Subject: RE: [IBIS-Users] C_comp value in IBIS model
From: Jeremy Plunkett (jeremy@serverworks.com)
Date: Fri Aug 01 2003 - 15:02:22 PDT
Kim,
actually a simulator which implements IBIS correctly will "cancel out" the
effects of C_comp on the output waveform. However the C_comp will be
present as part of the input loading that the buffer presents to the bus.
In practice there is probably a limit to how far you can tweak C_comp away
from the real value for most simulators. When I tried it in Hspice,
changing C_comp from the "correct" value to 0pF results in minimal changes
to the output waveform, but changing C_comp to 100x the correct value does
cause noticeable changes (but not what you might expect).
It's generally not possible to remove C_comp from the simulations when the
model is generated, since in most cases a large portion of C_comp is due to
the output transistors, and if you take those out of the netlist you won't
have much luck getting the IBIS data.
Jeremy
-----Original Message-----
From: owner-ibis-users@eda.org [mailto:owner-ibis-users@eda.org]On Behalf
Of Kim Helliwell
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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