[SI-LIST] Re: Do you ever measure any of the things you model?

Hello, Max,

Yes, most designers (model users as opposed to model makers) know how to
test hardware.  It's figuring out where and why a mismatch occurs that
drives us nuts (SPICE model? IBIS model built from SPICE? simulator?
hardware build? testing?).

IBIS models are known to be an issue.
http://www.vhdl.org/pub/ibis/summits/jan02/bell1.pdf lists some common IBIS
model problems.  Eventually, model users learn which companies put out
useless IBIS models, and simply cross them off their list of suppliers.

In a good methodology (used by many (all) of the best IBIS providers), the
models and hardware are checked.  Some use dedicated test boards, others
wait for prototype boards, some use both.  When a problem does occur, it is
easily debugged.

One problem is companies that assume IBIS model creation can be handed off
to a newbie along with s2ibis3 or another script.  These models ship without
running even ibischk4.  The only other phase of this "methodology" involves
Customer Support.

Another problem is "fabless" companies where models are created from SPICE,
but the company does not make chips or build boards (that's for their
customers to do...).  Unfortunately, model/hardware mismatches become more
difficult to resolve when company boundaries are crossed, although the IBIS
Quality Checklist has made communication of issues easier.
http://www.vhdl.org/pub/ibis/quality_wip/

Finally, some things are non-trivial to model or test, but can have
significant impact on system performance: pwr/gnd current densities near
chips having tens or hundreds of pwr/gnd pins; self-heating of an I/O due to
switching (data pattern); power droop and crosstalk at an I/O due to shared
pwr/gnd pins, crosstalk between vias in a board or package, overclocked I/O
switching, etc.

So we muddle along, produce our designs, and hope for better models.

Lynne

Next IBIS seminar May 1-2, Redmond WA.
"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 Max Artusy
Sent: Friday, March 28, 2008 4:32 PM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Do you ever measure any of the things you model?

I notice that you all are most concerned with your model.

Now by the scientific method, you have a hypothetical solution, produced by
your model.

The next step is to EVALUATE the result.

That is done with a very good scope.

No one ever seems to comment on this, leading me to suspect you only model.

You know they did that with the Hubble space telescope.

That technique produced one of the most expensive service calls in history.

Once modeled, they should have TESTED the thing.

Testing goes hand in hand with modeling, on the assumption that the model is
incomplete.

No measurement, saves lots of time. Any naive manager just loves to save
test time.

"Our models are perfect, you know" says he. ( He missed point of Scientific
method, entirely)

So what kind of scope and what kind of probing technique do you use?

How do you ground your probe setup to the PCB, as a place to start? 

How much noise on the ground??

What is the exact construction of the probe interface to the PCB?

This all counts, in high performance probing.


Max Artusy





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