Page 89 - The Art of Designing Embedded Systems

76 THE ART OF DESIGNING EMBEDDED SYSTEMS

Though the abstraction distances us from how things work, it enables us to
make things work in new and wondrous ways.
The art of guesstimating fails when we can’t or don’t understand the
system. Perhaps in the future we’ll need computer-aided guesstimating
tools, programs that are better than feeble humans at understanding vast in-
terlocked systems. Perhaps this will be a good thing. Maybe, like double-
entry bookkeeping, a computerized guesstimator will at least allow a
cross-check on our designs.
When I was a nerdy kid in the 196Os, various mentors steered me to
vacuum tubes long before I ever understood semiconductors. A tube is
wonderfully easy to understand. Sometimes you can quite literally see the
blue glow of electrons splashing off the plate onto the glass. The warm
glow of the filaments, the visible mesh of the control grids, always con-
jured a crystal-clear mental image of what was going on.
A 100,000-gate ASIC is neither warm nor clear. There’s no emo-
tional link between its operation and your understanding of it. It’s a pla-
tonic relationship at best.
So, what’s an embedded engineer to do? How can we reestablish this
“feel” for our creations, this gut-level understanding of what works and
what doesn’t?
The first part of learning to guesstimate is to gain an intimate under-
standing of how things work. We should encourage kids to play with tech-
nology and science. Help them get their hands greasy. It matters little if
they work on cars, electronics, or in the sciences. Nurture that odd human
attribute that couples doing with learning.
The second part of guesstimation is a quick familiarity with math.
Question engineers (and your kids) deeply about things. “Where did that
number come from?” “Do you believe it . . . and why?’
Work on your engineer’s understanding of orders of magnitude. It’s
astonishing how hard some people work to convert frequency to period,
yet this is the most common calculation we do in computer design. If you
know that a microsecond is a megahertz, a millisecond is 1000 Hz, you’ll
never spend more than a second getting a first-approximation conversion.
The third ingredient is to constantly question everything. As the
bumper sticker says, “Question authority.” As soon as the local expert
backs up his opinion with numbers, run a quick mental check. He’s prob-
ably wrong.
In To Engineer Is Human (1982, Random House, New York), author
Henry Petroski says, “Magnitudes come from afeel for the problem, and
do not come automatically from machines or calculating contrivances.”
Well put, and food for thought for all of us.