Page 181 - The Art of Designing Embedded Systems

168 THE ART OF DESIGNING EMBEDDED SYSTEMS

is not toggling. Scoping the pins will prove this one way or the other,
though now you’ll need another hypothesis and experiment to figure out
why the selects are not where you expect to see them.
Sometimes, though, the hypothesis-experiment model should be
much less casually applied. When Intel started shipping the XL version of
the 186 (supposedly compatible with the older series), I had a system that
just would not start with this version of the CPU. Scoping around showed
the processor to be stuck in a weird tristate, though all of its inputs seemed
reasonable. One hypothesis was that the 186XL was not coming out of
reset properly, an awfully hard thing to capture since reset is a basically
non-scopable one-time event. We finally built a system to reset the proces-
sor repeatedly, to give us something to scope. The experiment proved the
hypothesis, and a fix was easy to design.
Note that an alternative would have been to glue in a new reset circuit
from the start to see if the problem went away. Problems that mysteriously
go away tend to mysteriously come back; unless you can prove that the
change really fixed the problem, there may still be a time bomb lurking.
Occasionally the bug will be too complicated to yield to such casual
troubleshooting. If the timing of a PAL will have to be adjusted, before
you wildly make changes visualize the new timing in your mind or on
a sheet of graph paper. Will it work? It’s much faster to think out the
change than to actually implement it. . . and perhaps troubleshoot it all
over again.
Rapid troubleshooting is as important as accurate troubleshooting.
Decide what your experiment will be, and then stop and think it through
once again. What will this test really prove? I like experiments with binary
results-the signal is there or it is not, or it meets specified timing or it
does not-since either result gives me a direction to proceed. Binary re-
sults have another benefit: sometimes they let you skip the experiment al-
together! Always think through the actions you’ll take ufrer the experiment
is complete, since sometimes you’ll find yourself taking the same path re-
gardless of the result, making the experiment superfluous.
If the experiment is a nuisance to set up, is there a simpler approach?
Hooking up 50 logic analyzer probes or digging through a million trace
cycles is rather painful if you can get the same information in some easier
way. I’d hate to be in a lab without a logic analyzer, since they are so use-
ful for so many things . . . but I try to keep it as the tool of last resort, since
most often it’s possible to construct an easier experiment that is complete
in a fraction of the time it takes to connect the LA.
Don’t be so enamored of your new grand hypothesis that you miss
data that might disprove it! The purpose of a hypothesis is simply to