Subject: Re: Spice Model of IBIS

From: Kellee Crisafulli <[email-masked]>
Date: Sat, 28 Mar 1998 16:02:25 -0800

Hi John,

You asked basically why IBIS was created when SPICE already existed and why
can't it be used in SPICE.

  Really you should think of IBIS as somewhere between a true model and
specification.
It is really more a specification for what the parts do then a model.
Think of it
like a data book describing the characteristics of the part.  For example
there are
V/I curves just like in a data book.

  IBIS can be used with SPICE.  Many of the SPICE tool vendors have a
SPICE sub circuit for running the IBIS specification.  
Just ask your SPICE vendor for a sub-circuit model.  If your vendor
doesn't have one, find a better simulator package.

To answer your questions about IBIS it was specifically developed to
fix the major problems not addressed by spice for doing IC signal integrity
simualtion.  A large number of SPICE users were involved in the development
of IBIS to resolve problems that SPICE alone could not address i.e.

1) SPICE models were not portable even between spice tools since IC designers
  are very often using special transistor models not available in most spice
  packages used by systems engineers.
2) SPICE models gave away all the design tricks the IC companies wanted
protected.
  This meant they would not distribute models without non-disclosure
agreements
  which must be signed by a board member or V.P. or President level to be
valid.
3) SPICE has no rules so there is no minimum requirements to be a spice model.
  IBIS models provides sufficient detail about the component
characteristics to
  allow signal integrity, EMC analysis and high speed design work.  Since
SPICE
  models don't have any agreed upon standards there is no way to tell if the
  information needed will be present and where to find it.
4) There is no way to embed testing specifications in SPICE.  IBIS contains
testing
  constraints as well as device characteristics.
5) SPICE has no way to indicate best/worst case limits the manufacturer
wants you
  to design to, IBIS does.
6) IBIS has been extended to include resistor and capacitor packages as
well as
  diode packages.  This can be done in SPICE, but there are again no rules
about
  what information is present and how it is organized.  This works OK
(though a
  bit of extra work in a schematic level simulator) however it does not work
  very well if the model must be used at the board level and automatically
loaded.
7) IBIS is also being extended to do connector models.  This is an area
than can
  be done in SPICE, but lack of standards in SPICE  has prevented any standard
  method for modeling connectors from being developed.  IBIS will provide a
set
  of rules that everyone can follow to insure portable models that will
automatically
  load without user intervention.
8) There isn't any standards organization supporting SPICE format changes.
IBIS
  is an EIA specification and is a standard that everyone can use and
design to.

At 03:50 PM 3/28/98 -0600, John Synesiou wrote:
>I would like to know if there is any reason why an IBIS model of a gate
>cannot be modeled using a SPICE simulator, specifically a SPICE 3 compatible
>simulator. Since IBIS, refers to the I/O characteristics of the gate, why
>not use SPICE to model these characteristics? The package parasitics are
>easy to model in SPICE, and the V/I tables can be modeled using if-then-else
>statements.

>I wonder why a new syntax was developed for IBIS, rather than using SPICE
>syntax.
>
>I would like to think that there was some very compelling reason why a new
>syntax was necessary and why it is necessary for me to purchase yet another
>simulator. I see SPICE to IBIS converters are available, what about an IBIS
>to SPICE converter?
>
>Regards
>
>John Synesiou			[email-masked]
>U.S. Power, Inc			Phone (612)826-1111
>6497 City West Parkway	Fax (612)826-1003
>Eden Prairie			Date: 03/28/98
>MN, 55344			Time: 2:24 PM
>
>
>
>
>

---

From: "John Synesiou" <[email-masked]>
Date: Sat, 28 Mar 1998 19:25:24 -0600

Thank you Kellee for a very detailed explanation of the benefits of IBIS
syntax over SPICE. If you don't mind, at the risk of appearing
argumentative, I'd like to explore this issue further.

Here are some ideas I think may address some of the issues you raised.

Why not propose a template for a SPICE model that implements the IBIS
specification. All IC manufacturers could fill in key parameters and add V/I
tables required by the IBIS specification. A similar syntax checker could be
used to parse and validate the SPICE model.

This model would expose only the information required by the IBIS model.

Impose the same IBIS syntax rules on the SPICE model.

Use Tolerance for best/worst case and use Monte Carlo analysis.

I'm not sure how to address the embedded testing issue you raised, but maybe
one of the SPICE vendors can assist in this area.

I think you raise some very important issues, but why not take the time to
develop a SPICE compatible standard that addresses all these requirements,
rather than developing yet another standard.

My understanding is that SPICE has been around for years and will always be
far more flexible to analyze circuit dynamics than the relatively new IBIS
simulators. Why not take advantage of the wealth of analysis tools available
with SPICE simulators.

For the record, I am in no way affiliated with any SPICE vendor. I use
multiple simulators such as SPICE, VHDL, etc and I'd like to avoid investing
in yet another simulator if possible. Further more, I'd prefer to avoid
learning how to use another tools, and worse take the time to verify the
output of the simulator, you know garbage in garbage out.

-----Original Message-----
From:	Kellee Crisafulli [mailto:[email-masked]]
Sent:	Saturday, March 28, 1998 6:02 PM
To:	[email-masked]; [email-masked]
Subject:	Re: Spice Model of IBIS

Hi John,

1) SPICE models were not portable even between spice tools since IC
designers
3) SPICE has no rules so there is no minimum requirements to be a spice
model.
  be done in SPICE, but lack of standards in SPICE  has prevented any
standard

Regards

MN, 55344			Time: 6:32 PM

---

From: Fred Balistreri <[email-masked]>
Date: Mon, 30 Mar 1998 16:08:49 -0800

> >Eden Prairie                   Date: 03/28/98
My company supports both IBIS and Spice models. There are a variety of
mistakes and misconceptions in what Kellee says about spice models.

Statements 1 and 2 are true. In essence this is the whole reason for
IBIS.

Statement 3 is misleading. Spice is a simulator not a syntax or
standard.
The rules need to be developed by the people making the model not the
simulator. Spice models levels have been around for years and the
various
levels are clear about what is modeled and what is not. Berekey
documents
the models very well and they are public. IBIS as it stands has not been
approved for EMC and it's untested. Since there is no information about
the
power supply and gnd currents and their waveforms its doubtful that IBIS 
could be used in EMC at all. Repeated attempts to include this in the
spec
by Cadence and other IBIS knowledgeble people have been down played by
other
vendors who would be at a disadvantage technically by including the
current
specifications. Needless to say that Spice models do not suffer this
fate.

4. I don't have a clue what Kellee is talking about. Is the embeded test
the
   load resistance to test the model under? If so this has been a sore
spot
   in IBIS for a long time. John Powel can tell you why Quad requires 2
sets
   of TR/TF data in their simulator.

5. This is bogus since IC desigers have been doing this for at least 30
years
   using Spice. Most if not all currenty avaliable IBIS models are made
from
   Spice in the first place. Now how is that possbible if Spice doesn't
or
   can't indicate best/worst case limits?

6. Not only can Spice to this but it can do this elequently, both at the 
   schematic level and board level. Kellee you should learn more about
using
   Spice subckts. It works wonderfully well at the board level thankyou,
we've
   been doing this since 1992.

7. Spice connector models can be more robust than IBIS models. IBIS
cannot
   handle a matrix type solution since the committee was
unwilling/unable to
   come up with a coupling standard everybody would agree to. The result
is
   pin based parasitics which are inaccurate and practically useless in
some
   applications. It's true though that there is not set standard in
Spice.
   Using lumped LCR networks will work in any Spice package though. 

8. True. There is an upcomming EIA proposal from Japan that would
formally
   standardize Spice models without giving out the propietary
information.
   If it doesn't get killed by IBIS bigets it would be a good standard.

Now there is good reasons for having IBIS models. As mentioned
statements 1
and 2 are true. And there is some truth in most of the statements Kellee
made.
However Spice and Spice models should not be compared to IBIS data
sheets. 
They are NOT the same thing. There are applications where IBIS has
advantages.
Those advantages are in simulation speed and simplicity. There are areas
where
IBIS falls short. Those areas are in flexibility, modeling, and
accuracy. 
Syntax is the strength and weakness of IBIS. Since it takes more than
one
year to change the syntax/parser and everything that means it becomes a
weakness. IBIS is an emerging standard but it has not withstood the test
of
time. On the other hand I don't know of a main stream EDA or IC company
that
does not use SPICE. That's something to think about. How about it Bob
Ross
and company when are we going to get a SPICE standard? 

I'm not the most popular guy around and have no plans to be in the
future.
But somebody should stand up and tell the truth in this industry once
and
awhile. 

The opinions stated above are my personnal ones and do not reflect the
views
of Applied Simulation Technology.

Best regards to all!

---

From: Kellee Crisafulli <[email-masked]>
Date: Mon, 30 Mar 1998 19:18:39 -0800

Hi Fred,

  Ouch, here are my responses to your points:

>   IBIS models provides sufficient detail about the component characteristics to
>   SPICE models don't have any agreed upon standards there is no way to tell if the
>Fred Said: "IBIS as it stands has not been approved for EMC and it's untested"
Several companies are using IBIS models as stimulus for EMC
related products and have tested them.
Yes there are more physical package related issues that need to be added to
improve the accuracy.
SI analysis had many years head start on EMC analysis and I feel considerable
improvement can be made in this area in the future.

>    testing constraints as well as device characteristics.
>Fred Said: "I don't have a clue what Kellee is talking about. Is the embeded test
>            the load resistance to test the model under?"
These are things like threshold voltages, min and max specifications
Vmeasure specification for flight time adjustments which allow the simulator
delays to be used with timing tools.

>    wants you to design to, IBIS does.
>Fred said: This is bogus since IC desigers have been doing this for at least 30
>   Now how is that possbible if Spice doesn't or can't indicate best/worst case limits?
We are talking about board designers not IC designers and board designers
need to use min/max limits specified by the IC vendor.  And min and max design
limits are not provided by SPICE.  Even something as simple as the power supply range
is missing from spice models unless perhaps it is there as a comment.  Obviously spice
can be used to simulate and IC at best and worst case temperature and best and worse
case voltage etc.  But the user must choose the limits unless they are obtained automatically
from a specification like IBIS.  I don't see any reason why a SPICE package couldn't be
modified to do this though.

>    well as diode packages.  This can be done in SPICE, but there are again no rules
>    about what information is present and how it is organized.  This works OK
>    (though a bit of extra work in a schematic level simulator) however it does not work
>Fred said: Not only can Spice to this but it can do this elequently, both at the 
>   using Spice subckts. It works wonderfully well at the board level thankyou,
>   we've been doing this since 1992.
As I said schematic level can be done in SPICE.
If you have made some SPICE sub-circuits for some resistor packs at the board level
I would say great but I don't see any standard method for describing R,C,diode
packs emerging other than IBIS.

> 7) IBIS is also being extended to do connector models.
The IBIS specification is not complete in this area.
While SPICE can be used to model connectors there is no standards and
that includes lack of specification of standard methods for coupling matrix's.  

Hopefully we will make some progress with IBIS in creating a connector
specification this year.  I hope you will participate in this
effort as I know you and your company have a large amount of knowledge in this area.

-------------------------------------------------------------------------
Have a great day...
Kellee Crisafulli at HyperLynx
[email-masked]	http://www.hyperlynx.com
-------------------------------------------------------------------------