( ESNUG 467 Item 13 ) ------------------------------------------- [07/26/07]
From: Jay Pragasam <jpragasam=user domain=plxtech bot calm>
Subject: Why do PrimeTime-SI and VCS have 2 very different timing models?
Hi, John,
I have been wondering about the difference in timing results between VCS and
PrimeTime-SI for a while and I would like to know if some experts have a
good response.
These days pretty much every design that is sub 130 um gets analyzed using
OCV mode. So when PT-SI times a path for setup or hold, it tries to figure
out the variation in timing among the path elements and applies them to
find the worst case scenario. For setup checks, it makes the launch path
the slowest and the capture path the fastest to determine the worst possible
case and vice-versa for hold checks.
This data is captured accordingly in an SDF where every timing arc has a min
delay and a max delay for rise and fall. But when VCS reads this SDF, I am
told that it can read either max values or min values for a given arc, but
not both. This means that VCS' timing numbers would always be optimistic
compared to PT-SI. Isn't this a big discrepancy?
When I asked Synopsys, the AE said that he can file an enhancement request,
but I am surprised that this has not been raised before. Is the concept of
having two distinct delays for a timing arc impossible for VCS?
- Jay Pragasam
PLX Technology Sunnyvale, CA
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