used to control the serial EPROM or EEPROM to configure the CODEC 505 for an external computer system or microprocessor. Where an external serial EPROM or EEPROM for PNP capability is not available, the external CD-ROM interface is not accessed by the CODEC.

[1282] A. Digital Signal Processing Portion of CODEC Playback Path.

[1283] The CODEC playback DAC 514 (FIG. 44), and synth DAC 512 if synth DAC 512 is embodied within CODEC 505, each include an interpolation block 800 (FIG. 51), a noise shaper 802 and a semi-digital FIR filter 804 for left and right channel stereo audio data. Only the left channel is shown in FIG. 51 and described herein. Operation of the right channel is identical. The operation of CODEC playback DAC 514 will be described herein. The operation of synth DAC 512 is identical if embodied within CODEC 505, otherwise the operation of the synth DAC may deviate.

[1284] A 16-bit digital audio signal 806 is output from Format Conversion block 534 (FIG. 44), and is input as a signed data signal to interpolator block 800 (FIG. 51) of playback DAC 514 where the signal is up-sampled. After the first three stages of interpolation, the multi-bit up-sampled digital audio signal 840 is output to the input of noise shaper 802, where it is quantized and converted to a 1-bit digital output signal 842. The 1-bit signal 842 is then input to semi-digital FIR filter 804 which filters out undesired out of band frequencies and converts the signal to an analog audio signal 808, which is available at the output of playback DAC 514. The left channel analog audio signal 808 is available as an input to left channel CODEC playback mixer 678 (FIG. 45).

[1285] Referring to the front end of playback DAC 514 in FIG. 52, the 16-bit digital audio signal 806 is first interpolated, then quantized and noise-shaped. The playback DAC 514 receives as input, the 16-bit digital signal 806 at a sampling rate f_sand produces at the output of interpolator block 800 (FIG. 51) a 1-bit signal 840 up-sampled to 64 times the sample rate for the 16-bit input signal 806 (64 times oversampling). Interpolation is performed in three stages in interpolator block 800, since one stage would require too complex a filter. The complexity of the circuitry is minimized by performing the 64×up-sampling interpolation in three stages, with interpolation up-sampling factors of 2 in Interp.1 blocks 810 and 812, 2 in Interp.2 block 814, and 16 in Interp.3 block 816. The noise shaper 802 is operated at the rate of 64×f_s.

[1286] A typical input spectrum to Interp.1 block 810, 812 contains components of frequencies up to f_s/2, and their undesired images centered about integer multiples of f_s. See FIG. 53afor a typical input spectrum. To carry out the first interpolation in Interp. 1 block 810, to f_s=2×f_s, an FIR filter is preferably employed which has a passband extending to about 0.40 f_sand has a stopband beginning at about 0.60 f_s. Preferably, the passband extends to about 0.45 f_sand the stopband begins at about 0.55 f_s. The stopband attenuation of the filter is preferably greater than 100 dB, and the passband ripple is about +/−0.1 dB. This ensures that images of frequencies lower than 0.45 f_s, will be attenuated by at least 100 dB. Higher frequencies, however, will fall inside the filter's transition band together with their image, which will be attenuated less. The useful bandwidth is therefore about 3.6 KHz at f_s=8 KHz, or 19.8 KHz at f_s=44.1 KHz. The spectrum of the output of Interp. 1 blocks 810, 812, for the input shown in FIG. 53a, is shown in FIG. 53b. The impulse response coefficients used in Interp. 1 blocks 810, 812 are given in Table C13. The quantity of, and values associated with, these coefficients will be different if the passband or the stopband changes.

[1287] This interpolative filtering is performed digitally, to avoid filtering in the analog domain when operating at the lowest rate, which would require a complex, or sharp transition, analog filter. Without such an analog filter, the images would appear at the output. The analog filter would have to have variable cutoff to accomodate changes in the sampling rate, which is not an acceptable solution.

[1288] The second interpolation stage, performed by Interp. 2 block 814, changes the sampling rate to f_s″=4f_s. A sinc⁵filter is used in this stage, which provides approximately 30 dB of image attenuation. The spectrum of the output of the second interpolator stage 814 is shown in FIG. 53c.

[1289] The third interpolation stage, Interp. 3 block 816, changes the sampling rate further, by a factor of 16, to f_s″=64 f_s. A sinc²interpolator, with a differential delay of two, is used. This interpolator serves the following purposes: it attenuates the images around 4f_senough for the images to not exceed the noise levels introduced by the next block, i.e., noise shaper 802, and it also introduces a zero at 2 f_s, which together with interpolator stage 2814, provides enough attenuation for images around 2 f_s. The spectrum for the output of the third stage 816 is shown in FIG. 53d.

[1290] The final block in the front end of playback DAC, and the last stage of the interpolation filter, is a fifth order noise shaper 802 (FIG. 52). Noise shaper 802 converts the up-sampled multi-bit output 840 from the third interpolator stage 816 to a 1-bit signal 842. It shapes the noise according to a Chebyshev (equiripple) high-pass transfer function. The spectrum for the noise shaper 802 output appears in FIG. 53e. The operation of noise shaper block 802 is described herein.

[1291] The 1-bit signal from noise shaper 802 is then filtered with a semi-digital FIR filter 804 (FIG. 51). Semi-digital FIR filter 804 compensates for the attenuation caused by noise shaper 802, and also achieves a relatively flat noise floor extending to about 20 KHz when f_s=8 KHz. Noise shaper 802 has less than unity gain. The spectrum of the semi-digital FIR filter 804 analog output signal is shown in FIG. 53f. Time domain examples of a digital signal being processed by interpolator 800, noise shaper 802 and semi-digital FIR filter 804 are given in FIG. 54.

[1292] B. The Interpolator Processing Blocks (810, 812, 814 and 816).

[1293] A more detailed discussion of the processing blocks of the interpolator 800 follows.

[1294] 1. Interpolator L

[1295] Interp.1 stage, blocks 810, 812, is a symmetric (linear phase) FIR filter with 2N−1 taps (N distinct coefficients), with N equal to 40 in the preferred embodiment. The interpolation factor in this block is two. It is designed to have an attenuation of about 100 dB or more in the stopband, and approximately +/−0.1 dB or less ripple in the passband. The passband response also compensates for the rolloff introduced by the sinc⁵Interp. 2 stage 814, sinc²Interp. 3 stage 816 and the semi-digital FIR filter 804 used in the playback DAC 514 D/A conversion process, as well as the gain variation introduced by the noise shaper 802.

[1296] The FIR filter in this Interp. 1 stage 810, 812 includes passband compensation achieved by combining into one function all the frequency variations introduced by subsequent stages.

[1297] Referring to FIG. 52, when used as interpolator, the FIR filter acts on the input sequence of a digital values, 16-bit input signal 806, whereby every other data sample is equal to zero (for interpolation by 2). This means one odd output sample signal 832 is computed using only odd coefficients in Interp. 1 phase 2 block 812, and the next even output sample signal 834 is computed using only even coefficients in Interp. 1 phase 1 block 810, but on the same set of 16-bit input signals 806. This leads to a polyphase (in this case, 2-phase) implementation shown as Interp. 1810 and 812 in FIG. 52, in which two sub-filters execute in parallel, and the filter outputs 832 and 834 are interleaved by known methods to create the Interp. 1 signal output 836 which is then provided to Interp. 2 block 814.

[1298] In the time domain, the even and odd output signals 834, 832 from the two phases of Interp. 1810, 812 are: $y_1\left(n\right)=\sum _{k=0}^{\frac{N-1}{2}}{h}_{2k}x\left(n-k\right)$

[1299] for even output signal 834, phase 1 (even coefficients), and for odd output signal 832, phase 2 (odd coefficients). $y_2\left(n\right)=\sum _{k=0}^{\frac{N-1}{2}-1}{h}_{2k+1}x\left(n-k\right)$

[1300] All delays are at the input sampling rate.

[1301] The Interp. 1 blocks 810, 812 filter has phase linearity, which means the impulse response is symmetric with respect to the midpoint, with the symmetry condition given as:

[1302] h_k=h_N-1-k

[1303] k=0, . . . N−1 (N odd)

[1304] This is reflected in the structure of the filters 810 and 812, shown in FIGS. 55 and 56, respectively.

[1305] Typically, the impulse response contains coefficients which are very small. For large stopband attenuations, these coefficients are very important. To preserve the precision, the coefficients are scaled so the magnitude of each is between one-half and one. Then, in the summation circuit 818 (FIGS. 55, 56), the partial products associated with the smallest coefficients are added first, scaled, and then added to the products associated with the next higher-valued coefficient, and so on. This means the sums cannot be performed in an arbitrary order (e.g., in the same order as the taps are updated), unless the word width is further increased to preserve the precision.

[1306] 2. Interpolator 2.

[1307] The second interpolator stage 814, Interp. 2, is a sinc⁵interpolator filter. The interpolation factor in this block is two. Due to the attenuation that will be provided by the semi-digital filter 804, a high attenuation around 2×f_s, is not needed, and a relatively simple structure is used. The transfer function of the filter for Interp. 2 stage 814 is: $H_2\left(Z\right)={\left(\frac{1}{2}\frac{1-z^{-2}}{1-z^{-1}}\right)}^5=\frac{1}{32}{\left(1+z^{-1}\right)}^5$ $\text{expanding to,}$ $H_2\left(Z\right)=\frac{1}{32}\left(1+5z^{-1}+10z^{-2}+10z^{-3}+5z^{-4}+z^{-5}\right)$

[1308] Thus, the Interp. 2 filter 814 has only integer coefficients. The passband rolloff has to be compensated in Interp. 1 blocks 810, 812.

[1309] Since the Interp. 2 filter 814 interpolates by two, it operates on a sequence in which every other sample is zero, as illustrated below:

[1310] This leads to a two-phase implementation as shown in FIG. 57, similar to Interp. 1810, 812 blocks, where: $H_{2}a\left(z\right)=\frac{1+10z^{-1}+5z^{-2}}{32}=\frac{1+10z^{-1}+z^{-2}}{32}+\frac{4z^{-2}}{32}$ $H_{2}b\left(z\right)=\frac{5+10z^{-1}+z^{-2}}{32}=\frac{1+10z^{-1}+z^{-2}}{32}+\frac{4}{32}$

[1311] In H₂a and H₂b, the delays occur at the input sampling rate f_s. The common term in the transfer functions in both phases of Interp. 2 filter 814 results in some hardware savings. FIG. 57 shows an embodiment of the Interp. 2814 filter. A scaling factor of 2 has been applied throughout. The frequency response, normalized to DC, is shown in FIGS. 58 and 59.

[1312] 3. Interpolator 3.

[1313] The transfer function of Interp. 3 block 816 is: $H_3\left(z\right)={\left[\frac{1-z^{-32}}{1-z^{-1}}\right]}^2$

[1314] The interpolation factor in this block is 16. The differential delay is 2. The order is 2. One embodiment of the implementation of the transfer function is given in FIG. 60. The differentiators 839 run at a lower rate, while the integrators 841 run at a higher rate.

[1315] The differentiators 841 having 2 delays can be factored into a differentiator with one delay and a 2-sample accumulator, where: $1-z^{-2}=\underset{\text{ 2-sample accumulator}}{\left(1+z^{-1}\right)}·\underset{\text{ simple differentiator}}{\left(1-z^{-1}\right)}$

[1316] Another embodiment for Interp. 3 block 816 is shown in FIG. 61. Each signal sample is used 16 times by the integrator 846, which runs at the highest rate. A zero is introduced a 4 f_s. The double delay blocks 841A,B in FIG. 60 and 846A in FIG. 61 operate to introduce an additional zero at 2 f_s; which together with interpolator 2 sinc⁵filter 814, provides enough image attenuation and is more economical than using a sinc⁶filter for interpolator 2 filter 814. The frequency response of interpolator 3 filter 816, normalized to DC, is shown in FIGS. 62aand 62b.

[1317] C. Noise Shaper.

[1318] The final stage of the interpolator, noise shaper block 802 (FIGS. 51, 52), takes the multi-bit signal output from the third interpolator stage, interpolator 3 block 816 (FIG. 52), and converts it to a 1-bit signal while shaping the quantization noise according to a high-pass function. The block diagram implementation for the shaper 802, which is a preferably fifth order shaper, is shown in FIG. 63. The 1-bit output signal 842 is also input to integrators 822. Integrator 822 inputs must have suitable scaling factors, k1-5, to make the loop stable for a predetermined range of input amplitudes, as determined by the remainder of the digital path shown in FIG. 63. The simple additive noise model shown in FIG. 63 is used to represent the quantizer. Two transfer functions are defined for this circuit: a signal Transfer Function (STF) Y/X where X is the digital audio input signal 840 (FIG. 52), and a noise Transfer Function (NTF) Y/E, where E is the quantization noise (modeled as additive, white, uniformly distributed noise). Once the NTF is fixed, the STF is also determined. Since the system is not a FIR filter, the response is no longer strictly phase-linear. The phase variation in the passband, however, is very small, on the order of about 0.05 degrees, and the magnitude variation can easily be compensated in Interp. 1810, 812 block.

[1319] A signal flow graph (SFG) for noise shaper block 802 is shown in FIG. 64. The transfer functions are developed as follows:

[1320] Forward Path Gains:

[1321] The cumulative gains of all possible direct paths from input to output:

[1322] For X:

[1323] T₁=κ₁κ₂κ₃κ₄κ₅·I⁵

[1324] For E:

[1325] T₁=1

[1326] Loop Gains:

[1327] The gains of all closed loops.

[1328] G₁=A₁k₁I=A₁K₁·I

[1329] G₂=A₂k₁k₂I²=A₂K₂·I²

[1330] G₃=A₃k₁k₂k₃I³=A₃K₃·I³

[1331] G₄=A₄k₁k₂k₃k₄I⁴=A₄K₄·I⁴

[1332] G₅=A₅k₁k₂k₃k₄k₅I⁵=A₅K₅·I⁵

[1333] L₁=I²·B₁k₃k₄=B₁(K₄/K₂)·I²

[1334] L₂=I²·B₂k₁k₂=B₂K₂·I²

[1335] Non-touching Loops:

[1336] The products of the gains of sets of loops without any common nodes are calculated. First, pairs of non-touching loops have to be identified. Then, triplets are found, then sets of 4, etc. In the preferred embodiment, only pairs of non-touching loops exist.

[1337] L₁, G₁

[1338] L₁, G₂

[1339] L₁, L₂

[1340] Determinant:

[1341] This is defined in terms of the loop gains as

[1342] Δ=1−Σloop gains+Σgains of pairs of NTL−gains of triplets of NTL+ . . . NTL=non-touching loops

[1343] In the preferred embodiment, there are no triplets of non-touching loops, so $\Delta =1-\sum _{i=1}^5{G}_i-\sum _{i=1}^2{L}_i+L_1\left(G_1+G_2\right)+L_1{L}_2$

[1344] Sub-determinants:

[1345] Δ=Δ setting to zero gains of loops touching forward path k

[1346] For X:

[1347] All loops are touched by T1, so

[1348] Δ₁=1

[1349] For E:

[1350] Δ₁=Δfor T₁=1−L₁−L₂+L₁L₂

[1351] The transfer functions can then be constructed for X and E using Mason's rule, where $\mathrm{TF}=\frac{1}{\Delta }\sum _k{T}_k{\Delta }_k$

[1352] The transfer functions have the form: $\begin{array}{c}{H}_E\left(z\right)\equiv \frac{Y\left(z\right)}{E\left(z\right)}=\frac{1}{D}\{{\left(1-z^{-1}\right)}^5-\left(B_1\frac{K_4}{K_2}+B_2{K}_2\right){z^{-2}\left(1-z^{-1}\right)}^3+\\ \left(B_1{B}_2{K}_4\right)z^{-4}\left(1-z^{-1}\right)\}\\ =\frac{1}{D}\left\{{\left(1-z^{-1}\right)}^5-2C_1{z^{-2}\left(1-z^{-1}\right)}^3+C_2{z}^{-4}\left(1-z^{-1}\right)\right\}\end{array}$

[1353] for noise, and $H_X\left(z\right)\equiv H_{\mathrm{NS5}}\left(z\right)=\frac{Y\left(z\right)}{X\left(z\right)}=\frac{1}{D}$

[1354] for the signal, where $D={\left(1-z^{-1}\right)}^5+\sum _{k=1}^5{W}_k·{z^{-k}\left(1-z^{-1}\right)}^{5-k}$

[1355] Where, referring to FIG. 63, $W_1=-A_1{K}_1$ $W_2=A_2{K}_2-B_1\left(\frac{K_4}{K_2}\right)-B_2{K}_2$ $W_3=-A_3{K}_3+A_1{B}_1\left(\frac{K_1{K}_4}{K_2}\right)$ $W_4=-A_4{K}_4+A_2{B}_1{K}_4+B_1{B}_2{K}_4$ $W_5=-A_5{K}_5$

[1356] The coefficients are chosen to match a Chebyshev function, which yields equiripple quantization noise in the passband and a flat stopband. The values for Ai and the Bi are obtained from the Ci and Wi in the above equations by matching the noise TF to the desired shaping function.

[1357] Preferably, a function is chosen for the NTF which has zeros equally spaced inside the noise stopband (i.e., the signal band), and a flat high-frequency response. For the preferred embodiment, the stopband edge, the stopband attenuation and the filter order must be determined. Since the stopband attenuation is preferably at least 90 dB and the stopband edge is about 6 KHz for an input sampling rate of 8 KHz, or equivalently, about 36 KHz at the maximum sampling rate of 48 KHz, the filter order preferred is five. That is, the noise stop band for noise shaper 802 extends to at least 0.70 f_s, and preferably to about 0.75 f_swhich is about 0.25 f_spast the signal band. This allows the design requirements for the semi-digital filter to be less stringent.

[1358] First, the continuous time zeros and poles are obtained, where the zeros are given by: $s{z}_m:=j·{\omega }_r·\cos \left[\left(2·m+1\right)\frac{\pi }{2·N}\right]$

[1359] and the poles by: $\begin{array}{c}s{p}_m:=j·{\omega }_r·[\cosh \left(\frac{\mathrm{asinh}\left(\epsilon 1\right)}{N}\right)·\cos \left[\left(2·m+1\right)·\frac{\pi }{2·N}\right]+\\ j·\sinh \left(\frac{\mathrm{asinh}\left(\epsilon 1\right)}{N}\right)·\sin \left[\left(2·m+1\right)·\frac{\pi }{2·N}\right]]\end{array}$

[1360] where N=5, m ranges from 0 to 4, ω_r=stopband edge=2π. 36000, and ε1 is related to the attenuation G given in dB by: $\epsilon 1:=\sqrt{{10}^{-\frac{G}{10}}-1}$

[1361] The pole-zero diagram in the s-plane is shown in FIG. 65. A plot of the frequency response out to 300 KHz is shown in FIG. 66. Next, the discrete zeros and poles are obtained using the bilinear transformation: $z{z}_k:=\frac{1+\frac{T}{2}·s{z}_k}{\left(1-\frac{T}{2}·s{z}_k\right)}$ $z{p}_k:=\frac{1+\frac{T}{2}·s{p}_k}{\left(1-\frac{T}{2}·s{p}_k\right)}K=0,\dots 4$