Method and apparatus for support of multiple memory devices in a single memory socket architecture

[0002] This application is related to commonly owned U.S. patent application Ser. No. 09/163,154, now U.S. Pat. No. 5,982,655, entitled METHOD AND APPARATUS FOR SUPPORT OF MULTIPLE MEMORY TYPES IN A SINGLE MEMORY SOCKET ARCHITECTURE, by Steven J. Doyle.

[0003] This invention relates to computer memories and more particularly to a method and apparatus for accommodating different memory configurations within a single multi-pin DIMM socket.

[0004] Most microprocessor-based computing devices, used to execute a variety of application programs, include a microprocessor and a separate memory (RAM) each residing in a separate package or “circuit chip” on a printed circuit motherboard. The memory stores and retrieves data in a matrix according to a predefined addressing scheme. The microprocessor typically accesses data stored in multi-bit words at various locations within the memory over corresponding multi-bit address lines. According to the addresses provided to the memory, data words are output over an output bus that is, in turn, interconnected with the microprocessor and with a variety of other data handling devices both on the board and remote from the board.

[0005] To facilitate assembly, maintenance and replacement of computing components, each circuit chip package is applied to a multi-pin socket that is, in turn, permanently mounted on the motherboard. One popular socket arrangement for accommodating random access memories is the so-called dual in-line memory module (DIMM) socket. This socket generally defines one hundred individual pin connections in a preferred arrangement. Each pin is at a standardized location on the socket and is designated by a standard “pin number.” In general, the microprocessor, DIMM socket and other circuit components are connected by a series of discrete lines that are permanently placed on the motherboard, and that, consequently, are not reconfigurable. In other words, a particular pin of the microprocessor is permanently tied to a particular pin on the DIMM socket. This limits the ability of the dim socket to support memory devices other than those specifically designed to interface with certain interconnections or pins. However, the physical geometry of the dim socket is such that it can support several different types and sizes of memories. For example, the socket can accommodate the popular extended data out random access memory (EDO) available in a plurality of memory densities from 1 megabytes up to 32 or 64 megabytes. The addressing scheme for each of such devices is often slightly different. In particular, each different size, and array configuration (number of rows versus columns) requires more, less or different address pins to be employed. This means, that simply attaching different-sized memory into the same DIMM socket on a motherboard will not guarantee that the desired microprocessor address pins are connected with the proper addressing functions of the microprocessor. In particular, each EDO device may contain a different number of row versus column address pins. Some common configurations/sizes are detailed in the following table, which assumes one bank of memory per socket:

[0006] Accordingly, it is an object of this invention to provide an apparatus and method for supporting a variety of different memory configurations and sizes within a single socket. The underlying architecture should be simple to configure for the different size memories. It should not require excessive amounts of additional software or hardware to enable the configuration.

[0007] This invention overcomes the disadvantage of the prior art by providing a method and apparatus for supporting different sizes and configurations of random access memory in a single DIMM socket without altering the underlying physical connectivity between sockets/circuit components. In general, the address lines of the microprocessor are interconnected through a multiplexer and buffer arrangement. The multiplexer divides the connected address lines into two groups. The two groups of address bits, so divided, are selectively routed to predetermined pin connections of a DIMM socket that interconnect with predetermined address lines of the resident random access memory. Me address bits are transmitted to the pin connections during each of the row address cycle and the column address cycle of the memory. The interconnections between the multiplexer and the random access memory are arranged so that a variety of standardized address pin configurations are supported by the same socket. This, in part, results from standardized locations for address bits for a variety of memory devices, varying primarily in the presence or absence of higher order address bits for rows and/or columns. The architecture of this invention enables the microprocessor to be programmed to operate with a given size of memory, and for the appropriate connections to the desired memory address lines to be already present.

[0008] In this manner, the user can select an address configuration that is appropriate to a particular predetermined memory device without altering the physical connections between the microprocessor and the DIMM socket The addressing scheme provided by the microprocessor is constant, and the multiplexer to produces the desired addressing scheme at the DIMM socket to accommodate a given memory device. The actual address configuration of the memory device is “transparent” to the microprocessor.

[0009] In a preferred embodiment, the random access memory can comprise an extended data out dynamic random access memory (EDO) having a desired size and “width.” The microprocessor can comprise an MPC 860 PowerQuicc™ available from Motorola. Multiple sockets can be arranged in parallel in the form of discrete memory banks, having data and address busses arranged in parallel. Control of read/write, column and row addressing functions can also be arranged in parallel. In a typical arrangement, row address strobe (RAS) functions are used to select between particular memory banks, enabling each selected bank at a predetermined time.

[0010] The foregoing and other objects and advantages of the invention will become more clear with reference to the following detailed description as illustrated by the drawings in which:

[0011] FIG. 1 is a block diagram of a system for addressing multiple memory devices in a single DIMM socket according to this invention;

[0012] FIG. 2 is a more detailed schematic diagram of the system of FIG. 1;

[0013] FIG. 3 is a pin diagram of a multiplexer for use with the system of FIG. 1 and 2;

[0014] FIG. 4 is a table defining pin designations of the multiplexer of FIG. 3;

[0015] FIG. 5 is a truth tables defining logic states of the multiplexer of FIG. 3; and

[0016] FIG. 6 is a timing diagram of the addressing cycle for a memory device for use in the system of FIGS. 1 and 2.

[0017] FIG. 1 shows an overall system for accommodating a plurality of memory devices in a single DIMM socket according to a preferred embodiment of this invention. The socket 20 is configured with pin locations according to Joint Electron Device Engineering Council (JEDEC) standard No. 21-C, found at Page 4.4.8-1 of the JEDEC standards book. The illustrated socket 20 is a Molex 71251-5101 according to this embodiment, but DIMM sockets from other sources can be employed. These DIMM sockets generally should be 100-pin sockets in family that supports dynamic RAM (DRAM), synchronous dynamic RAM (SDRAM) and read only memory (ROM). The system 10 includes a microprocessor 12. The microprocessor in this embodiment is an MPC 860 PowerQuicc™ available from Motorola. It is contemplated that a variety of different types of microprocessors, or even external memory controllers located between the microprocessor addressing lines and memory, can be used in conjunction with the system of this invention if the particular well-known standards for addressing, clock, etc. are accommodated so that these other microprocessors perform within the spirit and scope of this invention. The MPC 860, hereinafter referred to generally as “the microprocessor,” can be configured to provide multiplexed memory address outputs based upon internal configurations. In this embodiment, the microprocessor 12 is configured so that addresses are multiplexed externally. In other words, addresses are output from the microprocessor in a conventional pattern on a long word boundary.

[0018] A multi-bit address bus 14 transfers address signals to the multiplexer 16 according to this invention. Similarly, a multiplexed address bus 18 transmits multiplexed address signals to the DIMM socket 20, which includes a selected EDO memory having a predetermined size and configuration according to this invention. While EDO is employed in a preferred embodiment is contemplated that other types of memory can be adapted to function according to this invention. These EDOs of various sizes shall be referred to generally as “the memory.” An optional additional memory 22 (shown in phantom), or a multiplicity of additional memories, can be provided in parallel banks to the base DIMM socket/memory 20. All memories in a bank output addressed data on a shared multi-bit output bus 24 that transmits the addressed data back to the microprocessor 12 and to other on-board and remote devices.

[0019] A variety of memory devices can be used in conjunction with the system of this embodiment. The following table (along with the table described generally in the Background above) refers to the types of EDO that are readily supported according to this invention. Note particularly the well-known address ordering scheme in which A29 is the least significant bit and A8 is the most significant bit:

[0020] Each of the foregoing EDO memories can be accommodated within a one hundred-pin DIMM socket. As noted above, however, the configuration of address lines in each do not always correspond. Some applications require two or more devices to make up a full thirty-two bit configuration. Also, one or more additional DIMM sockets like the socket 22 (shown in phantom) may be required for additional applications. In general each socket can support two separate banks of memory thereon. For the purposes of this description such banks shall be referred to as BankO and Bank1 within each socket 20 and 22. Each bank can be independently enabled as further described below. The refresh speed of various memory devices also varies. Refresh speeds of 1 K, 2 K or 4 K may be required. A refresh speed of 4 K is a common multiple. Hence, for the purposes of this embodiment, the refresh speed of 4 K is programmed into the microprocessor. Where devices have an 8 K refresh requirement, the program can deliver this as a minimum device refresh speed.

[0021] The memories used herein are addressed by the microprocessor for reading or writing operations according to a specific addressing cycle. This cycle will be described further below. In general, the cycle consist of an addressing of rows and columns of the memory array at different times using the same set of address inputs (A0-A12 for example). Complete row and column addressing requires that a row address strobe signal (RAS), a column address strobe signal (CAS) each be asserted in sequence at a predetermined time. During each RAS and CAS signal, addresses are provided by the address bus 16 of the microprocessor, to be read by the memory device 20. RAS and CAS control signals for operating the memory 20 are provided by the microprocessor over multibit control lines 26 and 28, respectively, extending between the microprocessor and the memory (or memories). A buffer 50 is connected along the RAS and CAS lines for delay-matching and signal-boost purposes as will be described further below. This buffer also handles non-multiplexed address signals QA7 and (optional) QA6 also described below. As will again be described below, the RAS signal is used to enable a specific memory in a bank of two or more memories in each socket 20 and 22. It this embodiment, a first memory is designated Bank0. A second memory is designated Bank 1. Either is selected, based upon the RAS signal configuration. In addition each socket is selected based upon the state of the RAS signal. In this arrangement, the CAS signal is unaltered, regardless of the memory bank being selected.

[0022] With reference also to FIGS. 2-6, the operation of the multiplexer 16 is further described. The diagram of FIG. 2 includes both the conventional signal/line names for each depicted circuit input and output, and a corresponding conventional socket pin number. The term TP denotes an unused pin or “test-point” and all resistors are designated by their respective resistance value in Ohms. In addition, pin connections (and their underlying functions) on individual circuit components that are not used according to this embodiment are not shown. In some instances, highly conventional connections such as power, clock, chip enable, etc. may not be shown, but are assumed to be connected in a well-known manner. Also, the conventional (manufacturer's) names for each individual chip/socket connection (e.g. RAS0-RAS3, SEL, OE, WE) are shown within the block representing the chip/socket adjacent the corresponding pin number.

[0023] The multiplexer 16 of this embodiment comprises a D-type latch device configured as a 12/24-bit wide unit. This embodiment employs a model 74ALVCH16260 circuit commercially available from Pericom Semiconductor of San Jose, Calif. for the multiplexer 16. This device defines the specific pin configuration 100 detailed in FIG. 3. The designation of various pins is further defined in the table 400 at FIG. 4. For the purposes of this description reference can be made to the manufacturer's data specifications found in publication PS8089A dated Mar. 25, 1997. In general, the microprocessor's address line inputs denoted QA8-QA29 (QA29 being the least significant bit) are carried on corresponding inputs 1B1-1B12 (1BN) and 2B1-2B12 (2BN) according to the connection order shown in FIG. 2. Input QA7 is carried separately as discussed immediately below. In particular, least significant address bit QA29 and adjacent bit QA28 are carried together on 2BN, with successive bits alternating by twos. Next adjacent bits QA27 and QA26, and successive alternating bits are carried on 1BN. 1B1 also carries the QA8 address input along with 2B2. Microprocessor address inputs QA7 and QA6 are also routed to the socket 20. As will be described further below, these values are routed through buffer 40 rather than through the multiplexer 16. In general, QA7 enables the use of a memory device in the socket 20 having a 13-row by 10-column configuration. Hence, this enables support of an 8 Mbyte×8 EDO with 8 K refresh speed. Note that four banks of such 8 Mbyte×8 devices yields up to 128 Mbyte DIMM arrangement on a given circuit board.

[0024] The address QA6 is also connected via the buffer 50 to the socket 20. This address is provided for upgrades that include wider address boundaries.

[0025] The order of multiplexed addresses is particularly defined as follows according to a preferred embodiment:

[0026] The unused multiplexer input 2B1 is tied to ground via a 220 Ohm resistor. To achieve the proper inputloutput setup, the multiplexer's latch enable inputs LE2B, LE1B, LEA2B and LEA1B, and B2N and B1N inverse output enable inputs OE2B and OE1B are tied to power of +3.3 V via a 4.7 K Ohm resistor. Similarly, the AN inverse output enable OEA is tied to ground via a 220 Ohm resistor.

[0027] The selecting signal DRAM_MUX_L (where “L” signifies a logical “Low” for assertion) is used to selectively drive the multiplexer's select input (SEL) to cause selected address values 1BN or 2BN to alternatively be output on output lines A1-A12 (AN). These values are carried to the DIMM socket 20 as multiplexed address lines MUXA0-MUXA11. It has been determined that, for the various memory devices shown in the table above, appropriate assertion of DRAM_MUX_L in either a high or low state provides the proper configuration of multiplexed addressed signals MUXA0-MUXA11 at the DIMM socket inputs A0-A11, respectively, so as to accommodate the given EDO installed in the socket. In other words, when the designated pins shown in FIGS. 2 and 3 are interconnected, the relationship of inputs (1BN and 2BN), defined in the truth table 500 of FIG. 5 will provide appropriate outputs (AN) to accommodate each of the variety of memory devices. Note, the table 500 uses H to signify a high logic state, L to signify a low logic state, X to signify an irrelevant notation and Z to signify a high impedance state (e.g. no signal from this output). Again, the multiplexer's output is particularly based upon the logical state of select (SEL) as governed by DRAM_MUX_L. In the preferred embodiment, the following relationship between multiplexer output and DIMM device address input is established:

[0028] The microprocessor address outputs of QA7 and QA6 (optional upgrade/expansion signal) are provided directly to the memory address inputs A12 and A13, respectively via the buffer 230 as described below. The buffer generates buffered signals B_QMA7 and B_QMA6, respectively which then are connected to the memory/socket 20. Memory address inputs A14-A15 are test points that remain unconnected according this embodiment.

[0029] It is expressly contemplated that a different microprocessors and memory arrays may require a different multiplexer configuration. Other commercial or customized multiplexer (such as programmable logic arrays) can be employed where appropriate.

[0030] The table appearing in the background gives a large list of EDO configurations that can be accommodated by the architecture described according to this invention. The following are some more particular examples of EDO memory devices, with specific address inputs defined according to this embodiment, and how these devices address configurations conform with the selected multiplexing scheme (wherein N is the memory width as specified by the manufacturer):

[0031] 1. 512 K×N array uses specified row addresses 1B12-1B3 (corresponding memory socket inputs A0-A9); column addresses 2B12-2B4 (corresponding memory socket inputs A0-A8). Note that contiguous microprocessor addresses QA29-QA11 are used.

[0032] 2. 1 Mbyte×N array uses specified row addresses 1B12-1B3 (corresponding memory socket inputs A0-A9); column addresses 2B12-2B3 (corresponding memory socket inputs A0-A9). Note that contiguous microprocessor addresses QA29-QA10 are used.

[0033] 3. 2 Mbyte×N array uses specified row addresses 1B12-1B2 (corresponding memory socket inputs A0-A10); column addresses 2B12-2B3 (corresponding memory socket inputs A0-A9). Note that contiguous microprocessor addresses QA29-QA9 are used.

[0034] 4. 4 Mbyte×N array uses specified row addresses 1B12-1B2 (corresponding memory socket inputs A0-A10); column addresses 2B12-2B2 (corresponding memory socket inputs A0-A10). Note that contiguous microprocessor addresses QA29-QA8 are used.

[0035] 5. 4 Mbyte×N array uses specified row addresses 1B12-1B1 (corresponding memory socket inputs A0-A11); column addresses 2B12-2B3 (corresponding memory socket inputs A0-A9). Note that contiguous microprocessor addresses QA29-QA9 are used.

[0036] 6. 4 Mbyte×N array uses specified row addresses 1B12-1B1 (corresponding memory socket inputs A0-A11); column addresses 2B12-2B3 (corresponding memory socket inputs A0-A9). Note that contiguous microprocessor addresses QA29-QA8 are used.

[0037] With reference again to FIG. 2, the RAS and CAS control is effected by a series of control signals that are routed through a set of parallel buffers 230, 232, 234 and 236. These buffers comprise four discrete buffer circuits on a multi-pin integrated circuit package 50 available from Pericom Semiconductor (or other suppliers) as model number 74LVT162244. Each buffer input line A0-A3 produces an output Y0-Y3 respectively, having the same logical state. While the depicted buffers 230, 232, 234 and 236 can be omitted in some applications, they serve to equalize the timing of signals from the microprocessor to the memory with respect to the multiplexer 16. In addition, where a bank of two or more memory devices are employed together, the buffers serve to provide sufficient signal strength in view of the internal impedance present in the memory bank to ensure adequate propagation of the buffered signals.

[0038] The addressing process occurs as follows. In general, DRAM_MUX_L begins in a high (deasserted) state 600. Various row addresses (MUXAN) are shown as either high or low 602 at this time. The RAS selection signal (DRAM_CSN) is asserted 604. This causes the memory to address the various rows identified by MUXAN. These rows are present on the 1BN inputs of the multiplexer 16. These inputs are allowed to pass through during the RAS phase of the cycle. Subsequently, DRAM_MUX-L becomes a low signal 608. A cross over occurs 610 in MUXAN and now the 2BN addresses are allowed the pass through the multiplexer 16. Subsequent to the cross over 610 the CAS signal (BS_AN) becomes low 612. At this time both row and column signals are transmitted in a cycle to the memory. Note that microprocessor address signals QA7 and QA6 (if used) are routed to the socket via the buffer (as respective buffered signals B_QMA7 and B_QMA6) while both the 1BN and 2BN mutiplexer inputs are routed to the socket. This results in a valid data bit 614 on each of the data lines QDN. The actual timing of valid data is an example since the timing for a read cycle generally differs from that of a write cycle. Following the reading or writing by the microprocessor and other devices of the valid data word 614, the cycle begins again at the time 618. The signals RASN, CASN, and DRAM_MUX_L each become high again, awaiting restart of the cycle for the next data word to be addressed.

[0039] With further reference to the buffer bank 50, containing the individual buffers 230, 232, 234 and 236, the control of the memory's RAS, CAS and read/write enable inputs is now described (see also FIGS. 1 and 2). Each of these signals is routed through one of the bank of buffers to provide a timing match with the multiplexer 16, and also to allow increased driving capability where a bank of two or more memories are employed. The microprocessor includes four conventional byte select outputs. These are asserted as signals BS_A3_L, BS_A2_L, BS_A1_L and BS_A0_L. These signals are used to enable conventional CAS signals CAS0, CAS1, CAS2 and CAS3, respectively on the memory according to known protocols.

[0040] Conventional chip selects on the microprocessor generate signals DRAM0_CS_L, DRAM1_CS_L, DRAM2_CS_L and DRAM3_CS_L, each passed through the buffer 234. DRAM0_CS_L is interconnected with RAS0 and RAS2 on the memory. This RAS signal is configured to select the memory socket 20 when needed. As noted there are two banks (Bank 0 and Bank1) available in each socket. When DRAM0_CS_L becomes low at the beginning of the RAS cycle, it enables Bank 0 on socket 20. Otherwise this signal remains high. DRAM2_CS_L is interconnected with RAS1 and RAS3 of memory socket 20. These RAS inputs enable Bank1 of socket 20 when they receive a low signal from DRAM2_CS_L. Note that DRAM1_CS_L is interconnected with RAS0 and RAS2 (actual connection not shown) of the next socket 22 (Bank 0) and DRAM3_CS_L is likewise connected with RAS1 and RAS3 of the next socket (Bank 1). The appropriate assertion of DRAM1_CS_L and DRAM3_CS_L signals, hence, selects Bank 0/Bank1 of socket 22 rather than Bank 0/Bank1 of socket 20. In general each existing socket and bank is enabled in turn by asserting the appropriate RAS signal. Other sockets and banks (not shown) can be controlled by further combining and/or multiplexing the chip select inputs to select specific sockets/banks based upon the particular combination of RAS signals.

[0041] The buffer 232 is interconnected with the microprocessor's read/write select signal denoted as RDWR_L. The derived buffered signal is then interconnected with the inverse memory write enable signal WE. A write operation can occur over the memory data lines when this signal is asserted low. Likewise, the buffer 236 receives the read/output enable signal DRAM_OE_L, which is then connected to the inverse output enable OE on the memory. When this signal is asserted low, the data lines output an addressed word.

[0042] Finally, each socket contains four parity bit outputs DPAR0-DPAR3. These outputs allow error checking using a well-known parity technique. EDO generally utilizes such a technique, and appropriate connections are made to carry out the parity function. Other types of memory may not utilize the parity function.

[0043] With reference to FIG. 2, all control and data lines that are shown interconnecting with additional memory sockets (socket 22, etc.) can be assumed to be tied to identical pin connections and perform identical functions as those described for socket 20.

[0044] Finally, the system that employs the foregoing architecture can be made to automatically recognize the size and configuration of memory device(s) being installed in the socket(s) by undertaking the following memory recognition steps upon boot-up:

[0045] 1. The system addresses the base memory location (e.g. row0 and column0) in each socket and each bank to determine whether any data is retrieved. If so, then the system has recognized whether and where each discrete memory device is located.

[0046] 2. The system begin polling memory addresses in, for example 1-megabyte increments to determine if data is present at each address. When data is no longer present, then the row/column boundary for each device populating the memory circuit board has been determined.

[0047] 3. These values are stored in the microprocessor or memory controller memory for future use.

[0048] The foregoing has been a detailed description of a preferred embodiment of this invention. Various modifications and additions can be made without departing from the spirit and scope of this invention. For example, the specific configuration of pin interconnections to the memory socket can be varied based upon the type of memory being employed. While a memory comprising a conventional EDO type is shown herein, different types of memories may employ different standards for pin configuration. The pin connections described herein can be altered to service a particular standard. The number of banks of memories can be altered depending upon the application, and the size of memory in each bank may be varied independently. While it is contemplated that the hardware configuration detailed herein can operate according to an existing memory control protocol, in some embodiments, modified memory control software may be necessary or desirable. Accordingly, this description is meant to be taken only by way of example, and not to otherwise limit the scope of the invention.