RE: [SI-LIST] : Clamp diodes in models (was Input switching threshold & CPCI)

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From: jrbarnes@lexmark.com
Date: Mon Jan 10 2000 - 13:22:20 PST


> We've measured 10's of thousands of I/O buffers to make models around here.
> Our internal spec is to push to part to at least 1 volt past the rail or at
> least 100 ma of current. In most cases we exceed these internal specs. It
> is rare that this causes any problems in the parts under test and in the
> cases where it has happened it seemed to be generic to a particular
> manufacturer's process. I cannot say what will happen over time from
> continous abuse like this. If you are seeing lots of failures then you are
> really putting more current into your devices than you may think.

Tom,
We had a field problem that showed up some 5 years after we had introduced our
first Token-Ring adapter, and about a year after we had ended production of it.
One customer had problems with these two-to-three year old first-generation
adapters "conking out" at random. They could revive an adapter just by
unplugging/replugging the power supply into the wall outlet. They didn't mind
this when it occurred maybe once every three months, but noticed that once it
occurred, that the adapter would act up at shorter and shorter intervals. When
it got down to a couple of weeks between the adapter conking out, they would go
ahead and replace it. After a while they noticed that they had quite a few of
these flaky adapters piling up in their repair shop.

We eventually traced the problem to a die-shrunk PAL, one input of which was
slowly damaged by a combination of:
* High supply voltage, close to the +5V +5% upper limit at this customer.
* High ambient temperature.
* Approximately 10^14 overshoot pulses of roughly 1.4V (swinging -1.4V to
+6.6V) on the rising-
   and falling-edges of this signal.

We wound up replacing hundreds of this customer's adapters for free, even though
they were well out of warranty, in the name of customer good-will.

Ever since then, before we let a new network adapter get out of Engineering
Verification Test (EVT), we check every signal for excessive overshoot on
rising- and falling-edges with a fast oscilloscope. We check resets, clocks,
and strobes at every input pin. We check other signals at at least one point,
preferably as far as possible from the driver pin(s). If the overshoot on
rising- or falling-edges exceeds 0.7V we flag the signal for further attention,
and may add/ adjust terminating resistors to reduce the overshoot. For
double-sided and 4-/6-layer cards, a 39-ohm series resistor within an inch of
the driver pin usually does the job. We sometimes go as low as 22 ohms, or as
high as 56 ohms, trading off risetime/falltime against the observed overshoot.
                                   ` John Barnes Advisory
Engineer
                                                   Lexmark International

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