Subject: [IBIS-Users] Re: Does a clock driver IBIS model need to incorporate steady states witching effects?
From: Ingraham, Andrew (a.ingraham@ieee.org)
Date: Thu Nov 06 2003 - 12:21:26 PST
In my opinion, at low to moderate frequencies, there isn't anything peculiar
to IBIS models that should make them particularly more or less accurate with
non-periodic waveforms than with periodic waveforms.
Someone could create an IBIS model that is tailored or optimized for one
particular frequency. This might be especially useful up in the GHz range
where a more "broadband" model might be somewhat less accurate, so its
author might have "tweaked" it for one target frequency.
There is a related issue about toggling IBIS models too fast when they have
V/T waveform tables. Switching the driver before its waveform table has
"played out" fully can cause erroneous results, depending on the simulator.
This would be true whether the waveform is periodic or not. To avoid that,
someone might make a modified IBIS model with truncated V/T tables for the
high toggle rate case.
Most of the contents of an IBIS model are found under steady-state or low
frequency conditions. The I/V tables are steady-state, and the V/T waveform
tables might be created with a single step or at low switching rates where
you can observe the full transient response until it settles. The
parasitics and tweaking might be done at higher frequencies.
At very high switching rates, where the device might be better characterized
as an amplifier than a logic gate (putting out a sine wave instead of a
rounded square wave), perhaps these measurements don't work well anymore and
model designers would need to tweak a lot more than the package parasitics
to get enough accuracy. That would depend on the device.
Regards,
Andy
> I have received some feedback recently that you would need
> a clock driver IBIS model to take into account the effects of
> steady state switching at some frequency. The feedback states
> that if you used the same silicon I/O cell for psuedo-random, or
> less frequent switching, then the IBIS model would need to be
> different to get the most accurate effect.
>
> Does anyone have any insight that could be helpful in explaining
> why this might be the case?
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