High speed FIFO synchronous programmable full and empty flag generation

[0002] The present application may relate to co-pending application Ser. No. 09/895,305, filed Jun. 30, 2001, Ser. No. 09/895,306, filed Jun. 29, 2001, Ser. No. 09/957,587, filed Sep. 20, 2001 which are each hereby incorporated by reference in their entirety.

[0003] The present invention relates to a method and/or architecture for generating flags in a memory generally and, more particularly, to a method and/or architecture for generating high speed almost empty status flags in a first-in, first-out (FIFO) memory.

[0004] First-in first-out (FIFO) memories provide a temporary buffer (or storage area) between communication systems. A programmable empty flag of a FIFO can be used as an interrupt to warn the system when the FIFO buffer becomes almost empty, which then blocks further read operations. Almost empty flags are generated in real-time to prevent underflow of the buffer. Traditionally, read counters, write counters and an offset register are used to track the status of the FIFO.

[0005] Referring to FIG. 1, an almost empty flag generation circuit 10 implementing a 3-input adder is shown. The circuit 10 includes a write counter 12, a read counter 14, a program value (i.e., an offset register) 16, a 3-input carry look ahead adder/comparator 18, an adder glitch filter 20 and a programmable almost empty flag register 22. The circuit 10 receives a FIFO write clock WRCLK and a FIFO read clock RDCLK. The write counter 12 presents a value to the adder/comprator circuit 18 in response to the FIFO write clock WRCLK. The write counter 12 tracks the number of writes. The read counter 14 presents a value to the adder/comparator circuit 18 in response to the FIFO read clock RDCLK. The read counter 14 tracks the number of reads. The offset register 16 stores a user programmed offset value. The adder/comparator circuit 18 also receives the offset value from the register 16. The adder/comparator 18 presents an output to the programmable almost empty register 22 via the adder glitch filter 20. The glitch filter 20 degrades performance of the circuit 10 (i.e., the glitch filter 20 is slow). The programmable almost empty register 22 then presents the full status flag EMPTY. The almost empty status flag EMPTY is obtained by the 3-input adder 18 which is in the critical path. The 3-input adder is slow and restricts the operational speed of the circuit 10.

[0006] Conventional almost empty flags that use a 3-input adder define the almost empty flag as EMPTY=(WR−RD<(offset)). Such an approach may have one or more of the following disadvantages of (i) being slow, (ii) consuming large area, and/or (iii) having wide glitches generated by the adder which need filtering, thereby degrading overall performance.

[0007] The present invention concerns an apparatus comprising a flag generation circuit configured to generate an empty flag signal in response to (i) a read clock, (ii) a write clock and (iii) a look ahead bitwise comparison configured to detect when a write count signal minus a read count signal is equal to 1.

[0008] The objects, features and advantages of the present invention include providing a method and/or architecture for generating high speed almost empty status flags in a FIFO that may (i) use a comparator to generate look ahead signals used by the almost empty flag generation, (ii) use state machines (e.g., asynchronous state machines) to generate the almost empty flag, (iii) implement a user programmable offset directly into the read counter upon programming, (iv) implement a shadow register for storing the offset value, (v) achieve high speed operation (e.g., 266 MHz) and/or (vi) minimize logic hazards (e.g., glitches).

[0009] These and other objects, features and advantages of the present invention will be apparent from the following detailed description and the appended claims and drawings in which:

[0010] FIG. 1 is a block diagram of a conventional programmable almost empty flag generation circuit;

[0011] FIG. 2 is a block diagram of a preferred embodiment of the present invention; and

[0012] FIGS. 3(a-c) are exemplary implementations of the present invention.

[0013] Referring to FIG. 2, a block diagram of a circuit (or system) 100 is shown in accordance with a preferred embodiment of the present invention. The circuit 100 may be configured to generate high speed almost empty status flags in a FIFO (to be discussed further in connection with FIGS. 3(a-c)). The circuit 100 may be configured to provide high speed FIFO synchronous programmable almost empty flag generation. The flag generation circuit 100 may comprise a comparator and an asynchronous state machine. Therefore, the flag generation circuit 100 may remove the traditional adder and filtering (as illustrated in the background) from the critical timing path.

[0014] The circuit 100 generally comprises a read load/counter block (or circuit) 102, a write counter block (or circuit) 104, a comparator block (or circuit) 106, a state machine 108, a state machine 110, a synchronization block (or circuit) 112 and a latch 114. The circuit 100 may be configured to receive a. FIFO read clock (e.g., RDCLK) and a FIFO write clock (e.g., WRCLK). The circuit 100 may also generate an output (e.g., EMPTY). The output EMPTY may be implemented as a programmable almost empty status flag.

[0015] The read load/counter circuit 102 may have an input 120 that may receive the read. clock RDCLK. The read load/counter circuit 102 may be preloaded with an offset value. The offset value may be programmable. The read load/counter 102 may then be incremented from the offset value. Programming the user defined offset value may allow the circuit 100 to monitor the programmable almost empty status flag EMPTY. For example, when the number of words in the FIFO drops below a predetermined threshold, the circuit 100 may pause further reads to prevent underflow. The read load/counter circuit 102 may present a signal (e.g., RC) to an input 122 of the comparator 106. The signal RC may be a read count signal (or pointer).

[0016] The write counter 104 may have an input 123 that may receive the signal WRCLK. The write counter 104 may be preloaded with a digital value (e.g., FFFF). The write counter 140 may be configured to count in response to the signal WRCLK. The write counter 104 may also present a signal (e.g., WC) to the input 122 of the comparator 106. The signal WC may be a write count signal (or pointer). The comparator 106 may be configured as a look ahead bitwise comparator. The comparator 106 may be implemented as a fast XNOR comparator. The comparator 106 may be configured to compare the signals WC and RC (e.g., WC−RC==1). The comparator 106 may then generate a signal (e.g., EQ). The signal EQ may be configured as a look ahead signal. The look ahead signal EQ may be used to generate the almost empty status flag EMPTY.

[0017] The state machine 108 may be configured as a programmable almost empty reset state machine. The state machine 108 may have an input 124 that may receive the signal WRCLK and an input 126 that may receive the signal EQ. The state machine 108 may be configured to generate a signal (e.g., RESET) in response to the signal RDCLK and the signal EQ. The signal RESET may be presented to an input 128 of the synchronization block 112. The synchronization block 112 may also have an input 130 that may receive the signal RDCLK. The synchronization block 112 may be configured to synchronize the signal RESET to the read clock RDCLK.

[0018] The synchronization block 112 generally comprises an SR latch followed by a register clocked by the write clock RDCLK (both of which are not shown). The SR latch of the synchronization block 112 may be reset after the register of the synchronization block 112 has been clocked high. The sync block 112 may present a signal (e.g., RESET′) to an input 132 of the latch 114. The signal RESET′ may be presented to a “set” (e.g., input S) of the latch 114.

[0019] The state machine 110 may be configured as a programmable almost empty flag set state machine. The state machine 110 may have an input 134 that may receive the signal EQ and an input 136 that may receive the signal RDCLK. The state machine 110 may generate a signal (e.g., SET) that may be presented to an input 138 of the latch 114. The signal SET may be presented to a “reset” (e.g., input R) of the latch 114. The latch 114 may be configured to generate the almost empty signal EMPTY in response to the signals RESET′ and SET. The latch 114 may be configured as an SR latch. However, the latch 114 may be configured as another appropriate type device in order to meet the design criteria of a particular implementation. Examples of the state machines 108 and 110 may be found in co-pending application Ser. No. 09/895,305, filed Jun. 30, 2001, Ser. No. 09/895,306, filed Jun. 29, 2001, U.S. Pat. Nos. 5,712,992; 5,809,339; 5,627,797; 5,850,568 and/or U.S. Pat. No. 5,852,748, each of which is incorporated by reference in its entirety.

[0020] The circuit 100 may be implemented to control a status of a FIFO. As the FIFO reaches almost empty (e.g., at a next read cycle the FIFO will be empty), the set state machine 110 may generate the pulse SET at a next rising edge of the read clock RDCLK. The pulse SET generally resets the SR latch 114 to generate an active LOW on the programmable empty flag EMPTY. The active LOW state of the signal EMPTY may indicate an inactive state of the FIFO.

[0021] Similarly, when the FIFO is not almost empty (e.g., the FIFO has more than offset+1 spaces available) the reset state machine 108 may generate the pulse RESET at a next rising edge of the write clock WRCLK. The write clock domain pulse RESET may then be synchronized by the read clock RDCLK to generate the signal RESET′, which then sets the SR latch 114 to generate an active HIGH on the programmable almost empty flag EMPTY. The active HIGH state of the signal EMPTY may indicate an active state of the FIFO.

[0022] The reset state machine 108 may be inhibited (or blocked) when the flag EMPTY is HIGH. The set state machine 110 may be inhibited (or blocked) when the flag EMPTY is LOW. Such a configuration may ensure that the SR-latch 114 may never receive simultaneous RESET′ and SET pulses that may lead into an illegal state for the SR latch 114.

[0023] Both the set and reset state machines 108 and 110 may enhance typical empty/full flag state machines. However, typical empty/full flag state machines may need additional logic to allow internal synchronous retransmit functions. For example, an addition SR-latch may need to be attached to the set path (e.g., RESET) to remember if the FIFO has ever gone from almost empty to not almost empty after a master reset cycle. Upon retransmit, if the signal RESET is active, the state machine 110 may trigger the pulse SET to recover an almost not empty status (e.g., a logic LOW) of the programmable almost empty flag EMPTY, if the FIFO is almost empty. The retransmit action may also trigger the read counter 102 to reload the user programmed offset value from a shadow offset register (not shown) that may be configured to store the value during programmable cycles (e.g., either preload, parallel or serial programming).

[0024] Referring to FIGS. 3(a-c), block diagrams illustrating an exemplary operation of the circuit 100 within a number of FIFOs 200a-200cis shown. Each of the FIFOs 200a-200nmay receive a write pointer (e.g., WR) and a read pointer (e.g., RD). The FIFO 200aof FIG. 3amay be empty. The almost empty flag EMPTY may be active LOW, indicating that the FIFO 200ais disabled. The FIFO 200bof FIG. 3bmay be almost full. The almost empty flag EMPTY may be active HIGH, indicating that the FIFO 200bis enabled. The FIFO 200cof FIG. 3cmay be almost empty. The FIFO 200cmay determine almost empty status in response to the write pointer WR, the read pointer RD and an offset value OFFSET. The state of the almost empty status flag EMPTY is determined as follows:

[0025] EMPTY=(WR−RD)<=OFFSET

[0026] The almost empty flag EMPTY of the FIFO 200cmay be a logic LOW, indicating that the FIFO 200cis disabled. The circuit 100 may allow the FIFOs 200a-200cto operate at a high speed (e.g., 266 Mhz, 3.76 ns cycle). The asynchronous state machines 108 and 110 may allow the circuit 100 to operate at high speeds.

[0027] The circuit 100 may implement the comparator 106 to generate the look ahead signal EQ needed by the almost empty flag generation circuitry (e.g., the state machines 108 and 110). The comparator 106 may be implemented as a fast XNOR comparator. The circuit 100 may implement the asynchronous state machines 108 and 110 to generate the almost empty status flag EMPTY. The circuit 100 may preload the user programmable offset directly into the read load/counter 102 upon programming. The circuit 100 may also implement a shadow register (not shown) to store the offset value, such that the value may be re-loaded back into the read load/counter 102 upon retransmit and partial reset operations. The circuit 100 may allow high speed almost empty flag generation in order to achieve high speed operation. Additionally, since the circuit 100 does not implement an adder, logic hazards (e.g., glitches) may be minimized.

[0028] The various signals of the present invention are generally “on” (e.g., a digital HIGH, or 1) or “off” (e.g., a digital LOW, or 0). However, the particular polarities of the on (e.g., asserted) and off (e.g., de-asserted) states of the signals may be adjusted (e.g., reversed) accordingly to meet the design criteria of a particular implementation. Additionally, inverters may be added to change a particular polarity of the signals.

[0029] While the invention has been particularly shown and described with reference to the preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made without departing from the spirit and scope of the invention.