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

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From: Tom Dagostino (tom_dagostino@mentorg.com)
Date: Mon Jan 10 2000 - 15:50:53 PST


I agree that long term effects can occur and thus my comment about them. I
was assuming, possibly wrongly, that the failures he was seeing were
immediate, like turn the thing on and it goes away. I have seen parts fail
by bringing input to about -0.7V, they latched up very quickly drawing
current up to the current limit set on our power supplies.

Tom Dagostino
ICX Modeling Group
tom_dagostino@mentor.com
503-685-1613

-----Original Message-----
From: owner-si-list@silab.eng.sun.com
[mailto:owner-si-list@silab.eng.sun.com]On Behalf Of
jrbarnes@lexmark.com
Sent: Monday, January 10, 2000 1:22 PM
To: si-list@silab.eng.sun.com
Subject: RE: [SI-LIST] : Clamp diodes in models (was Input switching
threshold & CPCI)

> 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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