0
\$\begingroup\$

I have an asynchronous FIFO (in a Stratix V FPGA) with two asynchronous read and write clocks of the same frequency of 100 MHz. As I understand, asynchronous FIFOs have a two-register resynchronisation stage.

Because the relative phases of the two clocks can wander up to 10ns, the jitter of the FIFO is at least 10ns. Am I correct? Is the jitter of an asynchronous FIFO at least one clock cycle? Is it more than one clock cycle?

\$\endgroup\$
1
\$\begingroup\$

Async FIFOs are characterised by their static latency, expressed as a number of complete cycles in the source domain (before the crossing) and in the destination domain. The crossing itself introduces jitter if there is not an exact phase and frequency relation between the two clocks. In your example you say the phase wanders, for this to happen the clock frequency must be slightly different. You are correct that this introduces 0-1 extra cycles on top of the static latency, depending on if the asynchronous grey-code counter signals land ahead of the setup window (0) or fall into the next cycle (+1).

Static values for the dcfifo megafunctions are in the Quartus Megafunction GUI and depend on the parameters you've set - otherwise you can measure see them in simulation. I think it's 1 cycles before, 2 cycles after, so 3 static in the show-ahead mode, so overall 30-40ns for your case.

\$\endgroup\$
0
\$\begingroup\$

There should be an answer to this question in the manufacturer's data sheet. I know it's a large document, but I suggest you dig through it. Also, jitter is not the right word to describe what you are talking about. Jitter refers to random variations in clock frequency on a single clock. What you are talking about is clock offset (which will vary based on the jitter of the two clocks, up to 10ns as you say). I will also say that I think what you are doing is exactly what that type of FIFO is designed for, so you are probably fine.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.