Re: Exact Timing Constraints vs. Over-Constraining

In article <[EMAIL PROTECTED]>, [EMAIL PROTECTED] 
says...
> In article <[EMAIL PROTECTED]>, Robert Sefton wrote:
> > "Larry Doolittle" <[EMAIL PROTECTED]> wrote in message
> >>
> >> Don't forget metastability slack.  In theory it does not apply to the
> >> purely synchronous nets; in practice I don't want to go through the
> >> work of separating them out, and it's a good excuse to add one more
> >> conservative assumption.
> > 
> > What is metastability slack and how do you apply it? Do you mean you
> > over-constrain your clock periods slightly to expand setup margins?
> 
> Yes and no.  Yes, I over-constrain my clock slightly (Peter Alfke's
> nominal number for modern chips and "typical" applications is 3 ns).
> The interpretation is to allow time after the clock edge for each
> flip-flop (that has an asynchronous input) to "choose" which state
> to land in.

Larry, your clocks must be pretty slow if you can afford to add 3 ns to every 
path in the design instead of just the few async boundary paths.  I would call 
that majorly, not slightly, over-constraining the design.

-- 
Rich Iachetta