altera cyclone and fifo synchronisation

clock

receiving

GL,

Your design sounds OK to me, but it's hard to be sure without seeing it. Using FIFOs to transfer data between asynchronous clock domains is of course standard usage, so the Quartus megawizard FIFOs will work fine for that.

I suspect we have an overzealous warning in the Design Assistant. The warning you're getting is that when you transfer a data word (that is wider than 1 bit), you have to be careful that all the bits arrive in the same cycle in the receiving clock domain. A FIFO will take care of that.

If you send me your design, I'll open a software problem report on it, get someone to check that the design analyzer isn't being too overcautious, and if it is we'll fix this case.

Regards,

Vaughn Betz Altera v b e t z (at) altera.com [Remove spaces and add @ to reach me]

Hi Peter,

Hmmm... interesting... so you joined Zilog during the past week? ;-)

Ben

Hi GL,

I talked to one of our design experts, and he is almost certain he knows what you're seeing (to be 100% sure he'd have to see the design, but he's pretty confident he knows the issue).

The asynchronous FIFO has a path between two grey coded counters, with appropriate metastability registers. The design assistant does not know that the bank of registers involved in this transfer are grey-coded counters, so it is flagging what appears to it to be a valid warning about a multi-bit transfer across clock domains. The grey-coding makes this safe in this case though, so you can ignore the warning.

There is already a software problem report open on this, and the long-term solution is to make the design assistant aware of the higher-level functionality of the FIFO.

If you want to make this warning go away, you can put a timing cut between the two banks of registers (or between the two clock domains, but that is more dangerous since it will cut all timing paths between the clock domains). That will stop the design assistant from analyzing this transfer.

Regards,

Vaughn Altera v b e t z (at) altera.com [Remove spaces and add proper @ to reach me]

There was a designer named Gray who spelled his name with an A He showed us a trick which counter to pick. Thank heavens for dear Mr. Gray.

Happy Holidays! Peter Alfke