First-in-first-out buffer

Abstract: A first-in-first-out buffer is described for transferring data between two asynchronous clock regimes. The buffer comprises a first register file, for transferring data from the slow to the fast clock regime, and a second register file for transferring data in the opposite direction. The write addresses for the first register file are produced by a counter in the slow clock regime, and converted back to standard binary, and a predetermined offset is added to the result, so as to produce a read address for the first register file. This is also used as the write address for the second register file. The read address for the second register file is produced by adding a predetermined offset to the output of the counter. Thus, the read and write addresses of the two register files are always in a fixed relationship, so that it is not necessary to provide special logic for detecting the buffer full and buffer empty conditions.

Description: Background to the Invention This invention relates to first-in-first-out (FIFO) buffers. In digital systems, it is sometimes required to transfer data between two asynchronous clock regimes; that is between first and second groups of logic where the first group is controlled by a first clock signal and the second group is controlled by a second clock signal which is faster than and not synchronised with the first clock signal. US Patent No. 4393482 describes an arrangement comprising a memory containing a plurality of data storage locations, a write counter synchronised to a clock signal, for producing a first address signal, the first address signal being used to select one of the locations for writing, and a logic circuit for deriving a second address signal from the first address signal, the second address signal being used to select one of the locations for reading. However, this referenced US patent does not deal with the problem of transferring data between two asynchronous clock regimes. The object of the present invention is to solve the problem of how to adapt such an arrangement for transferring data between two asynchronous clock regimes. Summary of the Invention According to the invention a first-in-first-out buffer comprises a memory containing a plurality of data storage locations, a write counter synchronised to a first clock signal, for producing a first address signal, the first address signal being for selecting one of the locations for writing, and a logic circuit for deriving a second address signal from the first address signal, the second address signal being for selecting one of the locations for reading, wherein said logic circuit comprises: (a) means for producing said second address signal by resynchronising said first address signal to a second clock signal which is faster than and not synchronised with said first clock signal; and (b) means for generating a data available signal indicating changes in said second address signal. Brief description of the drawings Figure 1 is a block diagram of a FIFO buffer. Figure 2 shows an address generation circuit in more detail. Description of an embodiment of the invention One FIFO buffer in accordance with the invention will now be described by way of example with reference to the accompanying drawings. Referring to Figure 1, the FIFO buffer provides a two-way interface between a slow clock regime 10 (with clock signal C1) and a fast clock regime 11 (clock signal C2). The boundary between the two clock regimes is indicated by the dotted line 12. The buffer comprises two register files: a slow-to-fast register file 15 and a fast-to-slow register file 16. Each of these register files contains eight individually addressable storage locations, each of which holds one word of data. Each register file has separate read address (RADD) and write address (WADD) inputs which allows data to be written and read simultaneously from different locations in the same register file. The write address input WADD of the slow-to-fast register file 15 received a 3-bit address signal WA -S from a counter 17. This counter is incremented at each beat of the slow clock signal C1, and produces a cyclic sequence of addresses, causing each location of the register file 15 to be addressed in turn in a cyclic sequence. Thus, successive input data words from the slow clock regime are written into a successive locations of the register file 15 in a fixed cyclic order. The output WA -S from the counter 17 is also fed to a logic circuit 18, which resynchronises this signal to the fast clock regime, to produce a read address signal RA-F. This signal RA-F is applied to the read address input RADD of the slow-to fast register file 15, so as to control the reading of data from this register file into the fast clock regime. The signal RA-F is also applied, as a write address signal WA-F, to the write address input WADD of the fast-to slow register file 16, so as to control writing of data from the fast clock regime into that register file. Thus, it can be seen that whenever one location of the register file 15 is addressed for reading, the correspondingly numbered location of the register file 16 is addressed for writing. The logic circuit 18 also produces a data available signal DAV which indicates that a data word is available from the output of the slow-to-fast register file 15. At the same time, the logic circuit 18 produces a data accept signal DACC which is applied to the write enable input WE of the fast-to-slow register file 16, causing a data word to be written from the fast clock regime into the currently addressed location of the register file 16. The signal WA -S from the counter 17 is also added to a predetermined offset value FS-OFFSET in an adder 19, to form a read address RA-S. This read address is fed to the read address input RADD of the fast-to-slow register file 16, so as to control the reading of data from this register file into the slow clock regime. Thus, it can be seen that reading from the register file 16 is performed in step with writing to the register file 15, at a location displaced by a predetermined offset. Referring now to Figure 2, this shows the logic circuit 18 in more detail. The input signal WA-S from the counter 17 is applied to a binary-to-Gray code conversion circuit 21, which converts the standard binary count sequence from the counter into a Gray code sequence. By a Gray code sequence is meant one in which, at each successive step, only one bit at a time changes. Thus, in the present example, the 3-bit count is converted into a 3-bit Gray code as follows: Count Gray Code 000 000 001 001 010 011 011 010 100 110 101 111 110 101 111 100 The output of the conversion circuit 21 is clocked into a register 22 by the fast clock signal C2. The output of this register 22 is in turn, clocked into a second register 23, at the next beat of the fast clock signal C2. These two registers provide a "de-dithering" effect; that is, they remove any uncertainty in the states of the signals which might arise if the input signal WA-S were to change at approximately the same time as the clock signal C2. The output of the second register 23 is fed to a Gray-to-binary code conversion circuit 24, which converts the signal back to standard binary form. The purpose of converting the address signal WA-S to Gray code at the point where it crosses the boundary between the two clock regimes is to avoid the possibility of corruption of the signal if the signal changes at approximately the same time as the clock C2. If the signal were in standard binary code at this point, a transition from, say, 001 to 010 at approximately the same time as the clock beat C2 might result in any of the following values being clocked into the register 22, depending on the tolerances of the circuits: 001 000 011 010 Of these, the first and last are acceptable, since they represent the values of the signal before and after the transition. However, the other two values are unacceptable, and represent a corruption of the data. On the other hand, when a Gray code is used, this corruption cannot occur. For example, a transition from 001 to 011 at approximately the same time as the clock beat C2 can lead to only one of the values 001 or 011 being clocked into the register 22, both of which are acceptable. The output of the code conversion circuit 24 is added to a predetermined offset value SF-OFFSET in an adder 25, to produce the address signal RA-F. It can be seen that the output of the circuit 24 (and hence the signal RA-F) is a conventional binary count sequence, except that some of the counts are repeated in successive beats of clock C2. This is because there are more fast clock beats (C2) than slow clock beats (C1). The occurrence of these repeated counts is detected as follows: The output of the circuit 24 is clocked into a register 26 by the clock C2. The outputs of the circuit 24 and the register 26 are then compared, in a comparator 27, which produces output signals DAV and DACC. In other words, the comparator 27 compares the current value with the previous value. If they are not equal, then both DAV and DACC are true; otherwise, they are false. As mentioned above, the signal DAV indicates that a new data word is available from the slow-to-fast register file 15, and it can be seen that this signal DAV goes true only when the read address RA-F changes. Similarly, the signal DACC indicates that a new word is to be written into the fast-to-slow register file 16, and it can be seen that this signal DACC goes true only when the write address WA-F changes. In summary, it can be seen that, in the buffer arrangement described above, the read and write addresses for both the register files are derived from the same counter, and hence are always in fixed predetermined relationship to each other. Hence, it is not necessary to provide any special logic for detecting buffer full and buffer empty conditions. The offset values FS-OFFSET and SF-OFFSET can be chosen so as to minimise the delays through the buffer. The optimum offset values will be determined by the ratio of the clock frequencies in the two regimes, and by the component tolerances. It has been found that the buffer can cope with wide variations in the clock frequency ratio. Moreover, the logic for addressing the register files does not have to be initialised: any illegal states occurring when power is first applied will disappear after an initial period. It will be appreciated that various modifications can be made to the system described above without departing from the scope of the present invention. For example, the register files could be addressed by a Gray code sequence, instead of by a conventional binary sequence. This would remove the need for a Gray-to-binary code conversion circuit 24, although it would introduce extra complication in the adder 25 for calculating the address signal RA-F, since this would have to perform addition of Gray-encoded values. Although the embodiment described above is a two-way buffer, it will be appreciated that the invention is also applicable to a one-way buffer arrangement, where data flows from a first clock regime to a second clock regime, but does not return in the opposite direction.

Claims: A first-in-first-out buffer comprising: a memory (15) containing a plurality of data storage locations, a write counter (17) synchronised to a first clock signal (C1), for producing a first address signal (WA-S), the first address signal being for selecting one of the locations for writing, and a logic circuit (18) for deriving a second address signal (RA-F) from the first address signal, the second address signal being for selecting one of the locations for reading, characterised in that said logic circuit comprises: (a) means (21-24) for producing said second address signal by resynchronising said first address signal to a second clock signal (C2) which is faster than and not synchronised with said first clock signal; and (b) means (26,27) for generating a data available signal (DAV) indicating changes in said second address signal (RA-F). A first-in-first-out buffer according to claim 1 wherein said memory (15) comprises a register file having separate write address and read address inputs (WADD, RADD) allowing data to be written and read simultaneously. A first-in-first-out buffer according to Claim 1 or 2 further characterised by: (a) a further memory (16) comprising a plurality of data storage locations; (b) means adapted such that said second address signal (RA-F) is for selecting one of the locations of said further memory for writing; and (c) means (19) for deriving a third address signal (RA-S) from said first address signal (WA-S), and further adapted such that said third address signal is for selecting one of the locations of said further memory for reading. A first-in-first-out buffer according to Claim 3 wherein said further memory (16) comprises a register file having separate write address and read address inputs (WADD, RADD) allowing data to be written and read simultaneously. A first-in-first-out buffer according to Claim 3 or 4 wherein said means (19) for deriving the third address signal (RA-S) from said first address signal (WA-S) comprises an adder for adding a predetermined offset value (FS-OFFSET) to said first address signal (WA-S) A first-in-first-out buffer according to any one of Claims 4 to 5 further including means for producing a write enable signal (DACC) for said further register file (16) whenever said data available signal (DAV) is true. A first-in-first-out buffer according to any preceding claim wherein said first address signal (WA-S) is in a conventional binary code, and wherein said means (21-24) for producing said second address signal comprises: (a) means (21) for encoding the first address signal into a Gray code; (b) means (22,23) for resynchronising said Gray code to said second clock signal (C2); and (c) means (24) for decoding the resynchronised Gray code back into a conventional binary code to produce said second address signal. A first-in-first-out buffer according to Claim 7 wherein said logic circuit (18) further comprises an adder (25) for adding a predetermined offset value (SF-OFFSET) to said second address signal. A first-in-first-out buffer according to Claim 7 or 8 wherein said means (26,27) for generating a data available signal (DAV) comprises means for comparing the value of said second address signal at each beat of said second clock signal (C2) with the value of said second address signal at the previous beat of said second clock signal (C2) and for producing said data available signal (DAV) when those values are different.

Inventors: FOX TREVOR ROBERT, FOX, TREVOR ROBERT
Applicants: INT COMPUTERS LTD, INTERNATIONAL COMPUTERS LIMITED
Application year: 1991
Main classification: G06F5
IPC codes: G06F5