Subject: RE: [IBIS-Users] C_comp value in IBIS model
From: Muranyi, Arpad (arpad.muranyi@intel.com)
Date: Fri Aug 01 2003 - 15:05:09 PDT
Kim,
I hate to argue with you, but your response is quite wrong.
It is not possible, or the least very hard to remove the effects
of die capacitance from a SPICE model. So the waveforms that
were generated from a SPICE simulation will always include the
effects of C_comp. Also, putting such a waveform and a C_comp
value into an IBIS model will not double count C_comp. This
is actually how IBIS simulators expect to get the data, because
they are the ones who do the compensation to avoid double counting.
So Giovanni did exactly the right thing.
The only thing he needs to watch out for is that if he does a
SPICE vs. IBIS correlation, he should have the value for C_comp
in the IBIS model that was measured on the SPICE model, and not
the data sheet number, because the two may not be the same value.
For correlation work. However, if the IBIS model needs to be
according to the data book, using the data book values is the right
thing to do. (Remember, the SPICE model may not have all the
layout related extra capacitances in the circuit, which is the
case most often when someone has a pre layout model).
Arpad Muranyi
Intel Corporation
=================================================================
-----Original Message-----
From: Kim Helliwell [mailto:kimgh@apple.com]
Sent: Friday, August 01, 2003 2:45 PM
To: Abril, Giovanni
Cc: ibis-users@eda.org
Subject: Re: [IBIS-Users] C_comp value in IBIS model
If the waveforms already include the effects of C_comp, putting
the C_comp value in the IBIS model will double-count the effect
of C_comp, which is probably not what you want.
I feel certain that the right thing to do is re-run the spice sims
with C_comp removed from the simulations, and put the correct
typ/min/max values of C_comp directly in the IBIS model
If I were your customer trying to use this model, I'd be really unhappy
to find that C_comp was (silently) being double counted. And I'd
be suspicious of your model if C_comp were 0 with no explanation.
At the very least, if you don't want to re-run things, you should set
C_comp to 0 but put in a comment that the reason it's zero is that
the effects of C_comp are included in the rising/falling waveforms.
But this is definitely a distant second-best, in my opinion.
Kim
On Friday, August 1, 2003, at 02:13 PM, Abril, Giovanni wrote:
Hello IBIS gurus,
I've created an IBIS model using a spice netlist from the vendor, and I'm wondering what to do with C_comp. My rising and falling waveforms include the effect of C_comp, but what should I put in the IBIS model? I have a spec sheet that says min C_comp is 2.8pF, max is 10pF. Currently I have 5.0pF for typ, 2.8pF min and 10pF max.
Thanks,
Giovanni
Kim Helliwell
Apple Computer
kimgh@apple.com
408 974 9936
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