Page 139 - The Art of Designing Embedded Systems

126 THE ART OF DESIGNING EMBEDDED SYSTEMS

the system had an unexpected reset. Don’t use a bit of clever watchdog
code to compensate for software or hardware glitches.

Should embedded systems have a reset switch?
It seems almost traditional to put a reset switch on the back
panel of an embedded system. When something horrible happens, hit
the reset and retry! Doesn’t this make the customer feel that we don’t
trust our own products? Electronic systems never had reset switches
until the introduction of the microprocessor. Why add them now?
A reset switch is no substitute for flaky hardware. It’s pretty
easy (or, at least possible) to design robust, reliable microprocessor
circuits. Any failure is most likely to be a hard fault that a simple
reset will not cure.
This argument implies that a reset switch is mostly useful to
cure software bugs. We have a choice of writing 100% reliable code
or adding some sort of an escape hatch for the user. I hereby pro-
claim, “We shall all now write correct code.”
The problem is now cured.
OK, so perhaps a bug just might creep in once in a while. My
feeling is that a reset switch is still a mistake. It conveys the message
that no one really trusts the product. It’s much better to include a
very robust watchdog timer that asserts a good, hard reset when
things fall apart. The code might still be unreliable, but at least we’re
not announcing to the world that bugs are perhaps rampant. Re-
member when Microsoft eliminated the Unexpected Application
Error message from Windows 3.1 . . . by renaming it?
No watchdog is perfect, but even a simple one will catch 99% of
all possible code crashes. Combine this percentage with the (ideally)
low probability of a software crash, and the watchdog failure rate falls
to essentially zero.

Making PCBs
In the bad old days we created wire-wrapped prototypes because they
were faster to make than a PCB, and a lot cheaper. This is no longer the
case. Except for the very smallest boards, the cost of labor is so high that
it’s hard to get a wire-wrapped prototype made for less than $500 to sev-
eral thousand dollars. Turnaround time is easily a week.