Page 187 - The Art of Designing Embedded Systems

174 THE ART OF DESIGNING EMBEDDED SYSTEMS

On another system the processor’s internal VO lost its con-
figuration every few minutes; all of the internal registers changed to
default states, yet the program continued to run fine, though all sys-
tem I/O was idled.
The culprit was again a reset glitch. In this case the pulse was
created by PCB crosstalk. Only one nanosecond wide, it was too
short to catch reliably on a 500-MHz logic analyzer. We sampled
dozens of the erratic resets, eventually creating a statistical view of
the glitch.
Though every processor has a minimum reset time at least
several clocks long, even very short glitches can drive CPUs and
peripherals into bizarre modes. The trick is identifying the source
of the problem. . . and never ignoring erratic results or hard-to-
diagnose symptoms.
Bob Pease, of analog design fame, recommends, “When things
are acting funny, measure the amount of funny.”
Diagnose all glitches. If the system behaves oddly, something
is wrong. Find the problem, or your customer will.

Learn to Estimate
At the peril of sounding like one of the ancients, I do miss the culture
of the slide rule. Though accurate answers might have been elusive, we did
learn to estimate the answer for every problem before attempting a solu-
tion. Alas, it’s a skill that is fading away.
Calculator abuse-computing without thinking-is now too in-
grained in our society to waste effort fighting. Bummer. Other instruments,
though, also tempt us to mentally coast, to do things without thinking.
Take the scope: I can’t count the times an engineer mentioned that he sees
the signal . . . but has no idea, when I ask, about the width of the pulse. Is
it 1 nsec? 1 pec? Perhaps a second wide?
Timing is critical in computers, yet too many of us use the scope as a
sort of logic probe. “Hey, the signal is there!” Which signal? If you expect
a lO-psec pulse every msec, then any deviation from that norm is simply
wrong. Know what to expect, and then ensure that the waveforms are ap-
proximately correct. A misused scope will generate a morass of misinfor-
mation.
Estimate the performance of firmware before writing it. Sure, it’s
tough to know how many microseconds an as-yet-unwritten function will