Page 160 - The Art of Designing Embedded Systems

Troubleshooting Tools 147

OrCillO~opeS

Emulators, ROM monitors, and the like are great for viewing your
code from the perspective of the CPU. Their tentacles into your target sys-
tem stop at the CPU socket, so events occurring beyond that point (say, in
an YO device) are almost invisible. You can see the IN and OUT instruc-
tions and the transferred data, but it’s pretty hard to check out timing rela-
tionships, or how the software interacts with the hardware.
Sure, most of these tools have external inputs that you can couple to
any point in the system. Few programmers use them. Perhaps this is be-
cause the display is so static. You have to actively recollect data and then
tediously sort it all out. For example, if you feed an external input to a real-
time trace buffer, you’ll collect tons of bus activity that may or may not be
important.
If all you really care about is the relationship between two events
(say, a switch closure and the resultant interrupt), why dig through thou-
sands of cycles? It is important to arm ourselves with as many tools as pos-
sible. No one tool is perfect for every problem.
One of my all-time favorite software debugging tools is the oscillo-
scope, colloquially known as the “scope.” Hardware guys seem to have a
scope attached as a pseudopod to one arm. Any development lab is invari-
ably filled with benches of scope-happy troubleshooters probing the mys-
teries of some electronic marvel. The software community seems less
comfortable with this tool, which is a shame because it can painlessly yield
crucial information about the operation of your code.
A scope is really nothing more than a device that displays one or
more signals. Most can simultaneously show two independent values.
The scope’s raison d’etre is displaying the signals’ voltage (ampli-
tude) over time.
A simple time-varying signal is the power coming from your wall
outlet. This is a 60-Hz sine wave (i.e., the voltage smoothly rises from 0 to
120 and back to zero again 60 times a second). It moves too fast to follow
with a voltmeter. On a scope display, the waveform’s voltage at any point
in time is crystal clear.
Software folks used to working with only a keyboard are sometimes
intimidated by the sea of knobs on any decent scope’s front panel. A bit of
experience makes working with this tool natural.
From the user’s standpoint the average scope has three major sec-
tions. A “vertical” amplifier sets the display’s up/down limits. The “hori-
zontal’’ portion controls the beam’s lefvright scanning. “Trigger” circuitry
synchronizes the scan to your input waveform.