Re[2]: Rising Waveform Loading Effects

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Richard,

Believe it or not, the kinds of comparisons that you are longing for are/were
being done by vendors (at least by Intel). When we began to use IBIS, (or our
IBIS-like behavioral models), the first thing we wanted to know was how accurate
they are. If they would have not provided the accuracy, we wouldn't have gone
out to simulation vendors to see if they are willing to sign up to a new
standard, just to embarrass ourselves... Also, we use these in our own
simulations. Do you think we would fool ourselves with a bunch of bogus data
and spend millions of dollars in making chips based on that?

I wonder how familiar you are with the IBIS modeling technique, at all. You say
you would rather take I-V curves or SPICE models from the vendors than IBIS
models. Do you realize that the IBIS models you do not want to accept contain
the same I-V curves (and more) that you are willing to take?

Arpad Muranyi
Intel Corporation
===============================================================================

The current crop of IBIS models are a far cry from being accurate enough
to really predict the kinds of things Jon is talking about, but many of
them are good enough to ensure that I don't drastically mis-terminate
the transmission lines on my boards.

     One almost does not need any models for that, IBIS or otherwise.
     If a simulator will not provide 200 mV and 500 psec accuracy
     while simulating a situation complex enough to be non-intuitive
     then it's hardly worth the trouble.

Another question to think about: do Ibis models derived from Spice models
really have the kind of accuracy that Jon needs?

     I don't know. How do we know how accurate such IBIS models
     are? That uncertainty is why I'm not rushing to embrace IBIS
     as a substitute for SPICE or measurement-derived I-V curves
     in simulation. When a vendor is willing to make direct
     comparisions between simulations (using their own nominal
     SPICE models, measured I-V curves and IBIS models) and
     measurements of actual circuits, and demonstrate at least
     200 mV/500 psec accuracy, then I'll start to have some
     confidence. Such simulations must address realistic situations
     in multiple cases (heavy vs light loading, lumped vs
     transmission line environments), and not merely 50 pF - 500
     ohm test cases. Until then I'll continue to press vendors
     for SPICE models or I-V curves, or generate my own.

     Richard Schumacher

     I do not speak in any capacity for the Convex Division of the
     Hewlett Packard Company.

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Subject: Re: Rising Waveform Loading Effects
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From: schumach@flare.convex.com (Richard A. Schumacher)
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