RE: [IBIS] Question regarding V-t tables

Thanks Chris, Bob, and Andy for your responses. Let me give a simple
example for the reason I was asking this.

I made an IBIS model for a differential buffer. Two V-t curves were
generated for it, one rising and one falling waveform. One of them
was saved from the non-inverting pad, and the other from the inverting
pad. The input was stimulated with only one edge. The IBIS model
correlates very well on every other transition, but not so well on
the rest of the transitions, see:

http://www.eda.org/pub/ibis/summits/oct03/muranyi.pdf on page 34.

I didn't verify this yet, but I suspect that if I would have generated
two more waveform tables with a stimulus going the opposite way and used
those waveforms for the transitions that don't match so well, I would
have gotten all transitions match well.

In this particular case I could probably do it with regular IBIS
syntax if I made two ***different*** [Model]s with the two sets of
waveforms
(one for the "P" pad and the other for the "N" pad) and associated them
with the normal [Diff Pin] keyword.

However, there may be other, more complex situations when this may not
be possible. In those cases I cold envision a *-AMS solution which
still
uses regular IBIS data, but adds a waveform selector logic that is based
on a set of logic equations which can detect data patterns and select
the
corresponding V-t tables on the fly for a given transition or pattern
from
the otherwise normal IBIS [Model].

This was the reason for my question. How could I put multiple waveform
tables in a [Model] with the same Rfixture and Vfixture parameters, and
tell the simulation engine which one to use depending on the input
stimulus (rise or fall), or depending on the data pattern on the input
(0101010, or 00001111, etc)? Please note that the I-V and V-t curve
processing algorithm would be the same as before otherwise, i.e. basic
IBIS algorithms would still work. This is just like a [Model Selector]
like Bob suggested, except that it does it on the fly, and could be
controlled by more complicated data pattern definitions.

I know, this could be done completely in *-AMS without touching the
legacy portions of the IBIS spec, so I am not pushing for this too hard.
I just wanted to gather some opinions to see if it would be worth
writing
a BIRD on this, or forget it and do it strictly in *-AMS.

Any (new) feedback would be appreciated.

Thanks,

Arpad
========================================================================

-----Original Message-----
From: owner-ibis@eda.org [mailto:owner-ibis@eda.org] On Behalf Of Andrew
Ingraham
Sent: Wednesday, September 08, 2004 9:35 PM
To: ibis@eda.org
Subject: Re: [IBIS] Question regarding V-t tables

Arpad,

Rather than the question "what would the simulator do with it," I find
the more important question is, "can IBIS even model a device that has
data pattern dependent behavior?"

I think the answer currently is no.

So one wonders why someone would even attempt to develop an IBIS file
for such a device. I think it's simply not appropriate.

Wouldn't this be like trying to represent an input-hold circuit by just
having multiple values in the same V-I table with the same voltages?

Regards,
Andy

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Received on Wed Sep 8 23:05:21 2004