Subject: RE: confusion about c_comp

From: "Lorang, David D" <[email-masked]>
Date: Thu, 21 Dec 2000 07:28:44 -0800

This is a great thread!  I think many of us, both SI tool users and
simulator developers, have struggled with this Ccomp problem.  

It is possible to match up the V-T curves using a piecewise linear ramp
waveforms, various controlled voltage or current sources, and perhaps some
resistors, but no storage elements.  If the simulator produces its internal
model to match the waveform in this manner, then afterwards adds Ccomp to
the model, then the Ccomp has been double counted and the final model would
be in error.
  
What should happen is that the simulator should match the IBIS waveform
using all of the ramps, sources, resistors, but with Ccomp in place also.
Then, because Ccomp is already in place in the simulator's internal model,
it must not be added in again.  

I have seen this problem, when trying to correlate IBIS based models with
the transistor level simulations they were developed from.  You see the
problem when waveforms do not correlate unless the Ccomp is manually "zeroed
out" in the model.

I'm not sure we can fault the vendors too much because it is easy to get
wrong and the IBIS Specification standard does not spend a lot of words
explaining all of this.   Perhaps some more clarification in the IBIS
Specification standard is in order.  Further thoughts?

Best regards,
Dave Lorang 

-----Original Message-----
From: Ingraham, Andrew [mailto:[email-masked]]
Sent: Thursday, December 21, 2000 7:14 AM
To: [email-masked]
Subject: RE: confusion about c_comp

> > Any simulator that uses the V/T data, must be careful enough to use
> > it correctly!
> > ...
> 
> It is actually very difficult to implement it correctly.
 
Yes, I'm sure it is!

> C_comp is included in the V/T data, but it is not included in the 
> pullup/pulldown data.  This means that to be truly correct, some kind 
> of add and remove algorithm is needed,
 
?  The pullup/pulldown (I/V) data is DC data.  C_comp is irrelevant, it has
no effect on the pullup/pulldown data.  Maybe I misunderstood you.

Regards,
Andy

---

From: Christopher Reid <[email-masked]>
Date: Thu, 21 Dec 2000 09:02:16 -0800

Hello IBIS land,

I don't see this as confusing at all.  The VT curves are what the
simulator should produce under the load conditions specified in
the IBIS model for each curve.  The simulation vendor must devise
an algorithm to "make it so."

The C_comp must be in place during the actual simulation because
it will affect the results in different ways for different loading
conditions.

Chris Reid
Mentor Graphics