Re: Spice Model of IBIS

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 jsynesio@us-power.com
>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
>
>
>
>
>
Received on Sat Mar 28 16:04:05 1998