From: Todd Westerhoff (email@example.com)
Date: Mon Jan 22 2001 - 10:02:07 PST
Thanks to everyone who replied to this thread on Friday. Sorry I didn't the
chance to reply myself - the day just got away from me!
There were a couple of threads that I wanted to comment on:
The issue of timing, as discussed, seems pertinent only to common-clock
transfer schemes - where Tco is valid and is an important of the timing
equation. The concept of timing changes significantly when you talk about
source synchronous transfer (or, so I think) - and you end up concentrating
on skew between receiver signals as opposed to flight time.
As an aside, we looked at various static timing tools for source-sync
analysis, and concluded that because they treat each ssync bus as a clock
domain unto itself, they really don't buy you much of anything at the system
level (for ssync). Anyone have a different idea?
I expected the issue of the "accuracy of IBIS" to come up - I would have
been surprised if it hadn't. And, as was pointed out - IBIS is just a spec,
and it's up to the tools as to what they do with it. So, we need to concern
ourselves with both the robustness of the spec, and the quality of the
... and, we got a couple of good methodology comments. Jon Dowling's
comments concur with my own experiences - the buffer designers are happiest
if you can give them targets to shoot at without burying them in detail.
But, my original question still remains ...
... what's the *difference* ?
I don't mean the difference in an theoretical sense, listing effects that
SPICE models and IBIS does not (and yes, behavioral vs. silicon level
analysis would probably be a better description). I mean - the difference
on a *practical* level. And yes, I realize it's an impossible question.
We've run several sets of parallel analyses in HSpice and SPECCTRAQuest for
circuits running at 200 MHz+, and most of the time, you can take the two
sets of waveform data and they lay right on top of one another. So the
point is, if I can run two different simulators and get almost the same
result, when do I run which one?
Here's one for discussion - receiver modeling and noise immunity. The IBIS
receiver model is simplistic, modeling the receiver as, effectively, a set
of voltage comparators and a capacitance. Real receivers have different
characteristics, and pass or filter pulses in a more complex manner. So -
SPICE analysis of the receiver circuit at some level is prudent if I'm
trying to understand critical die to die system timing.
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Billerica, MA 01821
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