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

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From: Greim, Michael (mgreim@mc.com)
Date: Thu Dec 07 2000 - 10:26:04 PST


Bo,

Todd makes some very good points and of course there
is the ole Murphy truth of 'If you don't check it, it
is broken by definition'.

Humans are fallable, what you said/wrote and what the
cad designer heard can be different, Tools don't always
properly interpret what you tell them and sometimes the
designer turns off the DRCs so things 'look' correct but
in fact are not.

Secondly there may be other effects that were not properly
modeled from the beginning, such as crosstalk, signals
crossing plane splits, signals with unanticipated impedance
changes, or power plane noise. All of these issues may be
uncovered with post route sim.

So, the short answer is, it depends. Almost always a certain
amount of post route sim is prudent. Then again some folks
enjoy the thrill of tracking down problems in the lab. I pre
fer having the debug/validation being as short as possible so
I can move on to the next fun thing. But that's just me.

best regards,

MG

-----Original Message-----
From: Todd Westerhoff [mailto:twester@hhnetwk.com]
Sent: Thursday, December 07, 2000 12:36 PM
To: si-list@silab.eng.sun.com
Subject: RE: [SI-LIST] : Post layout simulations: "To do or not do them
and when to do them..."

Bo,

You haven't answered the $99.95 question:

When you did your pre-layout analysis, you came up with layout rules, right?
Well, what were they, how did you communicate them to the CAD folk, and what
steps (if any) have been taken to ensure those rules were followed?

If your pre-layout analysis was exhaustive, AND the rules were passed to CAD
AND thoroughly followed, THEN the role of post-layout analysis is reduced.
BTW, "exhaustive" analysis isn't defined on purpose, because it varies
wildly by application. And, "reduced" is also undefined here, for the same
reason.

The broad point here is that if you don't have any good way of knowing that
layout rules have been followed short of simulation - then, yes, you have to
do traditional, complete post layout analysis. If you KNOW your rules have
been followed, then the types of things you look for in post-route analysis
changes.

Todd.

-----Original Message-----
From: owner-si-list@silab.eng.sun.com
[mailto:owner-si-list@silab.eng.sun.com]On Behalf Of Bo
Sent: Thursday, December 07, 2000 11:32 AM
To: si-list@silab.eng.sun.com
Subject: [SI-LIST] : Post layout simulations: "To do or not do them and
when to do them..."

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, isnt 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 cant 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 cant read automatically. But let assume that I can somehow
convert from one file format to another in fairly easy process (that
wouldnt
make mistakes when converting). So I am left with two solutions to perform
my
simulations:

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.

Regards,
Bo

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