Re: Exact Timing Constraints vs. Over-Constraining

"PO Laprise" wrote:

> I have seen much conflicting advice w.r.t timing constraints on this
> newsgroup, and I was hoping that the proponents of both camps might
> make explicit their reasons so that others (meaning I ;) can benefit
> from their experience.


Sometimes you use over constraining to force the tools to produce a certain
layout or routing.  For example, a 100MHz design where you take a data path
and call for something like a 600ps MAXDELAY.  There are only a few ways to
accomplish this.  You are effectively saying that you want the tools to use
short routing lines as well as close packing of data path elements.

Another case might be --a bit of conjecture here-- a design where you have
not taken the time to fully constrain all paths.  This meaning that there
might be multi-cycle paths as well as flat out TIG's.  These paths and those
that truly must make timing will compete for routing/placement.  By over
constraining you might be able to ensure that design iterations (as the
project evolves) don't necesarily break the important paths.  Like I said,
this might be stretching things a bit, but I've definetly seen designs that
don't work when constrained according to exact period values suddenly work
very well when over-constrained just a tad.


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Martin Euredjian

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