Block RAM with embedded FIFO buffer

[0001] This invention relates generally to digital logic devices, and more particularly to programmable logic devices and methods for configuring embedded random access memory (“RAM”) to function as a first in, first out (“FIFO”) memory system in the programmable logic device.

[0002] FIFO memory systems are used in a wide variety of applications, such as buffering high-speed serial data immediately after it has been parallelized, for temporary storage of a data packet during packet processing, or buffering data going to or coming from a disk. Data values that are sequentially written to a FIFO buffer are read from the FIFO buffer in the same order, namely the first data entered into the FIFO memory system is the first data read from the FIFO memory system.

[0003] FIFO buffers are implemented in RAM and the flow of data into and out of RAM is controlled by address counters that track the read and write addresses being used. The address counters coordinate the data flow into and out of RAM to insure that memory is available to accept incoming data to prevent overflowing the RAM, and that data is stored in RAM before a read operation executes.

[0004] Several status flags are available in FIFO memory systems, such as FIFO FULL and FIFO EMPTY, which indicates the RAM is either full or empty, which indicate that there is sufficient space in RAM for a WRITE to occur or sufficient data in RAM for a READ to be performed. For example, if a WRITE function is to be enabled, the FIFO FULL status flag will indicate whether the data can be written into memory. If a read is to be performed, the FIFO EMPTY status flag will indicate whether there is any data available in the FIFO.

[0005] FIFO memory systems can be synchronous or asynchronous. A FIFO memory system in which both the read address counter and the write address counter are clocked by the same clock signal is referred to as a synchronous FIFO memory system. In contrast, a FIFO memory system in which the read address counter and the write address counter are clocked by different clock signals is referred to as an asynchronous FIFO memory system. Asynchronous FIFO memory systems are extremely useful in digital data systems where different sides of the FIFO memory system are independently clocked, either at different clock rates, or at the same clock rate, but with a phase difference (“skew”).

[0006] In both synchronous and asynchronous FIFO systems, the read and write address counters are circular counters that wrap around to an initial address after the last address is accessed. The read and write address counter output signals are either multiplexed to address a single-port random access memory (RAM), or they are separately provided to address different input ports of a multi-port RAM (e.g. a dual-port RAM).

[0007] FIFO memory systems have been implemented in programmable logic devices (“PLDs”), such as a field-programmable gate arrays (“FPGAs”) and complex programmable logic devices (“CPLDs”). PLDs are semiconductor integrated circuits that have a “fabric” of programmable logic, and other functional portions, such as RAM.

[0008] FIFO memory systems have been implemented in PLDs using the fabric of the PLD to provide the FIFO control logic, such as generating the status flags, and providing the write and read addresses of FIFO memory systems. Such FIFO memory systems are described in co-owned U.S. Pat. Nos. 5,898,893 and 6,434,642, the disclosures of which are hereby incorporated in their entirety for all purposes. The logic for operating the FIFO memory system is typically developed in configurable logic blocks (“CLBs”) surrounding an embedded block of RAM (“BRAM”), and the embedded BRAM is operated as a FIFO buffer. In alternative designs, distributed memory is used in a FIFO memory system. However, in either instance it is often desirable to have CLBs available to perform other logic functions. Additional information regarding configuring FIFO memory systems in FPGAs is available in Application Note XAPP131, available from Xilinx, Inc., of San Jose, Calif.

[0009] Programming a CLB to provide the logic functions of a FIFO memory system can be challenging in certain applications, particularly generating status flags in asynchronous applications. Incorrect user programming can result in unreliable operation of the FIFO memory system. Another aspect of implementing the logic in CLBs is that the operating speed of the CLB is typically less than the operating speed of the memory, which limits the maximum speed that the FIFO memory system can operate.

[0010] Therefore, it is desirable to provide a PLD with higher operating speed and more reliable operation.

[0011] The present invention enables a PLD with an embedded FIFO controller to operate a FIFO memory system on the PLD. The embedded FIFO controller operates in conjunction with an embedded BRAM to provide a FIFO memory system on the PLD. Embedded BRAMs can be either dual- or single-port memory arrays. In a particular embodiment, the embedded FIFO controller is configurable to provide a FIFO memory system having a selected width and/or status flag limits. For example, the FIFO controller is configurable to generate a status flag indicating that the FIFO buffer is almost full at a selected limit, such as when the FIFO buffer is ¾ths full. In another application, the FIFO controller is configured to generate a status flag indicating that the FIFO buffer is almost full at a different selected limit, such as when the FIFO buffer is ⅞ths full. In a further embodiment, a register is added to the PLD to allow dynamic setting of the status flag limits. Embedded FIFO logic is capable of operating at higher speeds than similar FIFO logic configured in the fabric of the PLD, and with greater reliability over clock domains (asynchronous operation) and over manufacturing variations.

[0018] FIG. 1A is a simplified diagram of a PLD 100 with a FIFO memory system 102 having an embedded FIFO controller 104 according to an embodiment of the present invention. The FIFO controller 104 is embedded in the PLD 100, typically next to an embedded BRAM 124 (hereinafter “BRAM”). Data is provided to the BRAM 124 over data input line DIN and is clocked by the write clock signal on line WR_CLK. The write clock signal is provided to optional input registers 108, 110 and to the write counter 112, all of which are embedded in the PLD 100, typically next to the embedded BRAM array 124. In some embodiments these optional registers 108 and 110 may or may not be present. In one embodiment, the write counter 112 does not go through a register since its output is already registered. However, the ADDR input does go through register 110 (i.e., ADDR input goes directly into register 110 and the multiplexer 120 is moved in order to select between the output of register 110 and the output of the write counter 112). Optional output registers 114, 116, read counter 118, and multiplexers 120, 122 are also embedded in the PLD 100, typically next to the embedded BRAM array 124. In some embodiments these optional registers 114 and 116 may or may not be present. In one embodiment, the read counter 112 does not go through a register. However, the ADDR input does go through register 116 (i.e., ADDR input goes directly into register 116 and the multiplexer 122 is moved in order to select between the output of register 116 and the output of the read counter 118). “Embedded” means that the logic of these components is hard-wired in silicon, as opposed to being configured in the fabric of the PLD. The multiplexers and registers, which are relatively simple compared to the FIFO controller, are alternatively configured in the fabric of the PLD.

[0019] The BRAM 124 operates as a FIFO memory array that provides the memory used in the FIFO memory system. The BRAM 124 operates in a manner consistent with conventional dual-port RAM circuits, and is therefore not described in additional detail. Alternatively, a single-port memory array is used, as shown in FIG. 1B. Similarly, the write counter 112, read counter 118, registers 108, 110, 114, 116 and multiplexers 120, 122 are implemented using known techniques. Like for FIG. 1A above, registers 108 and 110 are optional. In one embodiment the outputs of write counter 112 and read counter 118′ need not be go through register 110. In addition output register 114 is also optional. The multiplexers 120, 122 allow selecting between external addressing for non-FIFO operation and write counter 112 for operation of the BRAM in a FIFO memory system. In other words, the write address comes from outside of the FIFO memory system on ADDR when the BRAM 124 is being used as a conventional memory array, and on line 113 from the write counter 112 when the BRAM 124 is being used in a FIFO memory system. The multiplexer 122 operates in a similar fashion with the read counter 118.

[0020] Data is read from the BRAM 124 over line DOUT, which is clocked by the read clock signal on line RD_CLK. The FIFO flags FULL, EMPTY, ALMOST FULL and ALMOST EMPTY are provided on FIFO_FLAGS to other elements (not shown) of the PLD or of a digital system (not shown) in which the PLD is used. For example, a FIFO FULL flag is provided to a data source of a digital system to indicate whether data can be written to the FIFO memory system, and a FIFO EMPTY flag is provided to a data destination of the digital system to indicate whether data is available from the FIFO memory system.

[0021] Register 108 allows pipelining of the input data to improve the speed of the FIFO memory system when writing data to the BRAM 124 when enabled by the write WR. Register 114 holds read data from the BRAM array 124 and allows pipelining of the output data to improve the speed of the FIFO memory system when enabled by the read RD. In one embodiment, the WR and/or RD signals may each go through an optional register (not shown). One reason the WR signal may be registered is so that the WR signal is time-aligned with the DIN signal(s). The read and write clocks are synchronous, skewed, or asynchronous in alternative embodiments. Optional registers 110, 116 are used to register the write and read addresses. These registers may be bypassed in some applications.

[0022] The write counter 112 and the read counter 118 provide the write and read address to the multiplexers 120, 122, and support different FIFO depths so that the FIFO flag logic will function correctly as the depth of the FIFO is changed. The depth of the FIFO memory is dependent on the width of the data bus (i.e., DIN or DATA_IN) of the FIFO memory system, which is 36 bits in a particular FPGA, but is generally variable from 4, 9, 18, or 36 bits in width.

[0023] The embedded FIFO controller 104 accepts user commands at boot-up (power on) to configure the FIFO memory system to a selected width and/or depth. Generally, for a memory array of a given size, selecting the data width will determine the depth of the memory array. Similarly, a user can program the embedded FIFO controller 104 to set the FIFO ALMOST FULL and FIFO ALMOST EMPTY flags at a selected level. For example, the user selects the FIFO ALMOST FULL flag to be generated when the FIFO memory array is ⅞-full in one application, and when the FIFO memory array is ¾-full in another application. Adding an optional register(s) to the FIFO logic provides dynamic setting of the levels of the FIFO status flags and the width of the FIFO.

[0024] The embedded FIFO controller 104 is designed for use with the BRAM array 124 to provide a FIFO memory system capable of operating at higher speeds than similar FIFO memory systems where the FIFO controller is configured in the fabric of the PLD. Embedding the FIFO controller in the silicon, rather than in the fabric, is estimated to provide a 10% to 30% improvement in the FIFO memory system operating speed because of the faster clock rate (typically 300 MHz to 600 MHz) for an embedded FIFO controller. The clock rate for a FIFO controller configured in a CLB of the fabric of the PLD 100 is currently about 250 MHz to 275 MHz. The increased speed of FIFO memory systems with embedded logic is particularly desirable as the clock rates of the input and output data increase.

[0025] FIG. 1B is a simplified diagram of a PLD 100′ with a FIFO memory system 102′ having an embedded FIFO controller 104′ according to an embodiment of the present invention using an embedded single-port memory array 124′ (i.e. a single-port embedded BRAM). FIFO memory systems using single-port memory arrays are familiar to those in the art, and many features are similar to the FIFO memory system illustrated in FIG. 1A, above. FIFO logic, such as write counter 112′, read counter 118′, FIFO controller 104′, multiplexer 120′, and registers 110′, 108′, and 114′ are embedded in the PLD 100′. Alternatively, the multiplexer and registers are configured in the fabric of the PLD.

[0026] A single clock signal is provided on WR_CLK to register 108′ and to register 114′ (to clock data in and data out, respectively). The read counter 118′ is coupled to multiplexer 120′ to provide the address to the single-port BRAM 124′ after a read operation. Thus, the address may come from the write counter 112′, the read counter 118′, or outside of the FIFO memory system 102′ on line ADDR, if the single-port BRAM is to be operated as a conventional single-port memory array. The increased speed of the FIFO memory system 102′ with embedded logic 104′, 112′, 118′ is particularly desirable as the clock rates of the input and output data increase.

[0027] Embedding the FIFO controller and other FIFO logic in silicon of the PLD, rather than configuring it in the fabric, also provides more reliable operation. When configuring the FIFO logic for an asynchronous FIFO memory system in a CLB of a PLD, the flags are often in transition due to some of the logic in the CLB being clocked in each time domain. The logic design required to generate FIFO status flags that function correctly without causing metastability issues can be very difficult to implement in the fabric. Flags in transition are more likely to result in operational errors.

[0028] FIFO logic configured in the fabric of a PLD may have timing errors arising from where in the fabric the logic is implemented (i.e. logic operating along different paths in the fabric), or timing errors that arise due to manufacturing variability of the PLD. With embedded FIFO logic, the FIFO logic design is verified to insure that the status flags are reliably generated for all specified operating conditions (e.g. asymmetric clock rates) and manufacturing tolerances. Similarly, embedding the FIFO logic in the BRAM relieves the user from having to generate code to configure the CLB to provide FIFO logic functions.

[0029] Embedding FIFO logic in a BRAM provides a high-speed, reliable FIFO memory system that operates in a consistent fashion in a variety of applications and greatly simplifies usage of FIFO memory systems in PLDS. The design cost and time is reduced because a FIFO memory system is provided as an operational component of the PLD. Embedding the FIFO logic also frees CLB resources for other functions. The amount of silicon area required to embed FIFO logic into a BRAM is modest, and much less than the equivalent silicon area that would be used configuring similar FIFO logic into the fabric of the PLD.

[0030] FIG. 2A is a simplified diagram of a PLD 200 having FIFO memory systems 202, 202′ using a single BRAM 204 array. The FIFO logic functions are configured in the fabric (not separately shown) of the PLD, such as in one or more CLBs, or alternatively are embedded in the PLD. FIFO logic is embedded within BRAM in some embodiments and outside of BRAM in alternative embodiments. In other embodiments, a portion of the FIFO logic is embedded, and another portion of the FIFO logic is configured in the fabric. The BRAM 204 is operated as a single-port memory array, or alternatively as a dual-port memory array.

[0031] Conventional PLDS with FIFO memory systems allocate an entire BRAM for each FIFO memory system. Each FIFO memory system has a single write counter and a single read counter coupled to the BRAM that provide the write and read addresses to a BRAM. A single FIFO controller (see, e.g. FIG. 1, ref. num. 104) generates the status flags for the FIFO memory system. When a PLD is used in a digital system having many different time domains, such as in a design where data is being passed from one function to another, the user often needs several small asynchronous FIFOs between processing blocks. Often, a user might only need a number of 64×18 FIFO memory arrays, and has to choose between allocating an entire BRAM (and use only about 1/16^th of a typical memory block of a 1 K×18-bit memory array) or consuming 16 CLBs to implement the FIFO memory system in distributed RAM. Thus, substantial CLB and memory resources can be consumed, even for FIFO memory systems requiring relatively little memory.

[0032] The single BRAM 204 is split into two FIFO memory arrays 205, 205′. The first FIFO memory array 205 operates as a first independent dual-port RAM and the second FIFO memory array 205′ operates as a second independent dual-port RAM. In other words, the BRAM 204 has a second, independent, set of data input/output ports, compared to a conventional BRAM. Alternatively, a single BRAM is split into two single-port memory arrays for use in two, independent FIFO memory systems.

[0033] In a particular embodiment, the PLD 200 is an FPGA and the BRAM 204 is a dual-port BRAM with a maximum I/O data width of 72 bits on each port and 12 address lines. Embodiments where the I/O buses are split into two dual-port 36-bit memory arrays for use in independent FIFO memory systems do not require additional I/O lines. The widths of the data buses should be independent between each portion of the split BRAM (FIFO memory array). The split FIFO mode replicates the flag and address counters for each side of the split BRAM (i.e. each FIFO memory system) and can use available unused address lines because the write/read addresses are generated locally by the FIFO logic. BRAMs are further divided in alternative embodiments, for example by splitting a BRAM into 3, 4, or more FIFO memory arrays, if sufficient data lines and ports are available.

[0034] It was found that many PLD users believe the total number of data ports is as important as the total amount of available memory (expressed in bits). Partitioning the BRAM 204 in to halves 205, 205′ and adding an extra set of counters (e.g. Read Counter B 206′ and Write Counter B 208′) and control logic (see FIG. 2B, ref. num. 224′) improves access to memory without increasing the amount of total memory or number of BRAMs on a PLD. The number of potential FIFO memory arrays is doubled, although the size of each individual FIFO memory array using the split BRAM is halved. If a user requires a large FIFO memory system, the portions of the BRAM can be combined so that they appear to function as a single, larger, FIFO memory array. Using split BRAMS, FIFO memory systems can be cascaded to provide a FIFO system with increased depth, or run in parallel to provide a FIFO system with increased width.

[0035] FIG. 2B shows additional details regarding the logic and ports of embedded FIFO memory systems 202, 202, using a split BRAM array 228 in the PLD 200. FIFO logic, such as FIFO controllers 224, 224′ are embedded in the PLD 200. Alternatively, FIFO logic is configured in the fabric of the PLD 200. The BRAM array 228 is split into sub-arrays 230, 230′. There is no physical division of the BRAM array 228, and it is shown as being divided by a dashed line only for purposes of illustration and discussion.

[0036] Additional logic has been embedded in the PLD 200 (compare FIG. 1A), such as registers 232′, 234′, 236′, 238′, FIFO controller 224′, write counter 240′, read counter 242′, and multiplexers 244′, 246′. Similarly, additional ports, such as a second write clock port WR_CLK_B (WR_CLK_A is alternatively considered the “additional” port), a second read clock port RD_CLK_B (RD_CLK_A is alternatively considered the “additional” port), and a second FIFO flag port FIFO_FLAGS_B are added. Registers 234, 234′, 236, 236′ are optional, as discussed above in relation to ref. nums. 110, 116 of FIG. 1.

[0037] In some applications, it is desirable to have FIFO memory systems that support the full data width on each half of the BRAM. In a PLD having a dual-port BRAM with a 36-bit data width on each port (i.e. 72 bits for data in and 72 bits for data out), splitting a BRAM to operate as two FIFO buffers does not require additional I/O lines if each half of the BRAM supports a FIFO memory system with an 36-bit data width.

[0038] BRAMs often have unused I/O lines. In a particular embodiment using a Virtex II™ FPGA, available from Xilinx, Inc. of San Jose, Calif., 24 unused address lines provide additional I/O lines for operating the FIFO memory system. Typically, only three to four lines are needed for each set of FIFO status flag; thus, no additional I/O lines would need to be added to the PLD to operate both FIFO memory systems 202, 202′ at full (36-bit) width.

[0039] The FIFO memory systems 202, 202′ shown in FIG. 2B are alternatively operated as a single FIFO memory system. When operating as a single FIFO memory system, DIN_A, WR_A, DOUT_B and RD_B are not used. Similarly, Writer Counter A 240, multiplexer 244, Read Counter B 242′, multiplexer 246′, and FIFO Control B 224′ are not used. The unused ports and logic may be different in an alternative embodiment with suitable line routing.

[0040] FIG. 3A is a simplified flow chart of a method of operating a programmable logic device as a FIFO memory system 300 according to an embodiment of the present invention. The FIFO memory system is optionally configured to operate at a selected data width or depth (step 302) when the programmable logic device is booted up. A write enable signal is provided to embedded FIFO logic (step 304) and the embedded FIFO logic generates a FIFO write address (step 306). Write data is input to a BRAM (step 308) at the FIFO write address.

[0041] A read enable signal is provided to the embedded FIFO logic (step 310), and the embedded FIFO logic generates a FIFO read address (step 312). Read data at the FIFO read address is output from the BRAM (step 314).

[0042] The FIFO memory system tracks read and write addresses to generate FIFO status flags. If the BRAM (FIFO buffer) is full, almost full, empty, or almost empty, a FIFO status flag is generated (step 316) and output to the fabric portion of the programmable logic device. The FIFO ALMOST FULL and FIFO ALMOST EMPTY FIFO status flags are optionally set at boot-up (e.g. step 302). In a further embodiment the programmable logic device includes a register(s) to dynamically set the width of the FIFO data and/or the levels of the FIFO status flags.

[0043] FIG. 3B is a simplified flow chart of a method of operating a programmable logic device 320 according to another embodiment of the present invention. An embedded BRAM of the programmable logic device serves as the memories, or buffers, for two independent FIFO memory systems. The first independent FIFO logic controls a first FIFO memory system and a second independent FIFO logic controls a second FIFO memory system. The first and second FIFO logic controllers independently track read and write operations and independently generate memory addresses. Thus, the first and second FIFO logic controllers generate independent FIFO status flags.

[0044] In one example, first write data is written to a BRAM (step 322), and is counted (step 324) with a first write counter. First read data is read from the BRAM (step 326) and is counted (step 328) with a first read counter. Second write data is written to the BRAM (step 330), and is counted (step 332) with a second write counter. Second read data is read from the BRAM (step 334) and is counted (step 336) with a second read counter. In a further embodiment, a first set of FIFO status flags are generated with a first FIFO controller (step 337) and a second set of FIFO status flags are generated with a second FIFO controller (step 338).

[0045] In a further embodiment, one or both FIFO controllers are configured to set the data width/depth of one or both memory arrays, and/or the limits for FIFO ALMOST FULL and FIFO ALMOST EMPTY flags. In one embodiment, the FIFO controller(s) are configured at boot-up for FIFO data width/depth and/or the first and second programmable FIFO status flags, ALMOST FULL and ALMOST EMPTY are set at selected levels of the FIFO depth. In a further or alternative embodiment, the FIFO controller(s) are dynamically configured (i.e. after boot-up, while the FIFO memory system(s) are operating).

[0046] Further details on embodiments of the present invention are disclosed in concurrently filed, commonly assigned, U.S. patent application, entitled FIRST-IN, FIRST-OUT BUFFER SYSTEM IN AN INTEGRATED CIRCUIT, by Wayson Lowe, Eunice Hao, Tony Ngai, Peter H. Alfke, and Jennifer Wong, the patent application of which is hereby incorporated by reference in its entirety.

[0047] In the foregoing specification, the invention has been described with reference to specific exemplary embodiments thereof. Other embodiments will be apparent to those of ordinary skill in the art. For example, specific embodiments have been discussed with reference to FPGAs and CPLDs; however, embodiments of the invention might be implemented in other types of PLDs. Thus, it is evident that various modifications and changes may be made thereunto without departing from the broader spirit and scope of the invention as set forth in the appended claims and their full scope of equivalents.