Abstract:
An apparatus comprising a first memory and a second memory. The first memory may be configured read and write words from a data stream comprising a plurality of words in response to (i) a first read enable signal and (ii) a first write enable signal. The second memory may be configured to read and write words from the data stream in response to (i) a second read enable signal and (ii) a second write enable signal. The first and second memories may be configured to read and write alternate words of the data stream.

Description:
FIELD OF THE INVENTION 
     The present invention relates to FIFO memories generally and, more particularly, to flag generation and cascadability of memories at double data rate (DDR) frequencies. 
     BACKGROUND OF THE INVENTION 
     Flag generation and cascadability of DDR first-in first-out (FIFO) memories are factors limiting the maximum operating frequency. One conventional approach is to implement an external ping-pong logic and use two slower FIFOs. Referring to FIG. 1, an example of a circuit  10  illustrating a conventional FIFO is shown. Two or more conventional FIFOs  10   a - 10   n , when implemented in parallel, illustrate such an external ping-pong logic approach. The FIFO  10  has an input  12  that receives an input data signal DATA_IN &lt;36:0&gt; and an output  14  that presents an output data signal DATA_OUT &lt;36:0&gt;. The circuit  10  also has an input  16  that receives a read enable signal RENABLE and an input  18  that receives a write enable signal WENABLE. The circuit  10  also has an output  20  that presents an empty flag EF and an output  22  that presents a full flag FF. The inputs to the FIFOs  10   a - 10   n  need to be connected in parallel and the outputs ping-pong every cycle, which results in (i) frequency degradation and/or (ii) the requirement of more board space. 
     Referring to FIG. 2, a timing diagram illustrating the operation of the circuit of FIG. 1 is shown. A time Tf is shown as the time between a rising edge of the signal CLK and the enabling of the signal FLAG. Time Tens is equal to the difference between the time that the signal FLAG is enabled and the next rising edge of the signal CLK. As a result, the cycle time Tcycle is generally equal to the time Tf plus the time Tens. An example of a 100 Mhz signal CLK, the signal Tcycle is equal to 8 ns (e.g., Tf)+2 ns (e.g., Tens), for a total cycle time of lons. 
     Another conventional technique for implementing DDR FIFOs is to implement logic by using a slower and wider FIFO. Such an approach has the disadvantages of (i) being difficult to implement bus matching, (ii) having latency of at least two words, (iii) not implementing an odd word read out and/or (iv) consuming more board space. 
     Referring to FIG. 3, a circuit  30  illustrating a conventional width cascading architecture is shown. The circuit  30  has the disadvantage of being difficult to implement with bus matching. The circuit  30  alternately writes a first word of the signal DATA_IN to a FIFO  32  and a second word to a FIFO  34 . The circuit  30  also alternately reads a first word from the FIFO  32  and the FIFO  34 . The circuit  30  has the disadvantages of (i) being difficult to route the DATA_OUT on the board, (ii) having frequency of operation that is lower because of the loading on DATA_OUT and (iii) crowbar issues. 
     Referring to FIG. 4, an example of a circuit  70  implementing a conventional cascading (depth expansion) architecture is shown. The signal DATA_IN is presented through a FIFO  72 , a FIFO  74  and a FIFO  76 . The circuit  70  has the disadvantage of high latency through each of the FIFOs  72 ,  74  and  76 . 
     SUMMARY OF THE INVENTION 
     One aspect of the present invention concerns an apparatus comprising a first memory and a second memory. The first memory may be configured read and write words from a data stream comprising a plurality of words in response to (i) a first read enable signal and (ii) a first write enable signal. The second memory may be configured to read and write words from the data stream in response to (i) a second read enable signal and (ii) a second write enable signal. The first and second memories may be configured to read and write alternate words of the data stream. 
     Another aspect of the present invention concerns an apparatus comprising a first memory, a second memory, a control circuit and a flag circuit. The first and second memories may each be configured to store data received from a first data input and present data to a first data output. The control circuit may be configured to control data stored in response to a write clock and control data presented in response to a read clock. The flag circuit may be configured to generate one or more composite flags in response to the first memory and the second memory. 
     The objects, features and advantages of the present invention include providing a memory architecture that may (i) double the operational clock frequency of existing FIFO technology by using internally slow divided clocks to drive internal odd and even FIFOs based on bus matching, (ii) improve performance that may be obtained by an automatic ping-pong of the data between internal odd and even FIFOs, which may essentially double the clock speed of the architecture, (iii) implement minimal logic that needs to operate at maximum frequency, (iv) allow at speed bus matching on a number of ports, (v) have two internal FIFOs working at half the frequency of an external clock, (vi) implement internal clock generation logic based on external bus matching mode, and/or (vii) be extended to multiple FIFOs with corresponding signals and flags to achieve an effective increase in the clock operating frequency. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     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: 
     FIG. 1 is a block diagram of a conventional FIFO memory; 
     FIG. 2 is a timing diagram of the circuit of FIG. 1; 
     FIG. 3 is a block diagram of a conventional architecture for cascading conventional FIFO memories; 
     FIG. 4 is a block diagram of a conventional architecture for cascading conventional FIFO memories; 
     FIG. 5 is a block diagram of a preferred embodiment of the present invention; 
     FIG. 6 is a timing diagram illustrating an operation of the circuit of FIG. 5; 
     FIG. 7 is a block diagram illustrating an operation of the circuit of FIG. 5; 
     FIG. 8 is a timing diagram illustrating an operation of the circuit of FIG. 5; 
     FIG. 9 is a timing diagram illustrating an operation of the circuit of FIG. 5; 
     FIG. 10 is a detailed block diagram of an alternate embodiment of the present invention; 
     FIG. 11 is a block diagram illustrating an operation of the circuit of FIG. 10; 
     FIGS.  12 ( a )- 12 ( b ) are timing diagrams illustrating an operation of the circuit of FIG. 10; and 
     FIGS.  13 ( a )- 13 ( b ) are timing diagrams illustrating an operation of the circuit of FIG.  10 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIG. 5, a block diagram of a circuit  100  is shown in accordance with a preferred embodiment of the present invention. The circuit  100  may be implemented as an architecture that may remove the flag logic from the critical path (e.g., the high speed data path). The circuit  100  may also allow convenient cascadability. 
     The circuit generally comprises a FIFO  102  and a FIFO  104 . The FIFO  102  may have an input  106  that may receive a data input signal (e.g., DATA_IN&lt;80:0&gt;). The FIFO  104  may have an input  108  that may receive the signal DATA_IN&lt;80:0&gt;. The FIFO  102  may have an output  110  that may present a portion of an output signal (e.g., DATA_OUT&lt;80:0&gt;). Similarly, the FIFO  104  may have an output  112  that may present another portion of the output signal DATA_OUT&lt;80:0&gt;. The signal DATA_IN&lt;80:0&gt; and the signal DATA_OUT&lt;80:0&gt; are shown as 80-bit signals. However, other bit-widths may be implemented accordingly to meet the design criteria of a particular implementation. The FIFO  102  and the FIFO  104  may be implemented on a single integrated circuit. 
     The circuit  100  may also comprise a logic circuit  120  and a logic circuit  122 . The logic circuit  120  may have an input  130  that may receive a read clock signal (e.g., RCLKB), an input  132  that may receive a read clock signal (e.g., RCLK), an input  134  that may receive a read enable signal (e.g., REN — 1) and an input  136  that may receive a read enable signal (e.g., REN — 2). The logic circuit  120  may have an output  137  that may present a first enabled read clock (e.g., ENR_CLK1) and an output  138  that may present a second enabled read clock (e.g., EN_CLK2). The read clock signal RCLK and the read clock signal RCLKB may be complementary clock signals. 
     The logic circuit  122  may have an input  140  that may receive a write clock signal (e.g., WCLKB), an input  142  that may receive a write clock signal (e.g., WCLK), an input  144  that may receive a first write enabl e signal (e.g., WEN — 1) and an input  146  that may receive a second write enable signal (e.g., WEN — 2). The logic circuit  122  may have an output  147  that may present a first enabled write clock signal (e.g., ENW_CLK1) and an output  148  that may present a second enabled write clock signal (e.g., ENW_CLK2). The write clock signal WCLK and the write clock signal WCLKB may be complementary signals. 
     The FIFO  102  may have an output  150  that may present an empty flag (e.g., EF — 1), an output  152  that may present a full flag (e.g., FF — 1), and an input  154  that may receive the signal ENW_CLK1. The FIFO  104  may have an output  160  that may present an empty flag (e.g., EF — 2), an output  162  that may present a full flag (e.g., FF — 2) and an input  164  that may receive a signal (e.g., ENW_CLK2). 
     The circuit  100  may be implemented with, in one example, two separate FIFOs  102  and  104 . The FIFOs  102  and  104  may each have distinct empty flags (e.g., EF — 1 and EF — 2), full flags (e.g., FF — 1 and FF — 2), read enable signals (e.g., REN — 1 and REN — 2) and write enable signals (e.g., WEN — 1 and WEN — 2) for each of the FIFOs  102  and  104 . In one example, the odd words may be written to and read from the FIFO  102  and the even words may written to and read from the FIFO  104 . By writing and reading the odd and even words to the separate FIFOs  102  and  104 , the words may be written/read on either edge of the reference clock (e.g., rising/falling). In general, the read enable signal REN — 1 and the write enable signal WEN — 1 are associated with the positive edges of the read clock signal RCLK and write clock signal WCLK while the signals REN — 2 and WEN — 2 are associated with the negative edges of the read clock signal RCLK and write clock signal WCLK. However, an opposite polarity may be implemented to meet the design criteria of a particular implementation. By associating particular enable signals with the positive and negative edges of the read clock signal RCLK or write clock signal WCLK, the circuit  100  may automatically ping-pong data between the two FIFOs. Additionally, the enable signals REN — 1, REN — 2, WEN — 1 and WEN — 2 may allow cascadability of the circuit  100 . 
     The empty flag EF1 and the full flag FF — 1 are generally associated with the odd FIFO (e.g., the FIFO  102 ) and the empty flag EF — 2 and the full flag FF — 2 are generally associated with the even FIFO (e.g., the FIFO  104 ). The empty flags EF — 1 and EF — 2 may be combined to form a single composite flag (not shown). The full flags FF — 1 and FF — 2 may be combined to form a composite flag. One way to combine the flags EF — 1 and EF — 2 or FF — 1 and FF — 2 may be with a logic gate, such as an AND gate. However, other gates may be implemented to meet the design criteria of a particular implementation. Additionally, other flags (e.g., almost full flags, almost empty flags, half full flags, etc.) may be implemented accordingly to meet the design criteria of a particular implementation. Examples of particular flag generation circuitry may be found in U.S. Pat. Nos. 5,712,992; 5,809,339; 5,627,797; 5,850,569 or 5,852,748, which are each incorporated by reference in their entirety. However, the present invention is not limited to such circuitry. 
     The logic circuit  120  may be used to generate the internal clock signal ENR_CLK2 from the read enable signals REN — 1 and REN — 2 and the read clock signals RCLK and RCLKB. Similarly, the logic circuit  122  may be used to generate the internal clock signal ENW_CLK2 in response to the write enable signals WEN — 1 and WEN — 2 and the write clock signals WCLK and WCLKB. 
     The circuit  100  may use two of each of the read enable signals REN — 1 and REN — 2, write enable signals WEN — 1 and WEN — 2, empty flags EF — 1 and EF — 2 and full flags FF — 1 and FF — 2 to attain a data rate that may be double the data rate of conventional FIFOs. The circuit  100  may allow a glue-less interconnect for depth expansion of FIFOs. The enable inputs (e.g., REN — 1, REN — 2, WEN — 1 and WEN — 2) may be associated with both edges of the FIFO clocks (e.g., RCLK, RCLKB, WCLK, and WCLKB). The empty/full flags (e.g., EF — 1, EF — 2, FF — 1, and FF — 2) may be associated with each of the internal odd and even FIFOs  102  and  104 , where the internal FIFOs  102  and  104  handle odd and even words, respectively. 
     Referring to FIG. 6, a timing diagram is shown illustrating an operation of flag circuits (not shown) of the circuit  100 . A time Tf1 is shown as a time between a positive edge  200  of the signal CLK (e.g., RCLK or WCLK) and a transition of the signal FLAG 1  (e.g., EF — 1 or FF — 1). A time Tf 2  is shown as a difference between a negative transition  202  of the signal CLK (e.g., RCLK or WCLK) and a transition of the signal FLAG 2  (e.g., EF — 2 or FF — 2). A time Tens1 represents the time between the transition of the signal FLAG 1  and the next transition (e.g.,  204 ) of the signal CLK. Similarly, the time Tens2 represents a time between the transition of the signal FLAG 2  and the next transition (e.g.,  206 ) of the signal CLK. Since both flags are generated, the total cycle time Tcycle is generally equal to the time Tf1 plus the time Tens1 divided by 2. Similarly, the total cycle time Tcycle may also be characterized as the time Tf2 plus the time Tens2 divided by 2. 
     Referring to FIG. 7, a block diagram illustrating data flow of the circuit  100  is shown. The circuit  100  may illustrate a particular flow of data (a cascading configuration (e.g., depth expansion) of the FIFO  102  and the FIFO  104  through a FIFO  180 ). Data may be read from the FIFO  104  in response to the signals REN — 1, RCLKB, REN — 2 and RCLK. Data may be written to the FIFO  102  in response to the signals WEN — 1, WCLKB, WEN — 2 and WCLK. The flow of data of the circuit  100  may be controlled by the FIFO  180 . 
     Referring to FIG. 8, a timing diagram illustrating an operation of the present invention is shown. The timing diagram illustrates a writing operation of the circuit  100 . The write enable signal WEN — 1 may allow data to be written to either an even or odd FIFO (e.g.,  102  or  104 ). The write enable signal WEN — 2 may allow data to be written to either an even or an odd FIFO (e.g.,  102  or  104 ). The write clock WCLK may allow data to be written to either the odd or the even FIFO (e.g.,  102  or  104 ). The write clock WCLK may cause data to switch from a present enabled writing FIFO (e.g., first even or odd) to a next enabled writing FIFO (e.g., remaining even or odd), if applicable. The write enable signal, WEN — 1 and WEN — 2 may be responsive to the write clock signal WCLK, which may allow ping-pong of data to be written. The even and/or odd FIFO, when full, may change a logic state of the full flag FF — 1 and/or FF — 2, respectively. 
     Referring to FIG. 9, a timing diagram illustrating the operation of the present invention is shown. The timing diagram illustrates a reading operation of the circuit  100 . The read enable signal REN — 1 may allow data to be read from, for example, the odd FIFO. The read enable signal REN — 2 may allow data to be read from, for example, the even FIFO. The read clock signal RCLK may allow data to be read from either the odd or the even FIFO. The read clock signal RCLK may cause data to switch from a present enabled read FIFO (e.g., even or odd) to a next enabled read FIFO (e.g., remaining even or odd), if applicable. The even and/or odd FIFO, when full, may change a logic state of the empty flag EF — 1 and/or EF — 2, respectively. 
     Referring to FIG. 10, an alternate embodiment of the circuit  100  is shown marked with primed notation. The circuit  100 ′ may be similar to the circuit  100 . The circuit  100  may comprise a number of registers  300   a - 300   n , a number of registers  302   a - 302   n , a number of latches  304   a - 304   n  and a number of clock generators  306   a - 306   n . The registers  300   a - 300   n  may be used to store the data in DATA_IN&lt;80:0&gt; and the data out DATA_OUT&lt;80:0&gt;. The clock generators  306   a - 306   n  may be implemented to generate read and write enable signals ENR_CLK and ENW_CLK, respectively. The registers  302   a - 302   n  may be implemented to store the enable signals. The latches  304   a - 304   n  may be implemented to latch the data in DATA_IN&lt;80:0&gt; and the data out DATA_OUT&lt;80:0&gt;. 
     The circuit  100 ′ may double a clock speed of the FIFOs  102 ′ and  104 ′. The circuit  100 ′ may maintain backward pin and functional capability to existing FIFOs. The circuit  100 ′ may allow at speed bus matching on both the input and output ports of the FIFOs  102 ′ and  104 ′. The circuit  100 ′ may provide two internal FIFOs ( 102 ′ and  104 ′) operating at half an external frequency. The circuit  100 ′ may provide an internal clock generation logic based on external bus matching modes. Additionally, the circuit  100 ′ may allow multiple FIFOs with corresponding signals and flags to achieve  2   n  speed-up in the clock operating frequency. 
     Referring to FIG. 11, a simplified block diagram of the circuit  100 ′ is shown. The circuit  100 ′ may illustrate a cascading configuration (e.g., depth expansion) of the FIFO  102 ′ and the FIFO  104 ′. Data may be written to the FIFO  102 ′ in response to the signals WEN — 1, WEN — 2, WCLK, and WCLKB. Data may be read from the FIFO  102 ′ in response to the signals REN — 1, REN — 2, RCLK, and RCLKB. Reading and writing of data from/to the FIFO  104 ′ may be implemented similarly. Each of the FIFOs  102 ′ and  104 ′ may include read and write logic (not shown). The FIFOs  102 ′ and  104 ′ may read and write without additional external logic. 
     FIGS.  12 ( a )- 12 ( b ) illustrate writing to the FIFOs with and without glue logic. The signal BMW_CLK — 1 may have a number of positive transitions  402   a - 402   n  that responds to every second positive transition  400   a - 400   n  of the signal ENW_CLK. The signal BMW_CLK — 2 may also have a number of positive transitions  404   a - 404   n  that may each respond to an alternate number of positive transitions of the signal ENW_CLK. As a result, the circuit  100  (or  100 ′) may write data at twice the speed of conventional memories. 
     In FIG.  12 ( b ), an implementation of the signals BMW_CLK — 1 and BMW_CLK — 2 illustrates an example where a number of positive transitions  402   a ′- 402   b ′ are shown on every second positive transition  400   a ′- 400   n ′ of the signal ENW_CLK. The signal BMW_CLK — 2′ may have alternate positive transition on every fourth positive transition  401   b ′ of the signal ENW_CLK. The writing in FIG. 12 a  generally occurs twice as often as the writing in FIG. 12 b.    
     The signal BMW_CLK — 1 and the signal BMW_CLK — 2 may be implemented for bus matching of the circuit  100  (or  100 ′). In one example, the FIFOs  102  and  104  may be implemented in an 80-bit mode, the internal clock (ENW_CLK) may be divided by two. The divided internal clock ENW_CLK may allow the circuit  100  (or  100 ′) to achieve twice frequency of operation, since there are two FIFOs  102  and  104 . If the FIFOs  102  and/or  104  is written to in a 40-bit mode, the internal clock ENW_CLK may be divided once more. 
     If the circuit  100  (or  100 ′) writes data into a first 40 bits (½ of the 80 latches) then a next 40 bits may be written to a remaining 40 bits. The signal BMW_CLK — 1 may be generated to write data to the FIFO memory  102  and/or  104  (shown in FIGS.  12 ( a )-(b)). The signal BMW_CLK — 2 may operate similarly to the signal BMW_CLK — 1. The circuit  100  (or  100 ′) may write data to the FIFOs  102  and  104  in the 80-bit mode, even while in the 40-bit mode. 
     FIGS.  13 ( a )- 13 ( b ) illustrate reading from FIFOs with and without glue logic. The signal BMR_CLK — 1 has a number of positive transitions  502   a - 502   n  that generally occur on every second positive transition  500   a - 500   n  of the signal ENR_CLK. The signal BMR_CLK — 2 has a number of positive transitions  504   a - 504   n  that generally occur on every second positive transition  501   a - 501   n  of the signal ENR_CLK. The positive transitions of the signal BMR_CLK — 1 are generally offset from the positive transitions of the signal BMR_CLK — 2. 
     In FIG.  13 ( b ), the signal BMR_CLK — 1 is shown having a number of positive transitions  502   a ′- 502   n ′ that respond to every fourth positive transition  500   a ′- 500   n ′ of the signal ENR_CLK. The signal BMR_CLK — 2 generally has a number of positive transitions  504   a ′- 504   n ′ that generally respond to every fourth positive transition  501   a ′- 501   n ′ of the signal ENR_CLK. In general, the signal BMR_CLK — 1 has positive transitions when the signal BMR_CLK — 2 have negative transitions. The reading in FIG. 13 a  generally occurs twice as often as the reading in FIG. 13 b . The signal BMR_CLK — 1 and the signal BMR_CLK — 2 may be implemented for bus matching. 
     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.