Subject: RE: [IBIS-Users] C_comp value in IBIS model
From: Beal, Weston (weston_beal@mentorg.com)
Date: Fri Aug 01 2003 - 16:05:45 PDT
Giovanni,
The best solution would be to use your favorite C_comp calculation algorithm to calculate the typical value. Then use the datasheet values for min and max. When I'm in a hurry or have a non-critical model I usually set the typical value to something a little bit below the average of the min and max values from the data sheet.
There have been a couple of presentations at IBIS summits about how to find values for C_comp. You can check the archives for those presentations.
Regards,
Weston
-----Original Message-----
From: Abril, Giovanni [mailto:giovanni.abril@ngc.com]
Sent: Friday, August 01, 2003 3:20 PM
To: Beal, Weston; kimgh@apple.com
Cc: ibis-users@eda.org
Subject: RE: [IBIS-Users] C_comp value in IBIS model
Thanks Weston.
Is there a way for me to vary the capacitance in accordance with the spec then? The way you explained it, I should have the same C_comp value for typ, min and max, an extracted value, right? I'll assume ICX does it this way, since you work for Mentor. If the measured value turns out to be say 6pF, how would I cover a spec of 2.5pF?
-----Original Message-----
From: Beal, Weston [mailto:weston_beal@mentorg.com]
Sent: Friday, August 01, 2003 3:16 PM
To: 'Kim Helliwell'; Abril, Giovanni
Cc: ibis-users@eda.org
Subject: RE: [IBIS-Users] C_comp value in IBIS model
The effect of C_comp is only double counted if you are using a substandard simulator. All the major simulators that I know of expect C_comp to be specified with the same (static approximation) value as what is included in the waveforms. Somewhere in the simulator this is reconciled. Usually it is done is such a way that the capacitive effect is calculated out of the waveforms and then the lumped capacitor is present at the pin during the transient simulation..
As Arpad noted, the capacitive effect is usually impossible to remove from the SPICE models used to derive IBIS data. It is part of the transistor models. Arpad and others have proposed methods of finding the best static value of the die capacitance to use for C_comp and this value needs to be there. If you put the value of 0 for C_comp then the simulator will try to back out the capacitance and put no capacitance at the pin. At first this seems OK because the waveforms still have all the capacitive effect. The problem appears when a reflected signal hits this pin or if this pin is hi-Z. There would be no capacitive loading and you would then have invalid simulations.
C_comp is your friend. Embrace it. ( and enhance it )
Regards,
Weston
-----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
408 974 9936
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