Page 117 - The Art of Designing Embedded Systems

104 THE ART OF DESIGNING EMBEDDED SYSTEMS

more address lines will toggle), and only then do the read test. Reads will
fail if the refresh logic isn’t doing its thing.
Though DRAMS are typically specified at a 2- to 4-msec maximum
refresh interval, some hold their data for surprisingly long times. When
memories were smaller and cells larger, each had so much capacitance that
you could sometimes go for dozens of seconds without losing a bit.
Today’s smaller cells are less tolerant of refresh problems, so a 1- to 2-sec-
ond delay is probably adequate.

A Few Notes on Sohare Prototyping
As a teenaged electronics technician I worked for a terribly under-
capitalized small company that always spent tomorrow’s money on
today’s problems. There was no spare cash to cover risks. As is so often the
case, business issues overrode common sense and the laws of physics: all
prototypes simply had to work, and were in fact shipped to customers.
Years ago I carried this same dysfunctional approach to my own
business. We prototyped products, of course, but did so leaving no room
for failure. Schedules had no slack; spare parts were scarce, and people
heroically overcame resource problems. In retrospect this seems silly,
since by definition we create prototypes simply because we expect mis-
takes, problems, and, well. . . failure.
Can you imagine being a civil engineer? Their creations-a bridge, a
building, a major interchange-are all one-off designs that simply must
work correctly the first time. We digital folks have the wonderful luxury of
building and discarding trial systems.
Software, though, looks a lot like the civil engineer’s bridge. Costs
and time pressures mean that code prototypes are all too rare. We write the
code and knock out most of the bugs. Version 1.0 is no more than a first
draft, minus most of the problems.
Though many authors suggest developing version 1.0 of the soft-
ware, then chucking it and doing it again, now correctly, based on what
was learned from the first go-around, I doubt that many of us will often
have that opportunity. The 1990s are just too frantic, workforces too thin,
and time-to-market pressures too intense. The old engineering adage “If
the damn thing works at all, ship it,” once only a joke, now seems to be the
industry’s mantra.
Besides-who wants to redo a project? Most of us love the challenge
of making something work, but want to move on to bigger and better
things, not repeat our earlier efforts.