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

  • From: "lgreen" <lgreen22@xxxxxxxxxxxxxx>
  • To: <weirsi@xxxxxxxxxx>, <kumarchi@xxxxxxxxx>, <si-list@xxxxxxxxxxxxx>
  • Date: Sat, 19 Mar 2005 19:58:16 -0800

Hi, Steve,

The real sticking point is that IBIS models is what I/O designers provide,
and IBIS does not model tightly coupled diff pairs.  If you get a SPICE
netlist, you are better off, of course.  But even SPICE models are not
foolproof - back when I used to design I/O buffers, I remember seeing some
flaky BSIM models from some of the foundries we dealt with.  ("Little"
things like discontinuities in I vs Vds.)

The links under Peter Lauritzen's page show a number of devices and circuits
where no macromodels could match the transient circuit characteristics,
while a well-built behavioral model provided a better match to data (i.e.
accuracy) and ran faster.

I guess the bottom line is to do whatever modeling it takes to get the
design out the door, while also verifying models whenever possible.

- Lynne

-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of steve weir
Sent: Saturday, March 19, 2005 7:29 PM
To: lgreen22@xxxxxxxxxxxxxx; kumarchi@xxxxxxxxx; si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: package SSN model accuracy requirements, now
Behavioral Modeling

Dr. Green, the trick of course is getting sufficient accuracy into the 
model, no matter the form.  I think that particular sticking point is what 
brings us back to Chris' position that such models are presently difficult 
to obtain in a form other than a SPICE netlist.

Regards,


Steve.


At 07:17 PM 3/19/2005 -0800, lgreen wrote:
>You have hit the nail on the head!
>
>Macromodels are not required to be implemented as current sources (one
>could, for example, use controlled V sources).  The simulator must still
>apply KCL and KVL to set up and solve the circuit matrix.
>
>The "philosophy" question is when to write behavioral models (equations)
and
>when to write macromodels (adding components).  The classic SPICE 2g6
matrix
>solution time increased as N_cubed, so if a macromodel added nodes, it
could
>greatly increase run time.
>
>I have seen comparisons for power devices, with an order of magnitude
faster
>run times for behavioral models in simple circuits with better accuracy
than
>the macromodel.
>http://www.ee.washington.edu/people/faculty/lauritzen/
>  I personally would love to see similar comparisons between the two
>approaches for circuits containing several I/O buffers.  Maybe someone will
>present these at future IBIS summits.
>
>- Lynne
>
>
>-----Original Message-----
>From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
>Behalf Of C. Kumar
>Sent: Saturday, March 19, 2005 3:50 AM
>To: lgreen22@xxxxxxxxxxxxxx; donaldt@xxxxxxxxxxx; 'Syed Huq'
>Cc: si-list@xxxxxxxxxxxxx
>Subject: [SI-LIST] Re: package SSN model accuracy requirements, now
>Behavioral Modeling
>
>lynn:
>
>Your definition makes sense. If we go by that we can
>the say the following about "macromodels" written in
>spice
>
>1. all spice elements are behavioral elements. They
>fundamentally model the current flow; all spice
>elements are some form of current sources. As you
>point out spice does not have "pure" equations.
>Everything has to be behavioral components connected
>in a netowork and spice imposes conservation at the
>nodes.
>
>2. For our purpose we can say the purpose of
>macromodels
>is
>  a. compactly reproduce the behavior of a complex
>device  using a collection of current sources (our
>behavioral primitives i.e all the 'elements' available
>in a given spice simulator).
>
>  b. like in your example op amp, include "non-ideal"
>effects like that may be contained in a post layout
>model.
>
>--- lgreen <lgreen22@xxxxxxxxxxxxxx> wrote:
>
> > Hi, Donald and Syed,
> >
> > In academia, a distinction is made between the two
> > modeling approaches,
> > although you are correct that the distinction is
> > often fuzzy in industry.
> >
> > "Macromodeling" involves adding components to a
> > model.  A textbook example
> > is adding components to an ideal op amp to add
> > effects like input offset
> > voltage, input bias current, output impedance, poles
> > and zeroes, etc.  A
> > macromodel's added components could be passive or
> > active, linear or
> > non-linear.  In this sense, all netlist-based tools
> > (such as SPICE) can do
> > macromodeling.
> >
> > "Behavioral modeling" involves creating a model from
> > performance
> > characteristics rather than topology, usually using
> > equations or data
> > tables.   IBIS 3.2 is the classic example, but there
> > are hundreds of
> > AMS-style models that have been created over the
> > past twenty years.
> >
> > There is also a "meet in the middle" approach, where
> > equation-based V and I
> > sources are added to a model.  The use of a V or I
> > component makes it
> > macromodeling (in the academic sense of the word).
> > SPICE 3f5 seems to be a
> > common subset of programmable sources for general
> > purpose analog simulators
> > (which means it is the most limited).
> >
> > There are advantages to both approaches.
> > Macromodeling is easier to do if
> > there is an existing model (such as an ideal op amp
> > or IBIS buffer) that is
> > almost what you need.  Behavioral models can
> > simulate faster (factors of 10
> > to 200, depending on how the model is coded), since
> > they do not add
> > extraneous components or nodes to the simulation
> > netlist.
> >
> > Best regards,
> > Lynne
> >
> >
> > "IBIS training when you need it, where you need it."
> >
> > Dr. Lynne Green
> > Green Streak Programs
> > http://www.greenstreakprograms.com
> > 425-788-0412
> > lgreen22@xxxxxxxxxxxxxx
> >
> >
> > -----Original Message-----
> > From: si-list-bounce@xxxxxxxxxxxxx
> > [mailto:si-list-bounce@xxxxxxxxxxxxx] On
> > Behalf Of Donald Telian
> > Sent: Friday, March 18, 2005 9:49 AM
> > To: Syed Huq
> > Cc: si-list@xxxxxxxxxxxxx
> > Subject: [SI-LIST] Re: package SSN model accuracy
> > requirements, now
> > Behavioral Modeling
> >
> > Syed,
> >
> > Thanks for helping us refine terminology.
> >
> > Let's be careful not to mix up concepts with tools
> > and languages.  While
> > it seems you want to confine "macromodeling" to
> > something only Cadence
> > tools do, I believe that if you look around you will
> > find that
> >
> > macromodeling =3D=3D behavioral modeling
> >
> > ...and the two are used interchangeably within
> > industry and academia.
> > If you reference every one of the research papers I
> > give links to on
> > slide 22 - as well as the quote from Franzon - you
> > will note that they
> > all use "macromodeling" in both the titles and paper
> > texts to describe
> > their work in "behavioral modeling".  They use the
> > term "macromodeling"
> > to refer to various implementations spanning
> > equation-based Hspice
> > models, VHDL-AMS models in ICX, SPICE models in IBM
> > PowerSPICE, etc.
> > And yes, "macromodeling" can also be done in
> > DML/Espice, Pspice,
> > Berkeley SPICE, Verilog-AMS, and a variety of other
> > tools/languages.
> >
> > Regarding your comments about Cadence tools, I'll
> > acknowledge that it
> > takes a fair amount of skill and craftsmanship to
> > construct effective
> > macromodels.  However, I've yet to find a device
> > that can not be
> > behaviorally modeled in the environment.  Kumar and
> > others have shown
> > considerable skill in adding things like
> > time-controlled sources, nth
> > derivative processing, multi-dimensional table-based
> > controlled sources,
> > and such into our tools to make this possible.
> > Thanks to their
> > contributions, users have had a number of published
> > successes (slide 21)
> > and implementing your BIRD95 ideas would be a snap.
> > If your team is
> > struggling with other particular implementations,
> > Cadence can provide
> > you assistance in a variety of ways and would be
> > happy to do so.
> >
> > During the years while it's simpler for IC companies
> > to encrypt and ship
> > their transistor layout-derived model, and model
> > users continue to ask
> > for faster behavioral solutions, I think it's
> > important that we keep an
> > open mind regarding the options before us.  Working
> > together, let's
> > encourage those working on the various tools and
> > languages as new
> > solutions emerge and keep the comments on the
> > positive side.  It's a
> > challenging problem to solve.
> >
> > Donald
> >
> >
> > >-----Original Message-----
> > >From: Syed Huq [mailto:shuq@xxxxxxxxx]=20
> > >Sent: Wednesday, March 16, 2005 2:44 PM
> > >To: Donald Telian
> > >Cc: si-list@xxxxxxxxxxxxx
> > >Subject: Re: [SI-LIST] Re: package SSN model
> > accuracy=20
> > >requirements, nowBehavioral Modeling
> > >
> > >Donald,
> > >
> > >Certainly our definitions are different. What you
> > described is=20
> > >what I would call behavioral modeling. There is a
> > difference=20
> > >and let me explain with an example:
> > >
> > >Let's pick your tool. SpecctraQuest uses this DML
> > scheme(which=20
> > >I call macromodeling), in practical applications
> > such as ours,=20
> > >we see the tool choking in many aspects in it's use
> > of macro=20
> > >modeling. Hence I would like to stay far away from
> > such an=20
> > >approach(macromodeling I mean). Maybe version 20.x
> > would have=20
> > >it all sovled. I am sure there are 10s of other
> > application=20
> > >where is excels quite well too.
> > >
> > >On the other side, behavioral modeling (atleast
> > again in our
> > >applications) has already proven it's capabilities
> > many times=20
> > >over and I am more than open to explore further
> > with tools to=20
> > >see how BIRD95 could be implemented thru
> > "behavioral modeling".
> > >
> > >Syed


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