olin_piclist@embedinc.com (Olin Lathrop) wrote:
> Hazelwood Lyle wrote:
> > It was myself who sent in the reference to AN910 a few days ago,
> > which is unclear at best when describing the need for multiple
> > voltage verification.
> 
> There is a lot of confusion around this issue, and I'm not totally
> clear on the real answer either.

[snip]

> I'm going to continue running two verify passes on anything I'm
> responsible for shipping. The cost of a single field failure is
> probably worth the extra few seconds on tens of Kunits.

Think of it as a specific instance of the more general process of margin
testing your overall design.

You DO margin test your designs, right? That is, operate samples of the
systems at all eight "corners" defined by supply voltage limits, frequency
limits and temperature range. This will idenfify any weak areas both in
individual components and in how they're put together in the overall
design.

Back when I was building motherboards for engineering workstations, we
would run the boards at +/- 10% the DC supply level (most components were
rated at +/- 5%). We would purchase crystals/oscillators that were +/- 5%
the nominal design freqeuncy. And we would put them in the environmental
chamber and see how far beyond our published temperature range they would
operate (at both ends). The additional test margins, beyond the published
specs, allowed us to be sure that any measurement inaccuracy in the testing
process did not affect our ability to meet our own published specs.

Anyway, verifying floating-gate memory technology at its Vdd limits is
a specific instance of margin testing, which covers both the programming
process itself (the amount of charge on the gates) and the readback
thresholds in the sense amplifiers. With modern chips, these two effects
may be indistinguishable, and may be covered by margins built into the
on-chip programming algorithms.

-- Dave Tweed
-- 
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist