Page 164 - The Art of Designing Embedded Systems

Troubleshooting Tools 1 5 1

Here’s another tip: When you’re using a scope, if a signal looks
weird, maybe there’s something wrong! Avoid the temptation to rational-
ize the problem. Instead of blaming the signal on a lousy ground, quickly
connect that ground clip and test your assumption.
Never accept something that looks awful. Either convince yourself
that it’s actually OK, or find the source of the problem.
Walk through your lab. You’ll find that most of the digital folks have
their vertical amplifiers set to 2 volts/division, which eases displaying two
traces simultaneously. Unfortunately, too many of us seem to think the
vertical gain knob is welded into position. It’s hard to distinguish a valid
zero from one drooling just a little too high with so little resolution. Flip to
1 V/division occasionally to make sure that zero is legitimate.
Every instrument is a lying beast, a source of both information and
disinformation. The scope is no exception. A 100-MHz scope will show
even a perfect 50-MHz clock as a sine wave, not in its true square form.
Digital scopes exhibiting aliasing sweep too slowly (below the Nyquist
limit) for a given signal, and that 50-MHz clock may look like a perfect
1 -kHz signal, causing the inexperienced engineer to go crazy searching for
a problem that just does not exist. Try this experiment: measure a 10- or
20-MHz clock on a digital scope. Crank the sweep rate slower and slower.
You’ll inevitably reach a point where the scope shows a near-perfect
square wave several orders of magnitudes slower than the actual clock fre-
quency. This is an example of aliasing, where the scope’s sampling rate
yields an altogether incorrect display. I’m sure many folks have heard a
claim such as, “This 16-MHz oscillator is running at 16 kHz! Can you be-
lieve it?” Don’t. Check your settings first.
We digital folks deal in ones and zeroes . . . and tristates. Each con-
dition means something. When troubleshooting, you’ve got to know which
of these three (not two) states a node is in. Our best tool is the scope, yet it
is inherently incapable of distinguishing the tristate condition.
In the good old days of LS technology you could be pretty sure a tri-
stated signal would show up at around 1.5 volts-somewhere between a
zero and a one. With CMOS this assurance is gone, yet most engineers
blithely continue to assume that zero volts means zero. It just ain’t so.
My solution is a little tool I made: a 1 k resistor with a clip lead on
each end. Mine is nicely soldered together and covered with insulation to
avoid shorts. To tell the difference between a legal state and high imped-
ance, clip the tool to the node and alternately touch the other end to Vcc
and then ground. If the node moves more than a trifle, something is wrong.
The scope, plus my tool, lets me identify all three possible states. Without