[SI-LIST] Re: package SSN model accuracy requirements, now Behavioral Modeling

  • From: "Muranyi, Arpad" <arpad.muranyi@xxxxxxxxx>
  • To: <si-list@xxxxxxxxxxxxx>
  • Date: Tue, 22 Mar 2005 15:29:04 -0800

Sorry for joining this thread so late, but I was out of my office for a
while.  I would like to add a few my thoughts for brain-food.

I am happy to note that the discussion finally concluded that basically
all models are behavioral, including the SPICE transistor or resistor
models.  Having said that I hope we can also conclude that the problems
we have with IBIS are not because it is behavioral.  In my opinion it is
simply a poorly written specification.  The limitations are mostly =
caused
by the rigidity of the spec, and the close relationship between the
keywords and their predefined (assumed) meaning.

Most people don't think about this, but SPICE is suffering from the same
exact problem.  It may be on a different level, but the problem is the =
same.
This was actually discussed several years ago in an IBIS Summit =
presentation:

http://www.eda.org/pub/ibis/summits/sep00/secasiu.zip

To sum it up, think about the various "level"-s of MOSFET (or any other
element) models.  When a semiconductor company invents a new process for
which the existing models are inadequate, a new set of equations and a =
new
model "level" needs to be invented.  The implementation of this can only
be done by the SPICE tool vendor, no one else.  The users of the SPICE =
tool
will have to wait, just like in the case of IBIS.  This is where the =
*-AMS
languages come to a rescue.

Another thing that most people don't seem to consider is that the =
equations
of the SPICE primitives can be, and have already been converted to *-AMS =
in
several projects.  Therefore an AMS simulator can be used the same exact =
way
as if it was a SPICE simulator.  But why bother going to AMS if we have =
SPICE?

Well, the advantage of AMS over SPICE is that we get access to the =
equations.
We don't have to wait for the tool vendor to implement anything for us, =
we
can write it any time we want it to.  Also, having that access to the =
equations,
the model maker can decide what level of abstraction they should use for =
their
model.  They can choose to do it as we know it from SPICE (using circuit =
elements,
such as R, L, C, transistor, diode, etc), but they could also go to =
higher or
lower levels of abstraction.  If they wish, they could write equations =
for
the electron behavior in the crystal structure with all kinds of quantum
effects, etc., but they could also do as IBIS does and write equations =
for
larger building blocks, if they like.  IT IS ALL UP TO THE MODEL WRITER.

Another one of the advantages of AMS over SPICE is that we can also =
write
truly digital equations, and mix them in with analog equations.  These =
can
simplify and speed up the modeling and simulation of logic equations and =
state
machines tremendously, compared with their analog counterparts.

I agree, writing AMS equations may not be for everyone, but it doesn't =
have
to be.  To illustrate this from the SPICE world, another example comes =
to my
mind.  Think about how many people actually write those so-called =
process files.
(Remember, a SPICE transistor primitive is useless by itself without the
underlying process parameters).  Well, in my company there is a special =
group
of a handful of people who does that, while there are probably several =
thousand
of circuit designers who write schematics.  This discussion thread made =
it seem
that SPICE is easy because it is so familiar, yet no one mentioned that =
none of
those thousands of people designing circuits would get anywhere without =
those few
who provide the most important part of all, the process files!  Also, if =
this
wasn't such a sensitive subject, I could probably tell horror stories =
about what
happens when they don't get it right, because no one using SPICE knows =
how to
fix those parameters...

I see a similar distribution of work in AMS.  There will be a few people =
who
will be able to write templates, primitives, and there will be lots of =
people
who will be using them.

Arpad Muranyi
Intel Corporation
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