Answers Database
Old part works on board, but the new Xilinx part does not! --Asynchronous design.
Record #1686
Problem Title:
Old part works on board, but the new Xilinx part does not! --Asynchronous design.
Problem Description:
The design has been working with a particular lot of Xilinx parts, but when a new lot of parts are u
sed (same part/speed), the design fails. What happened?
This usually indicates asynchronous design practices. For instance, at some point in the design, a c
ircuit might depend on a path being *slow*, to prevent a race condition. This may indeed work someti
mes, but you should realize that Xilinx parts are periodically fab'd on a faster process, so you can
not assume a particular parameter will not speed up later on.
Even if you have the exact same speed grade, one part may still
be significantly faster than another, since Xilinx guarantees
worst-case parameters only.
Examples of problematic design practices include gated clocks
(which introduces significant delay to the clock net), flip-flops with the asynchronous reset/preset
driven by internal logic (flip-flop's state could be reset before clock edge in one part, after clo
ck edge in another), combinatorial loops (potential race condition, if loop's state is clocked befor
e feedback is completed in one part, and after feedback has completed in another).
Solution 1:
You can run a DRC check to report on possible asynchronous
paths in your design by going into "xde" and selecting
Programs > DRC -Informational.
The real solution is to make the design fully synchronous,if possible. You can, of course, add more
delay to paths that need
it, but that is obviously an inferior solution since you will likely have to deal with this problem
again when the parts become faster.
End of Record #1686 - Last Modified: 04/08/97 13:05 |
