Subject: Re: [SI-LIST] : IBIS Modeling

From: Kellee Crisafulli <[email-masked]>
Date: Wed, 25 Mar 1998 01:00:30 -0800

At 11:22 AM 3/24/98 -0700, D. C. Sessions wrote:
>In brief, the answer is that IBIS device models are *not* good enough.
>You absolutely need to model the substrate and the terminators (DIMM
>series resistors) on them.  IBIS 3.1 includes the capability of
>defining Electrical Board Descriptions, which are basically simplified
>netlists of the PWB.  The examples given are in fact memory modules.
>D. C. Sessions
>[email-masked]

We have built DIMM and SIMM module modules using IBIS 2.1 that do
a reasonable simulation job.  It requires a bit of playing with the
package model and it is somewhat simulator dependant.  Our simulator
interpretes
large RLC package values as transmission lines instead of lumped LC parameters
which allows us to make a reasonable DIMM or SIMM module module using
IBIS 2.1 To accomplish this we create a full circuit boad model of the
DIMM or SIMM and then tweak an IBIS package model until it looks
the same under several simulation conditions.  The die model input capacitance
must also be increased to account for the extra loading.

While I agree IBIS 3.x makes this relatively easy and provides a more accurate
result it is possible to get usable results with 2.1 prior to the release of
the 3.x parser.

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From: Kellee Crisafulli <[email-masked]>
Date: Wed, 25 Mar 1998 01:00:30 -0800

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From: Weber Chuang <[email-masked]>
Date: Wed, 25 Mar 1998 17:34:58 +0800

We are PC chipset vendors, so we need to handle kinds of I/O pads and
modules, so far I use real module database and then combine it with
mainboard to run a backplane-style simulation for AGP and PCI and
memory, though we only consider capacitance effect for connector or
socket. Using lumped RLC to substitute for the trace yields odd and
inaccurate waveforms.

  Best Regards

 Weber Chuang(ChingFu Chuang)
 SI Engineer, System Team.
 VIA Technologies, Inc.
 Taipei, Taiwan, ROC 
 http://www.via.com.tw
 Very Innovative Architecture

	********************    Re: [SI-LIST] : IBIS Modeling
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From: Weber Chuang <[email-masked]>
Date: Wed, 25 Mar 1998 17:34:58 +0800

Best Regards

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From: Andrew Ingraham <[email-masked]>
Date: Wed, 25 Mar 1998 11:28:44 -0500

>IBIS 2.1 

I don't mean to criticize your approach ... but this seems like a
dangerous way of creating a model.

In an ideal world, IBIS models *ought* to be portable such that they can
be used on different platforms and different simulators and yield
basically the same result, within the accuracy limits of those
simulators.

If there are big differences such as this (where large RLC values are
treated in a fundamentally different way on some simulators), that seems
like a risky thing to take advantage of.  It ties your IBIS model to
that simulator.  How do you prevent the model from being used on another
simulator?

In essence, you have created a proprietary *.ibs model, similar to an
Hspice model that won't work correctly on someone else's simulator.

As a side note, is it even the right thing to treat large RLC package
values as a transmission line?  What if they represent something like a
long bondwire and a big bonding pad?  How do you make that choice
between which way to model it in the simulator?

Should the IBIS spec make such loopholes possible?

Regards,
Andy Ingraham

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From: Kellee Crisafulli <[email-masked]>
Date: Wed, 25 Mar 1998 12:46:12 -0800

Hi Andrew,

>basically the same result, within the accuracy limits of those
I agree completely.

>that simulator.  How do you prevent the model from being used on another
In this instance we made the models for our own customers only.
However the real issue is how good or bad it works using either
method.  Please try it, demonstrate the magnitude of whatever problem
you are concerned.  Based on comparative
simulations we ran we felt the result were acceptable and useful until the
version 3.x models are available and simulators are all using them.  I was
one of the most vocal advocates of getting modeling into IBIS for
DIMM and SIMM modules and made several proposals to the IBIS committee to get
this done.

>Hspice model that won't work correctly on someone else's simulator.
You have a valid point. However unlike Hspice
it will run on all the simulators that use IBIS with slight differences in
the package effects but at least they will have a model v.s.
no model.  Do you think it is better not to do any modeling and have
users guess what the signals look like.  And yes I think this sort of
model is an order of magnitude better than guessing.

>between which way to model it in the simulator?
IBIS has never specified that the LC pin values be used as lumped values.
As you may know some packages are modeled better as a lump and some
model better as a transmission line. Neither is perfect for all packages.
however it generally isn't possible to get devices to work if the pin is
really a large lumped L so I don't see much danger here.

>Should the IBIS spec make such loopholes possible?
I don't see this as a loop hole.  Each vendor needs to use the IBIS
data to create the best results they feel possible.

I don't believe we need to spend a great deal of time on this thread Andrew.
I only wish to point out that it is possible to create a useable model for
a DIMM or SIMM in V2.1 IBIS.  
I lobbied to get better SIMM and DIMM modeling into the spec. for over a year
so I agree it is a much better method.  I was the one that insisted that the
changes proposed by Intel for their processor models also work for DIMM and
SIMM
modules.

I understand your concerns and they are reasonable.
Best wishes..
Kellee