The FPGA Programming Handbook_Second Edition by Frank Bruno | Guy Eschemann

Author:Frank Bruno | Guy Eschemann
Language: eng
Format: epub
Tags: COM041000 - COMPUTERS / Microprocessors, COM025000 - COMPUTERS / Expert Systems, COM067000 - COMPUTERS / Hardware / General
Publisher: Packt
Published: 2024-04-22T09:37:49+00:00

Using Gray coding for passing counter values

Binary counts increment by adding a 1 to the lowest bit. This results in a count such as the following:

00 – 01 – 10 – 11

Gray coding only allows one bit to change at a time, such as the following sequence:

00 – 01 – 11 – 10

Gray-coded counters have a limited range as they must always only have one-bit change at a time. A power of 2 is always safe to implement, but other combinations, such as 2n + 2m, also work.

Let’s look at a FIFO using a Gray code:

Figure 6.18: Asynchronous FIFO with Gray coding

By adding a Gray code module and synchronizers across each clock domain, we can compare the gray-coded values against each other or convert them back to binary on the destination clock.

Since Gray code only allows one bit to change at a time, we are guaranteed to either capture the old value or the new one and not capture a transitional value that would cause a FIFO empty, full, or word count error.

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