FreeLists / si-list / [SI-LIST] Re: IBIS Driver Models, Simulators and CComp

["Todd Westerhoff" <twester@xxxxxxxxxxx>]

Hi all,

I received a number of replies privately, and thought I should summarize
some of what I learned for the group:

There is a numerical technique that allows an IBIS simulator to place the
Ccomp capacitor across the driver output (so the reflections are correct)
AND compensate for the fact that the V-T curves ALREADY have the effect of
Ccomp included.

When the driver is switching, for each time step, the simulator calculates
the amount of current that would flow into/out of Ccomp based on the desired
V-T value, and "adjusts" the current information from the IBIS model to
compensate.  That way the correct driving V-T behavior is produced, even
with Ccomp in the circuit, AND subsequent reflections off the driver are
correct.  Or - more appropriately - reflections off the driver are as
correct as they can be, given that Ccomp is the only parameter modeling the
die parasitics.

The scheme isn't perfect, but it gets very close to what HSpice predicts.

From various comments I've received, it looks at though Cadence
SPECCTRAQuest, Mentor ICX and the Avant! HSPICE IBIS implementation all use
this scheme.  This is based on user feedback and educated guesses on my
part - and doesn't necessarily reflect the official positions of the
vendors.  I've seen no commentary one way or the other about the remaining
SI tools; maybe we'll learn more in the days to come.

The remaining problem is with a bidirectional device where Ccomp is
mode-dependent - since IBIS only allows a single value to be specified.
Whether or not that actually occurs, and how often, I don't know.  We're
still working with our HSpice model in question to try and understand if
we're seeing mode-dependent capacitance or something else ...

Todd.

Todd Westerhoff
SI Engineer - Hammerhead Networks
5 Federal Street - Billerica, MA - 01821
email:twester@xxxxxxxxxxx - ph: 978-671-5084
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