Page 88 - The Art of Designing Embedded Systems

Real Time Means Right Now! 75

theory, in place of which they employed their own experience, that of their
colleagues, and rule of thumb.”
The flip side of a “feel” for a problem is an ability to combine that
feeling with basic arithmetic skills to very quickly create a first approxi-
mation to a solution, something often called “guesstimating.” This won-
derful word combines “guess”-based on our engineering feel for a
problem-and “estimate”-a partial analytical solution.
Guesstimates are what keep us honest: “200,000 bits per second
seems kind of fast for an 8-bit micro to process” (this is the guess part);
“Why, that’s 1/200,000 or 5 microseconds per bit” (the estimate part).
Maybe there’s a compelling reason why this guesstimate is incorrect, but
it flags an area that needs study.
In 1995 an Australian woman swam the 110 miles from Havana to
Key West in 24 hours. Public Radio reported this information in breathless
excitement, while I was left baffled. My guesstimate said this is unlikely.
That’s a 4.5 MPH average, a pace that’s hard to beat even with a brisk
walk, yet the she maintained this for a solid 24 hours.
Maybe swimmers are speedier than I’d think. Perhaps the Gulf
Stream spun off a huge gyre, a rotating current that gave her a remarkable
boost in the right direction. I’m left puzzled, as the data fails my guessti-
mating sense of reasonableness. And so, though our sense of “feel” can
and should serve as a measure against which we can evaluate the mounds
of data tossed our way each day, it is imperfect at best.
The art of “guesstimating” was once the engineer’s most basic tool.
Old engineers love to point to the demise of the slide rule as the culprit.
“Kids these days,” they grumble. Slide rules forced one to estimate the so-
lution to every problem. The slide rule did force us to have an easy famil-
iarity with numbers and with making coarse but rapid mental calculations.
We forget, though, just how hard we had to work to get anything
done! Nothing beats modem technology for number crunching, and I’d
never go back. Remember that the slide rule forced us to estimate all an-
swers; the calculator merely allows us to accept any answer as gospel with-
out doing a quick mental check.
We need to grapple with the size of things, every day and in every ave-
nue. A million times a million is, well, The gigahertz is a period of one
nanosecond. A speed of 4.5 miles per hour seems high for a swimmer. It’s
unlikely your interrupt service routine will complete in 2 microseconds.
We’re building astonishing new products, the simplest of which have
hundreds of functions requiring millions of transistors. Without our amaz-
ing tools and components, those things that abstract us from the worries of
biasing each individual transistor, we’d never be able to get our work done.