RE: [IBIS-Users] Conditions for clamp and pullup/down tables

Arpad,

Thank you for your explanation.
It was not that I have a need for better modelling of the clamp. In fact, 
it was a customer who raised the question why the clamp curves were 
crossing. They seem to be satisfied with the answer I gave them. But it 
made me re-think the way I make IBIS models.

Kind Regards,

René Voshol









"Muranyi, Arpad" <Arpad_Muranyi@mentor.com> 
Sent by:
owner-ibis-users@server.eda.org
2008-08-08 07:09 PM

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Subject
RE: [IBIS-Users] Conditions for clamp and pullup/down tables






Rene,
 
You are raising very good questions, and your observation
is correct.  We were aware of this problem, but didn't
concern ourselves too much because this clamping diode
behavior is only effecting the overshooting signals.  In
general, designs with overshoot should not rely on the
clamping diodes of the devices for various reasons, and
because of that we shouldn't expect hair splitting accuracy
from the simulations based on clamping diode behavior.
 
In addition to that, the parasitic clamps of MOSFET models
in SPICE tools are quite often very inaccurate due to
inadequate modeling techniques which adds another reason
for not relying too much on the claming effects.
 
Also, more modern signaling schemes use terminations which
means that the signals will not get into the active region
of the clamping diodes in which case they can be neglected
in the models.
 
In summary, even though your observations are correct, we
didn't feel that this was an important deficiency in the
IBIS specification and models.  If you can show us an example
which would justify to make corrections to this situation
we could certainly address this issue.
 
Thanks,
 
Arpad
=============================================================

From: owner-ibis-users@server.eda.org 
[mailto:owner-ibis-users@server.eda.org] On Behalf Of Rene Voshol
Sent: Friday, August 08, 2008 7:25 AM
To: ibis-users@server.eda.org
Subject: [IBIS-Users] Conditions for clamp and pullup/down tables


Hi, 

I have a question about the conditions used to extract the data for the 
clamp and the pullup/down curves. I understand that the temperature range 
given in the model should correspond to the min. and max. conditions of 
the device. Meaning that for the pullup/down curve of a CMOS device  you 
will have maximum performance with low temperature, and vice versa. But 
the clamp of this same the device will most likely be a diode, of some 
sorts, which will have the reverse temperature behavior. Since both curves 
are substracted, you need to apply the same conditions when extracting the 
curves. In which case I use the pullup/down curve as reference. 

My questions are: 
1.      Is this correct thinking? (Or am I perhaps overthinking things 
here?) 
2.      What about an input model? This has only clamp curves. Is it OK to 
use a different temperature ranges for input and output of the same 
device? 
3.      Or a switch where the clamps and the series current are in 
separate models?

Clarifications on this matter will be highly appreciated. 

Kind Regards, 

René Voshol
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