Subject: RE: [IBIS-Users] Modeling input receivers with on-die termination
From: Todd Westerhoff (twesterh@cisco.com)
Date: Fri May 09 2003 - 13:34:25 PDT
Tom,
I don't disagree with the comment "fix the model, not the simulator",
although we should recognize that there are models and tools (e.g. s2ibis2)
out there already, and getting all of them fixed isn't necessarily simple.
Easy if we have people of your caliber doing the work - but we don't.
I was actually proposing the simulation algorithm to see what response it
drew. In theory, it solved both the problem of the existing models/tools,
and would work with corrected models - with the caveat about the curve
endings that I mentioned.
In actual fact, I am somewhat dubious about the wisdom of changing the
simulators, if no other reason than we'd go through a period where different
tools were doing different things, whereas they are (relatively) consistent
with regard to this issue at the moment. Trying to understand how the
different tools work is hard enough already; I wasn't really anxious to make
it more complicated.
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"
-----Original Message-----
From: owner-ibis-users@eda.org [mailto:owner-ibis-users@eda.org]On
Behalf Of Tom Dagostino
Sent: Friday, May 09, 2003 3:52 PM
To: 'Kim Helliwell'
Cc: 'Todd Westerhoff'; 'Ibis-Users (E-mail)'
Subject: RE: [IBIS-Users] Modeling input receivers with on-die
termination
Kim
My comment was directed towards having the simulator handle the data
differently:
> 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)
................
Getting the simulator companies to change their simulators would be harder
in my opinion than just fixing the model that is causing a user a problem.
What fix and how does it affect other models and does it cause additional
issues, etc. Fix the model, the problem, not the simulator.
Tom Dagostino
Teraspeed Consulting Group LLC Teraspeed Consulting Group LLC
2926 SE Yamhill St. Device Modeling Division
Portland, OR 97214 13610 SW Harness Lane
Beaverton, OR 97008
http://www.teraspeed.com 503-430-1065
tom@teraspeed.com
-----Original Message-----
From: Kim Helliwell [mailto:kimgh@apple.com]
Sent: Friday, May 09, 2003 11:15 AM
To: tom@teraspeed.com
Cc: 'Todd Westerhoff'; 'Ibis-Users (E-mail)'
Subject: Re: [IBIS-Users] Modeling input receivers with on-die
termination
Tom:
I completely agree with making the models correct.
Todd was answering my earlier question of what to
do when a vendor supplied model is NOT correct.
For that case, a band aid is needed, particularly when
the vendor is recalcitrant about fixing the model...
On Friday, May 9, 2003, at 10:19 AM, Tom Dagostino wrote:
> This sounds like a band aid, not a solution. Let's make the models
> correct
> and not double count the currents.
>
> Tom Dagostino
> Teraspeed Consulting Group LLC Teraspeed Consulting Group LLC
> 2926 SE Yamhill St. Device Modeling Division
> Portland, OR 97214 13610 SW Harness Lane
> Beaverton, OR 97008
> http://www.teraspeed.com 503-430-1065
> tom@teraspeed.com
>
>
> -----Original Message-----
> From: owner-ibis-users@server.eda.org
> [mailto:owner-ibis-users@server.eda.org]On Behalf Of Todd Westerhoff
> Sent: Friday, May 09, 2003 8:06 AM
> To: Ibis-Users (E-mail)
> Subject: RE: [IBIS-Users] Modeling input receivers with on-die
> termination
>
>
> 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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Kim Helliwell
Apple Computer
kimgh@apple.com
408 974 9936
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