Subject: RE: [IBIS-Users] Modeling input receivers with on-die termination
From: Todd Westerhoff (twesterh@cisco.com)
Date: Fri May 09 2003 - 08:05:30 PDT
Kim, et al,
I'm a little surprised that none of the tool vendors have checked in by this
point. I've had users tell me what the different tools do, but no word from
the tool-makers themselves.
It seems to me the rub is that s2ibis2 assumes the clamp curves will be
"added" one way (which is backed up, at least partially, by the IBIS spec,
as noted by Stephen) - and the simulators do something else by extrapolating
the power clamp curve. Simply put, s2ibis2 assumes that clamp current
should be zero if voltage point is not specified in the particular clamp
curve - the simulators do not.
There is a different algorithm the simulators could use to avoid double
counting with the existing models:
1. Check the input voltage
2. If the input voltage, <=VDDQ, look at the ground clamp curve first.
Otherwise, look at the power clamp curve first.
3. If the voltage point exists in the curve you're looking at, use the
corresponding entry for the current. Done. (this takes care out
double-counting and the overlap case)
4. If the voltage point does not exist in the first curve, check the second.
If the voltage point exists, take the corresponding current value. Done.
5. Only when the voltage point exists in neither curve (the gap case),
extend
each curve by extrapolation to find the current at the point in question.
This will provide two (potentially slightly different) current values -
use the average of the two.
If the simulators worked this way, we wouldn't have to change s2ibis2 at all
(or, so it seems to me). The proposal 1 technique from my original mail
would still work, but proposal 2 would need to be modified so that the
individual curves ended once they hit the zero current point.
Todd.
Todd Westerhoff
High Speed Design Specialist
Cisco Systems
1414 Massachusetts Ave - Boxboro, MA - 01719
email:twesterh@cisco.com
ph: 978-936-2149
============================================
"When did the choices get so hard, with so much more at stake?
Life gets mighty precious when there's less of it to waste"
- Bonnie Raitt, "Nick of Time"
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