( ESNUG 363 Item 9 ) --------------------------------------------- [01/25/01]
From: Ajay Kumar Sinha <ajay@siliconsystems.co.in>
Subject: PrimeTime 2000 Uses Dangerously Optimistic Models For SPF Timing?
Hi John
With the new 2000 versions of pt_shell, we recently moved to doing timing
with a SPF (our designs have been too large to attempt this before). Doing
this however has some implications. PrimeTime claims to calculate an
effective capacitance form the distributed RC_network in the SPF. For this
it uses some algorithm, which assumes that there's a montonically increasing
relationship between input transition/output capacitance with the cell delay
and this should be expressed in the cell delay table. If it is not so, it
says it found a negative driver resistance and failed to compute effective
cap in which case it will use the lumped capacitance. This is optimistic
for best case conditions (that is if we care!!)
This is a a pretty bad assumption (and we know it!! after doing a lot of
spicing!!)
I reported it to Synopsys AE. They first didn't believe me, then when I
gave them the library example, they wanted a test case which we couldn't
give them because the library is from Avanti. So we asked them to come on
site. They have been missing since then.
I just wanted to know if other people have encountered this in PrimeTime
2000 and does Synopsys acknowledge issue?
- Ajay Kumar Sinha
Silicon Systems India
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