Page 165 - The Art of Designing Embedded Systems

152 THE ART OF DESIGNING EMBEDDED SYSTEMS

the tool I’m guessing, and guessing while troubleshooting always sends
you down time-consuming blind alleys.
You can use a variation of this approach when troubleshooting an in-
termittent problem. If the silly thing refuses to fail when you’re working on
it-a sure bet, given the perversity of nature-run your fingers over the
board’s pins. A purely digital board should continue to run despite the
slight impedance changes brought about by your fingers, yet these may be
enough to drive a floating pin to the other state, possibly creating the fail-
ure you are looking for.
On SMT boards it’s tough to get at a device’s pins. If there’s one pin
you are suspicious of, touch it with an X-Acto knife. The sharp blade will
precisely align with any tiny pin, and its metal handle will conduct your
body impedance to the node. Sometimes 1’11 connect my trusty pull-
up/pull-down clip lead to the knife itself to exercise the node more deter-
ministically.
No scope will give decent readings on high-speed digital data unless
it is properly grounded. I can’t count the times technicians have pointed
out a clock improperly biased 2 volts above ground, convinced they found
the fault in a particular system, only to be bemused and embarrassed when
a good scope ground showed the signal in its correct 0- to 5-volt glory.
Yet most scope probes come with crummy little ground lead alliga-
tor clips that are impossible to connect to an IC. Designers all too often in-
sert a clip lead in series just to get a decent “grabber” end. Those extra 6 to
12 inches of ground lead will corrupt your display, sometimes to such an
extent that the waveform is illegible. Cut the alligator clip off the probe and
solder a micro grabber on in its place.
Ask an experienced scoper to work with you for a couple of hours.
Have the mentor randomly shuffle the controls; then try to bring the dis-
play back and stabilize it. Try probing around a battery-operated radio
(where there are no dangerous voltage levels!). Look at signals. Fiddle
with the trigger controls and time base to stabilize and examine them.

Fancy Tools, Big Bucks?
As an ex-tool vendor I can’t count the times I’ve heard, “Well, we re-
ally need decent equipment, but my boss won’t let me spend the money.”
It matters little what equipment we’re talking about. Once I wrote an
offhand comment about companies who won’t upgrade computers. An
avalanche of email filled my electronic in-box, from developers saddled
with 386-class machines in the Pentium age. We live in front of our com-
puters, spending hours per day with them. It’s incomprehensible to me that