Subject: RE: [SI-LIST] : IBIS Subcommittee Formed to Review Spice to IBIS

From: "Beal, Weston" <[email-masked]>
Date: Mon, 7 Jun 1999 18:11:52 -0500

Kellee,

You're disagreeing a mute point.  Your point was the same point I was trying
to state by saying, "the IBIS creator tool should have an algorithm to
remove points in linear regions."  We should delete all these extra data
points.  The problem that s2ibis has now is that all points are equal
difference.  They should be closer in knee points and very sparse is linear
regions.  The IBIS creator tool should have an algorithm to clean out extra
data.

So, I agree with you and now you can agree with me.

Later,
Weston Beal
Signal Integrity Engineer
Compaq Computer Corp.

		-----Original Message-----
		From:	Kellee Crisafulli [mailto:[email-masked]]
		Sent:	Monday, 07 June, 1999 2:19 PM
		To:	Beal, Weston; [email-masked]
		Subject:	RE: [SI-LIST] : IBIS Subcommittee Formed to
Review Spice to IBIS

		Hi

		I have a comment on Weston's comment on the number of data
points.
		I disagree on one point.  Instead of increasing the number
of points generated
		in the IBIS file the spice to IBIS tool should improve the
quality of the
		points
		choosen.  

		I can hand build files with 50 points that out perform files
with 400 points.
		The big gain is that the FILE SIZE is reduced with higher
accuracy.
		Large IBIS files will take up many megabytes on 1000's of
hard drives.
		The V/I and V/T tables are the largest part of IBIS files.  
		Increasing them 4 x will generally increase the file size 3
x or more.

		>The third thing that I changed was the number of points in
the time
		>waveforms from 50 to 101.  I have to remove one point to
comply with the
		>golden parser, but I get better detail.  On the issue of
number of points in
		>a curve or waveform, I think the user should be able to
define any degree of
		>granularity and the IBIS creator tool should have an
algorithm to remove
		>points in linear regions.  If the resulting curve still has
too many points,
		>then increase the linearity tolerance and try it again.

		Best wishes...
		Kellee

---

From: [email-masked]
Date: Tue, 08 Jun 1999 09:26:20 -0700

I agree with Weston.

Either that or remove the quite artificial limit of number of points in an IBIS
file.
I have actually been faced with the problem that even with software to remove
extra points I cannot
create IBIS models to the accuracy that I desire with the 100 point limitation.

jon powell
Viewlogic Consulting Services

---

From: Bob Ross <[email-masked]>
Date: Tue, 08 Jun 1999 11:44:06 -0700

To All:

We the 100 point VT limit up at the last IBIS Meeting based on the fact
that several people felt that accuracy could be compromised.  The general
consensus was to consider a conservative 1000(1) point limit for a future
Version of IBIS.  An expanded discussion is in the minutes stored at

  http://www.eda.org/pub/ibis/minutes/min1999/m052899.txt

I think we should consider more flexability regarding the number of
points (including going beyond the limit) in the upgraded s2ibis2/3
utility.

Bob Ross
Mentor Graphics

---

From: Scott McMorrow <[email-masked]>
Date: Tue, 08 Jun 1999 17:00:31 -0700

I second Jon's opinon.

There is no real reason to limit the number of points in
IV or VT tables.  I have often used a simulator which allows
an arbitrary number of points over 100 to create high
resolution simulations.  256 to 512 is often useful to
capture rising edge timing information accurate to 1ps, along
with driver overshoot behavior to within 1 mv of HSpice
simulations.

Model size makes no difference to me.  It is the result that
matters.

Scott McMorrow
SiQual

---

From: Kellee Crisafulli <[email-masked]>
Date: Tue, 08 Jun 1999 21:14:09 -0700

Hi all,
 
  At the risk of repeating myself there are several very good
reasons to limit the number of points in the IV and VT tables.
e.g. disk size, transfer time, overall accuracy improvement.

The last one may surprise you but I do feel that including for
example a 3rd VT table buys more than increasing the number of points
for most models.

I do agree the number of points could be increased to say 256 to
pick yet another arbitrary spot on the wall.  I would prefer to
see a model creator like Arpad propose a new limit based on 
actual demonstrated requirements where some form of data reduction is
used.

I feel we should keep this number as small as we can and still create
accurate models to insure the files sizes and download times are reasonable.

I think the biggest problem here is spice to ibis.  People are always
generating models with the maximum number of points.  I see many many models
that will work EXACTLY the same with 1/3 this many points if they are placed
sensibly instead of using a linear spread.

I am worried that if we set the limit at 1000 we will have all the new
models with 1000 points......
Just doesn't make sense to me sorry...  especially with more and more
VT tables being used.
best wishes...
Kellee

>> >                 >then increase the linearity tolerance and try it again.
---------------------------------------------------------
Have a great day....
Kellee Crisafulli 
HyperLynx, a division of Pads Software Inc.
SI,EMC,X-talk and IBIS tools for the Windows platform
E-mail: <mailto:[email-masked]>
web:    <http://www.hyperlynx.com>
---------------------------------------------------------

---

From: [email-masked]
Date: Wed, 09 Jun 1999 08:59:25 -0700

Here is a comment from a model creator:

I have written extensive software to deal with the ideal selection of
non-redundant data. I have found that the 100 point limit is over restrictive for
both IV and VT curves. (that is, I have example models that cannot be constructed
inside the 100 point limit and still be as accurate as I want (ie. as accurate as
the XTK model I made at the same time)). I don't think that model size or
transmission time should really be a major consideration. Disk space if free.
Bandwidth is soaring. The IBIS format should be relieved of artificial barriers.
Our goal should be to make it easy for people to make good IBIS data sets, not a
big pain.

jon

---

From: Al Davis <[email-masked]>
Date: Wed, 09 Jun 1999 07:10:12 +0000

> create IBIS models to the accuracy that I desire with the 100 point limitation.

I don't believe even 10000 points would provide the accuracy you desire,
because the limits to accuracy are usually elsewhere.

For example ....
100 points chosen to be uniformly spaced in voltage, modeling a 5 volt
rising waveform, provides a data point every .2 volts.  Assuming that
the interpolation is completely bogus, this leads to a worst case error
of .1 volts.  In reality, interpolation is much better than that, and
even this choice of data points is far from optimal, so the accuracy is
much better than this.  Remember, this isn't DC, but the dynamically
changing voltage during a high speed transition.

Some questions ....

Is your Spice model that accurate?  (I don't think so.)

If you measure one real unit with perfect accuracy, and compare to the
next one, are they this close to each other.  (I don't think so.)

Are your instruments this accurate?  (Don't forget that you change the
reading when you attach the probe.)

What if the real load is different from the test load?  Since the
driving impedance is nonlinear, and not specified anywhere, even with
perfectly accurate and complete data for the nominal load, all bets are
off for anything else.

Sure, the voltage is right, into a resistor, but what about reflections
and noise?

How accurate is the rest of the simulation?

There are other ways to improve the accuracy.  One obvious one is to
carefully select the data points (as Kellee said).  Another is to add
another VT table with a different load.  (As Kellee also said)  The
third table does much more to improve accuracy than more points will
do.  Tying the load resistor to a middle voltage does a good job at
modeling the overlap.

I wouldn't object to removing the limit entirely, because whatever the
limit is, most files will use it, without any valid reason.  With no
limit at all, it is up to the modeler to decide what trade-offs to use. 
Maybe with nothing in the spec saying how many points to use, we might
actually get some good 20 point models, that are more accurate than the
256 (or whatever) point models that we will get if the limit is raised,
and the capability is there for those rare cases when you really do need
the extra points.  So, maybe the answer is to remove the hard limit, but
add a comment that tables with more than 50 points are rarely worth the
extra space.

---

From: "D. C. Sessions" <[email-masked]>
Date: Thu, 10 Jun 1999 16:31:55 -0700

Aside: maybe we should take the [SI-LIST] out of the Subject?

> create IBIS models to the accuracy that I desire with the 100 point limitation.

I'm having a problem with this, Jon.  With inflection points chosen using
the usual spline algorithms, 100 points should be enough for almost ANY
monotonic transition.  Oddly enough, I don't disagree with increasing
the point count, but "you can't model it accurately otherwise" just isn't
obvious to me.

Could you give us an example?

-- 
D. C. Sessions
[email-masked]