Mike Hord wrote...

>Everyone always says that- "You're so lucky it works at all!"
>NONE of my electronics labs even mentioned using bypass caps.

Nevertheless, if you make a habit of building stuff without
bypass caps (or without enough of them) and it still works, you
indeed ARE lucky.

The basic problem of bypass caps is that the kinds of
malfunctions you see with inadequate bypassing a) tend to be very
intermittent (meaning they may show up for the first time not on
the test bench, but out in the field), b) usually exhibit
symptoms that make absolutely no sense whatsoever, and c) are
actually caused by physical phenomena (i.e., glitches on the VDD
pins of one or more ICs) that are EXTREMELY difficult to observe
directly without very expensive equipment.

These form a deadly combination that can result in hours and
hours of wasted time troubleshooting what appear to be baffling,
oddball problems with symptoms that constantly change and appear
unrelated to one another.  And time equals money.  And capacitors
are cheap.  And a mind is a terrible thing to lose on account of
a couple of missing capacitors.

>Is it simply a matter of no caps + low speed = success?  After all,
>most of my labs involved EXTREMELY low speeds (10 Hz or less).

When it comes to logic devices, clock speed has absolutely,
positively, completely, and utterly NOTHING to do with it.  The
reason for this is that the glitches occur on the clock edges
when the signals are changing state, not during the interval
between edges.

The only difference between an inadequately bypassed circuit
being clocked at 10 Hz and one being clocked at 10 MHz is that
the latter can screw up more frequently.

Dave D.

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