[SI-LIST] : Post layout simulations: "To do or not do them and when to do them..."

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From: Bo (bo_pfc@yahoo.com)
Date: Thu Dec 07 2000 - 08:31:49 PST

Hi Everybody,

I am facing a problem that may seem an easy one at first but not so easy when
you think about it in more detail.
Here is a “skeleton” question surrounding the problem:
“Should I do post layout simulation of the board?”
Seems easy to answer this question, isn’t it? Well here are the facts that
make this problem little bit more complicated:
Few weeks ago someone asked me for an opinion: “Should we use tool X from
company Y to do post layout simulation of our boards?” I was to already
opening my mouth to give my opinion when I abruptly stopped to think about the
issues surrounding this question.
Here are the issues:

1) Is doing post-layout simulations waste of time? This must have been the
easiest issue I had to ask myself regarding this problem. The answer was
easily: “NO!” The hard question that followed was: “When post layout
simulations are NOT necessary? Is it when you have a lot of margin, the layout
is not complex (e.g. the board is not well populated), etc?”

2) Why would I have to use for post layout simulations tool X when I already
did pre layout simulations with tool A? In my case I use tool A (spice based
tool) to do most of my pre layout simulations. The greatest quality and at the
same time greatest limitation of spice (my opinion) is that is allows a great
level of detail when defining circuits. You can define all the little things
in the circuit but at end you may not be able to run your simulation due to the
complexity of the circuit you have created. Here comes in play a tool X that
can read a layout of the board (my spice tool can’t do this) and perform
simulations on the actual layout. The problem is that this (and I think other
similar tools; I may be wrong) use IBIS models. Now my models are in spice and
I am not an expert at converting Spice to Ibis (there is I think enough
literature around explaining how to convert Spice to Ibis). So for me to
perform simulations of this board with tool X I would have to convert my models
from Spice to IBIS (not so easy task from my standpoint) and then perform them.
 To make things a little worse may board layout file may be in another format
that my tool X can’t read automatically. But let assume that I can somehow
convert from one file format to another in fairly easy process (that wouldn’t
make mistakes when converting). So I am left with two solutions to perform my

a) Convert my spice models to Ibis and perform simulations using tool X, or
b) Extract a layout using tool X (or any other tool) to get exact layout and
then modify my pre layout spice simulations in order to reflect the real layout
of the board.

Is a) or b) solution better? Typically tool X (by using IBIS models) will
allow way more simulations to be done in shorter amount time than my spice
tool. Yet I am not sure how does this affect accuracy of the simulations. And
then if I am not concerned that much with accuracy of the simulations is there
a point in doing post layout simulations? About the only thing that comes to
mind then regarding tool X is to use this tool to verify that routing was done
properly. Am I correct?

What is your opinion on this problem? Feel free to make any comments. I will
appreciate any comments you might send.


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