Page 195 - The Art of Designing Embedded Systems

182 THE ART OF DESIGNING EMBEDDED SYSTEMS

Delayed sweep is essential when working on any embedded sys-
tem-let’s look at a couple of cases.
Suppose your microprocessor crashes immediately after RESET.
Traditional troubleshooting techniques call for hooking up the logic ana-
lyzer and laboriously examining all of the data and address lines. Person-
ally, I find this to be too much trouble. Worse yet, it tends to obscure
“electrical” problems: the analyzer might translate marginal ones and ze-
roes into what look like legal digital levels. Logic analyzers are great for
purely digital problems, but any problem at power-up can easily be related
to signal levels.
Only a scope gives you a view of those crucial signal levels that can
cause so much trouble. Trigger channel 1 on the RESET input and probe
around with channel 2. Look at READ: every processor starts off with a
read cycle to grab the first instruction or startup vector. You may find a
puzzling phenomenon: if the reset is provided by a source asynchronous to
the processor’s clock (as is the case with an RC circuit, a Vcc clamp, and
even with many watchdog timers), READ will bounce around with re-
spect to RESET. You’ll never get a nice high-resolution view of READ
this way.
Triggering off READ will not help. You need to catch thefirsr read
after reset (to look at the first instruction fetch), not any arbitrary incarna-
tion of the signal . . . and no doubt there will be millions of reads between
resets.
The answer is delayed sweep. Put RESET into the scope’s external
trigger input and fiddle the knobs until you get a stable trigger. (I like to put
one scope channel on the external trigger while doing this initial setup to
make sure the trigger is doing what I expect.) Then connect channel 1 to
your processor’s READ output and crank the time base until it appears
over toward the right side of the display. Go to delayed (A intensified by
B) mode, and rotate the B time base trigger adjustment until the bright part
of the trace starts on the leading edge of the bouncing READ signal.
At this point time base A starts the sweep going on the asynchronous
RESET, and time base B triggers the intensified part of the sweep when the
first READ comes along. Flip the Horizontal Mode switch to B (to show
only the intensified part of the sweep-that part after the B trigger), and a
jitter-free READ will be on the left part of the screen. Cool, huh?
With the now stabilized scope display you can use channel 2 to look
at the data lines, ROM chip selects, and other signals during the read cycle.
It becomes a simple matter to see if the first instruction gets fetched
correctly. A lot has to be perfect for this to happen. Very often a power-up