Re: 100-points limit discussion

A few points need to be clarified in Arpad's statements. The 100 point
limit did not
come from the SPICE world. It came from a restriction in the particuliar
brand of
SPICE being used by ????? at the time. As an example my company is a
SPICE vendor with
no restriction on PWL number of points. Our derivitive is Berkeley SPICE
3C1 circa
1980's something. Basing a standard on third party EDA vendor
restrictions is @#*%#$#@!
in my opinion. My point here is don't blame this on SPICE. There is
enough blame going
around the SPICE world. This is an IBIS problem. B source may be new in
some SPICE
programs but its been around at least since Berkeley 3C1 SPICE.

Secondly as a model builder my two cents are that 100 points were fine
in the original
IBIS when there was no v/t information. If done correctly 100 points
for I/V
information is more than enough. The addition of v/t information may
have changed all
that.

Thirdly, s2ibis is a public program written by students at UNC, and at
that its
origin is now maybe 3 or 4 years ago. Why is the whole IBIS world
relying on this
tool? There are a few multi hundreds of million of dollar in sales EDA
companies and
at least one over 1 billion in sales EDA company that claim support for
IBIS. Yet all
of these companies I am led to believe rely on an old public domain
software program.
Oddly enough there is enough complaints and questions brought up by
s2ibis that an
innocent bystander would have plenty of doubts about this program! My
point here is that IBIS is the standard NOT s2ibis....and there are
alternatives to
s2ibis albiet not free.

In my opinion the 100 point limit can be lifted as it seems that its a
problem
already.

Muranyi, Arpad wrote:
>
> All,
>
> I agree with all of you, and at the same time a also disagree with some of
> the points raised (no pun intended).
>
> 1) I believe that we are mixing at least two issues here. One is on the
> subject of the number of points limit in the IBIS specification, the other
> is regarding how s2ibis generates points. The third one relates to the
> intelligence and experience of the model maker. (There is no need to run
> a waveform simulation for 20 ns at 1 ps step time generating 20k points
> when the edge reaches steady state in 2-3 ns and 3k points would be
> sufficient,
> as someone already mentioned it).
>
> IBIS does not need uniformly spaced data, in fact, in Section 3 the spec
> says
> the following:
>
> | 9) The V/I data tables should use enough data points around sharply
> curved
> | areas of the V/I curves to describe the curvature accurately. In
> linear
> | regions there is no need to define unnecessary data points.
>
> It is not stated, but this also applies to V-t curves. The problem is that
> s2ibis doesn't know how to pick the best 100 points, so it generates
> uniformly
> spaced data with a bunch of redundant, unnecessary points in the linear
> regions.
> This uses up the 100 points quickly without much benefit, just because the
> software doesn't have a best 100 point picker routine. We should not blame
> the IBIS spec for this deficiency.
>
> 2) On the other hand, YES, I do agree that we need to raise the points
> limit
> in IBIS because the way things are now DOES limit much needed accuracy in
> some
> cases. I demonstrated this in a test case when I overlaid the waveforms of
> a
> SPICE and its equivalent IBIS model. The question is what should be the new
> magic number.
>
> 3) History / current situation: The real reason for the 100 points came
> from
> the 100 point limits of the PWL sources in the SPICE world. We didn't want
> to
> exclude SPICE vendors and users from behavioral simulations. To make that
> possible without having to ask each SPICE vendor to rewrite all of their PWL
> sources, we had to introduce the 100 point limit. There were no technical,
> engineering reasons behind this rule.
>
> We found the first problem when I was writing my "Best 100-points" algorithm
> (for my IBIS Center program that we use internally to make IBIS models). I
> realized in this process, that there is a difference in the outcome when we
> count the X-axis points, or the Y-axis points because the typ., min., and
> max.
> tables use a common X-axis. When the three sets of Y-axis data are curved
> in
> different areas of the X-axis, in the theoretical extreme we could end up
> with
> 300 X-axis points while each of the Y-axis has only 100 points. The IBIS
> 1.1
> specification did not say anything about how the 100 points were supposed to
> be
> counted. We did discuss this in the Open Forum Teleconference, but choose
> not
> to do anything about it in terms of changing or clarifying the IBIS spec.
> It
> was verbally said, however, that we should count X-axis points to be safe.
>
> I don't remember whether I found this before IBIS 2.1 was ratified or later,
> but
> somehow the most important IBIS 2.1 addition, the V-t curves ([... Waveform]
> tables) inherited the same problem.
>
> IBIS 3.x has a few I-V curve related additions. It seems that in effort to
> make
> the spec clearer we not only defined it better for the new keywords, but for
> the
> sake of consistency we also changed the wording in the usage rules section
> of the
> old keywords, such as the [Pulldown], [Pullup], [GND Clamp] and [POWER
> Clamp].
>
> | ...first and last voltage points on any V/I table. Each V/I
> | table must have at least 2, but not more than 100, voltage
> | points.
>
> where "voltage points" defines that the counter should count 100 X-axis
> points.
> It is interesting to note that the [... Waveform] keywords were still left
> vague
> in the IBIS 3.x spec:
>
> | ...parses down the table. The waveform table can contain a
> | maximum of 100 data points. A maximum of 100 waveform
> tables
> | are allowed per model. Note that for backward
> compatibility,
>
> 4) Now, why did I write this all down? Well, to give some sense for the
> proposal
> that I am about to make here. It can be seen from the above history that,
> for one,
> the spec is still inconsistent regarding how to count the points in the
> waveforms
> and I-V curves, but more importantly, there is no technical reason for the
> X-axis
> counted 100 points in the I-V curves.
>
> There is another piece of information that is necessary before I make my
> proposal.
> I have not seen any generic PWL source in any SPICE flavor which can handle
> one
> X-axis column with multiple Y-axis columns. So if someone uses generic
> controlled
> sources in SPICE for building behavioral models which use IBIS data, they
> will have to
> separate the typ., min., and max. columns of the IBIS model into three
> separate PWL
> sources. Therefore, if we still want to support any SPICE tool by observing
> their 100
> point limits in the PWL sources, it is sufficient to count the points in the
> Y-axis.
>
> On the other hand, if there are newer implementations in SPICE that support
> IBIS data
> directly, such as the new B-element in HSPICE, chances are that being new,
> they can
> easily implement any (new) IBIS rule.
>
> 5) So, based on all this, I propose that we should allow for counting 100
> points
> in the Y-axis. This will still work with any old SPICE PWL source, yet
> would
> significantly raise the accuracy of the curves. By doing this, we are
> really
> not changing the original intent of the spec.
>
> One problem that Bob raised in a private conversation to me is that this
> will force
> NA-s into the Y-columns, which is not desirable, even though it is perfectly
> legal.
> For this reason he suggested that we should just allow 300 points as counted
> in the
> X-axis, which essentially yields the same level of accuracy. The problem I
> see with
> this method is that this way the Y-axis can also end up with 300 points,
> which may
> prevent the data be used in old SPICE PWL sources.
>
> 6) On the other hand, if we do not care about SPICE PWL sources, we can
> raise the
> limit to practically any number. The things we need to consider then are
> what some
> already brought up in this discussion, file size, distribution, but I would
> add
> speed of simulation also. My understanding is that the larger the tables
> get, the
> more if-then-else evaluations will need to be done which can slow down the
> simulator.
>
> 7) I tend to agree that unlimited points may not be a good idea, because
> (forgive me)
> there is no cure for laziness and stupidity. We may end up with a lot of
> large files
> with no reason, or added accuracy.
>
> Please comment.
>
> Arpad Muranyi
> Intel Corporation
> ============================================================================
> =======

-- 
Fred Balistreri
fred@apsimtech.com
http://www.apsimtech.com