Subject: Serious s2ibis problem?

From: [email-masked] (Scott Schlachter)
Date: Tue, 5 Nov 96 16:25:12 PST

Hello IBIS folk:

We've been trying to figure out why s2ibis produces a Pull-down (and
Pull-up) curve that is VERY different from our self-produced
tables (from SPICE), in particular at voltages from Vcc to 2*Vcc.  
This led me to the FAQ under the /man directory of S2IBIS.  Here,
I found:

  QUESTION (6): "How does s2ibis generate tables?"

  ANSWER: 
	s2ibis uses the following scheme for determining what tables 
	need to be generated for a particular pin:
	0) For POWER, GND, and NC ...
	
	--snip--

 	3) For each I/O or 3-state pin...

	--snip again--

->	pulldown(-5->0):subtract the associated ground clamp table from 
		the pulldown table

->	pulldown(0->Vcc): just use the pull-down table as simulated

->	pulldown(Vcc->2*Vcc):extrapolate using the point at 
	Vcc and the point five points before Vcc in the pulldown table

A similar explanation is provided for pulldown table generation.
My questions are: WHY DO THEY DO OPT FOR EXTRAPOLATION WHEN THEY HAVE
		ALL THE DATA NESSESSARY IN THE SPICE OUTPUT FILES?
		WHY ISN'T THE TRI-STATE DATA SUBTRACTED FROM THE
		ENTIRE VOLTAGE RANGE?

After reviewing all of the files that are created (and left) by 
s2ibis, I found that it creates V/I tables (***from -Vcc to 2*Vcc***)
for an I/O in: 1)Tri-state (un-enabled) conditions, 2)Pull-up conditions,
and 3)Pull-down conditions.    

In searching through all of the IBIS documentation I could find, the best
explanation for how to produce proper Pull-down and Pull-up curves (for
either SPICE sim data or real data) was in the V2.1 Spec., under 
"Other Notes:" of the [Pulldown], [Pullup], [GND Clamp], [Power Clamp]
Keyword section:

... The clamp curves of an input or I/O buffer can be measured
directly with a curve tracer, with the I/O buffer 3-stated.
However, sweeping enabled buffers results in curves that are
the sum of the clamping curves and the output structures.
Based on the assumption outlined above (which makes sense), the pull-up
and the pull-down curves of an IBIS model must represent the DIFFERENCE
 of the 3-stated and the enabled buffers curves. (Note... ...)
This requirement enables the simulator to sum the curves, without the
danger of double counting, and arrive at an accurate model in both the
3-stated and enabled conditions. ...

This explanation seems good for producing V1.1 tables as well (and 
perhaps just didn't make it into documentation until the V2.1 Spec).

So:
Why does s2ibis not subtract the ***Tri-state (un-enabled)*** data
from the entire range of both pull-down and pull-up data to derive the
corresponding [Pulldown] and [Pullup] tables?  Instead, it appears that it 
takes EXTRA steps to produce seemingly innacurate tables...  Is this a 
serious mistake with s2ibis, or are am I missunderstanding something. 
Thanks ahead for all feedback.

Regards,
-Scott Schlachter
 Design Engineering
 Actel Corporation
 Sunnyvale, CA

---

From: [email-masked]
Date: Fri, 8 Nov 1996 10:46:30 -0500 (EST)

<snip>
> Thanks ahead for all feedback.
<snip>

Scott:

While I didn't write the version of s2ibis that you refer to, I think I
can answer your question.

The answer lies in the voltage ranges of the pullup (or pulldown) and
power clamp (or ground clamp) curves, and the assumption that a tool
will add the pullup and power clamp curves of a tristate device together
to get the correct total behavior.

If you look at the voltage points in the pullup curve, you'll see that
they vary from -Vcc to 2*Vcc, while those in the power clamp curve only
vary from Vcc to 2*Vcc. When the simulator adds these two curves
together, then, it only adds values to the pullup curve in the range Vcc
to 2*Vcc, not the whole range of the curve.

If one were to subtract the tristated pullup curve from the active
pullup curve over the entire -Vcc to 2*Vcc range, the IBIS tool would be
unable to reconstruct the correct behavior of the model, as the required
data (i.e. the tristated data from -Vcc to Vcc) would not be present in
the IBIS file.

Hope that clears things up.

Regards,

-- 
Alan Glaser                         "It's not a competition,
ECE Dept.                            it's just a mint..." - K
North Carolina State University