[SI-LIST] : PCI clamp spec

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From: George Borkowicz ([email protected])
Date: Mon Jun 12 2000 - 07:02:20 PDT

Hi everybody.

Long before you and me and IBIS Intel (Arpad Muranyi I believe) came up
with the behavioral model of the 5V PCI spec. To these days the model and
the template are included as a sample in HSpice suite. Well, now that
HSpice supports IBIS (for better or worse) and other people here do not
use HSpice it seemed a good idea to recreate this effort in IBIS. And
so was done.

Now the issue. All flavors of PCI spec seem to specify one minimum
clamp limit roughly as a 15 ohm resistor offset by .625 V. This size
is supposed to fit all (weak and strong). It probably does so on the
short, slotted buses ("speedway" they were called) but not so on the
long stub-less runs which happen to be the reality of clusters of large
ASICs and multi-board systems("subway" was the name). If one applies
such a clamp with anything but the weakest driver one usually ends up
with the broken bus. To damp the reflections, systems with strong drivers
must rely on the clamps to do the job. So to be realistic, in the spec
I need to associate stronger clamps with strong (and typical) buffer.
Short of using an ideal diode does anyone has an idea what would
be a good resistance number here?

George Borkowicz

Nortel Networks

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