Re: [SI-LIST] : Edge rates

Roy Leventhal (Roy_Leventhal@mw.3com.com)
Mon, 19 Jul 1999 14:03:46 -0500

Mike,

I'll take the position that data sheet specsmanship problems have relatively
little to do with technology issues and quite a lot to do with people protecting
themselves. I'll explain myself.

But first, let me say, up front that switching behavior into a transmission line
would be a welcome addition to data sheets. I say addition, because people tend
to cling to baselines. In this case switching behavior into a lumped load. This
gives them, so they feel, a comparison with something they are familiar with and
understand. Then also, folks who are standards and procedures oriented like
component (yep, been there - done that) engineers, test engineers, etc., are
control oriented and like repeatability. So, even if people start testing into
transmission lines don't expect the old methods to disappear any time soon.

Your story brings to mind a recent simulation I ran where the question was the
correct Vol - the value from the data sheet or the prediction of the IBIS model?
The issue was simulating the correct values for noise margin. Inquiring into
this at the Silicon vendor we were informed that "The value on the data sheet is
guard banded by 200 mV because we guarantee it."

Of course, try collecting on those guarantees even if you test for them. I can
recall characterizing processes and publishing many semiconductor device data
sheets. Disclaimers were standard fare in any of them as they are today. Or,
open any IBIS or SPICE file and look at the disclaimers therein. All of what I
am asserting is, I believe, a result of the usual adversarial nature of the
relationship between supplier and customer. Also, a result of people wishing to
avoid responsibility for their own actions. Specifically, I include specs that
are set far wider than they need to be, parts that are misapplied in a lazy
design and anything else where it doesn't take rocket science to come up with a
better answer. As far as I'm concerned the whole specsmanship discussion plus
the "why do anything until the customer absolutely demands it" discussions sound
like rationalizations for doing a second class job.

If you want to break out of the adversarial loop and have the numbers to mean
anything at all, you have to look two things:

1. Communicate what you really need in a Purchase Spec under your control.

2. Get buy-in to that Spec by building trust, understanding - especially
technical understanding, etc., with the supplier. Be sure you both listen
absorbtively.

I can only re-emphasize the many fine suggestions by D.C., yourself and many
others regarding specs and the exchange of technical information.

But, there are three other points I've been trying to make:

1. The earlier the "what-if" simulation the better even if you have to "suppose
(invent)" the model characteristics that will work, loosely based on a real
device.

2. Early efforts to desensitize the design to device variations and
characteristics - shared with your device supplier.

3. Then listen to your supplier and respond to their suggestions until you come
to the most mutually beneficial agreement = specification.

Finally, take steps to deal with those things that cannot be solved effectively
at the device level and don't believe that expected problems will magically cure
themselves.

Regards,

Roy

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