Subject: Re[2]: Rising Waveform Loading Effects

From: Arpad Muranyi <[email-masked]>
Date: Fri, 01 Nov 96 11:25:00 PST

Text item: 

Dileep,

If we go by your suggestion, we would have to specify every simulation algorythm
in IBIS.  The tool vendors would not subscribe to that, as I see it.  Even the 
way IBIS is now with some of the rules on how I-V curves "must" be handeled was 
debated at length in the Open Forum meetings.  I am referring to the clamp 
curves being subtracted from the pullup and/or pulldown curves, and the Vcc 
relative notation, for example.

Take, for example, the Vcc-relative curves in IBIS.  My intension was to enable 
GND bounce and Vcc droop simulations by properly referencing which pin these I-V
curves get their currents from, with the added benefit of a nice symmetry.  
Also, with this notation all pullup curves go through the same point when Vds is
0, regardless of what the Vcc voltage is for each given curve (typ., min., 
max.), just like in the case of pulldown curves.  This makes it a lot easier to 
compare them with each other.

However, since it is fairly unusual to say that the origin of the plot is not at
the GND potential, but at Vcc, many people have (or had) problems with this 
notation.  (Even though ECL or the old tube technology does basically the same 
with all those negative voltages).  Anyway, this and similar issues prompted 
lengthy debates on what the "right" way of doing things is.  One argument I 
heared frequently was "give me raw data, and my tool will do the necessary 
things to it the way I believe is the right way".

This is just an example for why you can't specify in the IBIS spec what the tool
should do with the data.  Tool vendors want to be able to differentiate 
themselves from others, because they believe they know it better how to do it 
right.  And there is nothing wrong with it.  The customer has to figure out 
which tool he believes before he invests.

And, again, IBIS is not a device physics algorythm spec, but a data format spec.

Now, if there is something missing in it in order to be able to simulate 
correctly with the given data, we can consider to add new data definitions to 
it.  One example for this with respect to the V-t curves was the attempt to add 
a keyword that defined whether the device was CMOS or Bipolar.  I don't remember
where that issue stands now, but it seems to me that it was dropped.

Without trying to be funny or sarcastic(!) this situation is similar to 
politics.  Which is better, dictatorship, or freedom?  In dictatorship you loose
your freedom, but everything is unified, controlled, etc.  In freedom you loose 
control, and things have a tendency to become caotic, etc.

Anyway, since I am getting outside the range of my expertise, I better shut up 
and let the experts take over this topic.

Sincerely,
Arpad Muranyi
Intel Corporation
===============================================================================

Chris Rokusek, Quad Deisgn Technology wrote:
>
>There are a few "dimensions" of knowledge not explicity present in the
>.ibs model which may be useful to a simulator:

That is precisely my point. You are making certain assumptions and guesses,
which are OUTSIDE the ibis specification. In fact you are trying to guess
the information which ibis is trying to hide. What do you do if you encounter
some data which does not fit any of your guesses?

All the knowledge "dimensions" should be utilized in developing the
specification,
but once the specification is developed, it should have any loose ends.

Let us assume that you have a simulator which can predict waveforms for any
arbitrary load, given ONE waveform for one RESISTIVE load. But the ibis
specification does not specify that. A model developer can specify a waveform
into a general RLC load (as shown in the example on page 19 of 2.1
specification)
and claim that they have provided a perfectly legal ibis model and now it is
up to the simulator to do the rest. What does
the simulator do in this case with the waveform with all the ringing,
overshoot,
undershoot etc.? The simulator should not place restrictions in ADDITION to
the ibis specification. Therefore, if one waveform into one resistive load
is all that is required, then it should be stated explicitly. Otherwise
different people can make different assumptions and guesses. If certain things
are REQUIRED to carry out a simulation, then the ibis model should SPECIFY
those things instead of leaving them to guesswork.
------------------------------------------------------------
Dileep Divekar
Applied Simulation Technology, Inc.
2188 Bering Drive
San Jose, CA 95131

Phone - (408)-434-0967 x 100
Fax   - (408)-434-1003
Email - [email-masked]
------------------------------------------------------------

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Cc: [email-masked]
Subject: Re: Rising Waveform Loading Effects
To: [email-masked]
From: [email-masked] (Dileep Divekar)
Date: Wed, 30 Oct 1996 08:30:53 +0800
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From: [email-masked] (Dileep Divekar)
Date: Fri, 1 Nov 1996 14:34:18 +0800

>And, again, IBIS is not a device physics algorythm spec, but a data format spec.

I am not saying that IBIS should specify WHAT the simulator should do with the
data. I am saying that IBIS should make sure that SUFFICIENT data is provided.
Thus, it should not be necessary to make assumptions or guesses OUTSIDE of the specification.
Again, I am not saying that IBIS should specify the algorithms. The simulator
can use any algorithm as long as it makes sure that the characteristics specified
in the IBIS model are reproduced.

My understanding was that when a model developer makes an IBIS model, it is
simulator independent. If IBIS specification does not want to make sure
that enough data is provided and it wants to give freedom to the simulators, then
is it acceptable for each simulator to have its own requirements, as long as
the data is provided in the IBIS data FORMAT? Because some simulators may want
to base the simulations on assumptions and guesses, whereas some may want to have
more data.

Again, going back to the curve fitting example, if the specification provides
three data points, then the simulators can have their own algorithms and
differentiate themselves by either fitting a straight line thru those three
points, or fitting a quadratic. And if they want to be more agressive and
make some assumptions and guesses, they can even fit a higher order
polynomial. This is where the freedom part comes in. But if the specification
provides only one data point, there is clearly insufficient information to do
anything BUT only guesswork.
------------------------------------------------------------
Dileep Divekar
Applied Simulation Technology, Inc.
2188 Bering Drive
San Jose, CA 95131

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From: "Boeckmann, Ed (Edward F)" <[email-masked]>
Date: Mon, 4 Nov 1996 09:33:44 -0600

Hello to all,

We have had many interesting and informative responses to the subject.

I would like to add a comment that I think IBIS is an important and
useful modeling technique.

We should strive to make it as accurate and useful as possible and
especially it should be as simulator independent as possible.  In this
regard, is there  some standard test IBIS model that would produce
verified standard responses (Golden Waveforms?) under several different
standard conditions if the simulator is working correctly? 

Best Regards,

Eduard Boeckmann
Intergraph Corp. 
Huntsville AL 35894
(205)-730-6219
[email-masked]

---

From: "C. Kumar" <[email-masked]>
Date: Mon, 4 Nov 1996 11:14:18 -0500

The v-t curves themselves provide such golden data. But unfortunately it is only
one locus. 
> From [email-masked] Mon Nov  4 10:57 EST 1996
> Received-Date: Mon, 4 Nov 1996 10:57:04 -0500
> From: "Boeckmann, Ed (Edward F)" <[email-masked]>
> To: "[email-masked]'" <[email-masked]>
> Subject: RE:Rising Waveform Loading Effects
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> Hello to all,
>