Re: Subtracting clamp curves from pull-up/down curves

Bob,

I think there are two obvious reasons for keeping the clamps separate:

1. It was intended that models be usable for a range of supply voltages
with the pull-up and power clamps tracking with the supply voltage
and the pull-down and ground clamps tracking with the ground rail.
This was also the rational for referencing the former to the supply voltage.

2. Separating the clamps from the pull-up/down allows modeling of
high/low/tristate
for devices that support these modes.

Recent feedback has indicated that the characteristics of many devices change
markedly with supply voltage, so using an IBIS model for a supply voltage
other than at which it was designed, can give inaccurate results

Ian Dodd
Cadence Design Systems

"Haller, Robert" wrote:
>
> Mike,
> Not trying to play devil's advocate, but I have always wondered
> why the model maker has to subtract the clamps out, then the software vendor
>
> puts the clamps back in ? Why not just have syntax (in IBIS) for a pullup
> and pulldown model WITH the clamp ? Thus reducing everyone's work ?
>
> My old (internal) simulator and models were defined this way (IV curves
> included clamps)
> and it seemed a lot easier. We could feed bench data from a curve tracer (or
> semiconductor analyzer)
> directly into the model without any math, and when you looked model curves
> they were intuitive.
> I know its probably now water over the dam, but I was just wondering....
>
> regards,
> Bob
>
> Cereva Networks
> 100 Locke Drive
> Marlboro MA. 01752
> Phone: 508-486-9660 X 3365
> FAX: 508-486-9661
> Email: rhaller@cereva.com
>
> -----Original Message-----
> From: Mike LaBonte [mailto:mikelabonte@cadence.com]
> Sent: Friday, July 07, 2000 8:28 AM
> To: ibis-users@eda.org
> Subject: Re: Subtracting clamp curves from pull-up/down curves
>
> Stephen,
>
> Arpad's reply indicates that both clamp curves are indeed subtracted
> to produce pullup and pulldown curves. I can verify the other side of
> your question; that simulators (at least Cadence SPECCTRAQuest)
> re-combine the curves as follows:
>
> low state: pulldown + powerclamp + groundclamp
> high state: pullup + powerclamp + groundclamp
>
> So the clamps are never disabled (unless Submodel is used).
>
> Mike LaBonte
> Cadence
>
> Stephen Nolan wrote:
> >
> > Hello IBIS gurus,
> >
> > When creating a model for a three-state device, it is necessary to
> subtract the
> > clamp curves from the pull-up/down curves to avoid double counting, as the
> EDA
> > tool adds the clamp-curves back in to the pull-up/down curves to obtain
> the
> > devices output charactersitics in the enabled state. Right so far?
> >
> > The question is this, do you only subtract the corresponding clamp curve
> from
> > the related pull-up/down curve, or do you subtract BOTH clamp curves from
> each
> > pull-up/down curve?
> >
> > For example, if I am creating the pull-down curve, do I subtract only the
> > ground-clamp curve, or do I subtract both the power-clamp and ground-clamp
> > curves? Does the EDA tool add only the ground-clamp data back in, or does
> it add
> > both curves back in?
> >
> > --
> > Regards,
> > Stephen M. Nolan
Received on Fri Jul 7 08:10:29 2000