Subject: Non Monatomic waveform, practically, how important?

From: "Krull, Nick J" <[email-masked]>
Date: Tue, 10 Nov 1998 09:29:25 -0700

Has anybody run test cases against popular simulators to determine
how important/sensitive monotonicity is to:

1) Convergence
2) Accuracy

We are frequently running into models with non-monotomic features.  The
simulator
vendors recommend that models be monotomic and I believe the IBIS
spec demands it.  Screening these features out is time consuming and
testing the resultant model is even more time consuming.  IBIS simulators
generally do not have the knobs that SPICE simulators do.  Is there a 
mathematical reason for this, or is it more a customer issue?

Anybody have a few pearls of wisdom on this?  What is the history behind it?

Nick Krull
StorageTek
11/10/98

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From: Jon Powell <[email-masked]>
Date: Tue, 10 Nov 1998 09:23:25 -0800

The classic reason we argued against allowing non-monotonic IV curves
was because the non-monotonic behavior requires a feedback path (and
thus a delay) that we have never been able to properly define. If you
don't know the feedback path delay then you do not know the inherent
non-stable point for the driver. We have tried to work with IC companies
that have this problem (and I think we did get a few non-real examples
from some people) but the actual resolution of the feedback delay has
never been completely addressed.

I think that the current IBIS simulators take one of two approaches:
1) Allow the non-monotonic behavior and assume 0 delay.
2) Filter out the non-monotonic behavior automatically.

Which one is right? Depends on the driver and the feedback delay.

regards,
Jon Powell
Manager, High Speed Consulting Services, Viewlogic Systems.