Re[2]: [SI-LIST] : Output Impedance

Arpad Muranyi ([email protected])
Thu, 02 Apr 98 08:15:00 PST

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I would like to remind you that the IBIS "model" is not rally a model, but a
data exchange vehicle to provide modeling information about devices. It depends
entirely on the tool how it is being utilized.

I mentioned this several times in our IBIS Open Foruum meetings, and let me
repeat it again. There are usually three I-V curves in an IBIS model, taken at
different supply voltages. If you think it through, you will realize that the
supply voltage really is the gate voltage of the driver transistor. Now, if
your simulator simulates GND bounce or Vcc droop, it should also have the
instanteneous supply voltage at every iteration. Using those values, your
simulator could also switch dynamically between these curves and anything
inbetween or even outside them (in other words scale them), depending on the
instantenous supply voltage.

I don't want to go into more detail here, the point I am trying to make is that
there is information in an IBIS model that could be used to model these effects.
It is entirely up to the tool vendor's algorithms. (I did implement this effect
in my HSPICE behaviorals and it works pretty well).

There is an issue on how accurately these effects can be modeled, but that is
another subject.

Arpad Muranyi
Intel corporation

I'm convinced IBIS works great if you have no rail collapse so that the IV
curves are valid. I don't think they are valid under heavy bus switching
with significant rail collapse, although I hear people claim otherwise. I
just don't see how IV curves provided at, say 3.3V, work when you have 0.5V
of collapse. Any comments?

Brian Young

Larry Smith wrote:

> ... I have compared the results of IBIS simulation to that of full
> HSPICE model simulation and hardware measurements for several different
> transmission line problems. IBIS model simulation compares very well to
> both.

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Subject: Re: [SI-LIST] : Output Impedance
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