Page 200 - Embedded Microprocessor Systems Real World Design

If you are using an emulator, often there is an external input that you can break-
point on. By connecting the trigger output of the logic analyzer or DSO to this
external input, you can force a breakpoint.
If you are not using an emulator or have no external breakpoint input, you can
stop the software by using an otherwise unused interrupt, using a timer as an inter-
rupt (see Chapter 5), or even using an NMI. Another possibility is to use the reset
input, but you need a means to keep the software from reinitializing everything
after the reset. Using the reset technique will not let you see what the software does
after the event, but it will let you look at the state of the software just before the
event.
Finally, you can use this technique to capture the contents of a rotating trace
buffer when a real-world event occurs. If you use any of these techniques to connect
a logic analyzer or DSO trigger to your system, be sure the trigger output is com-
patible. You may need to invert the polarity or change the pulse width to make
everything work.

Memory Dumps

I once worked with a consultant who was interfacing an add-on to our equipment
for a third-party reseller. My job was to make sure that nothing we did or failed to
do would keep him from doing his job. This individual had an interface subsystem
that consisted of dual microcontrollers communicating via hardware FIFO buffers.
Every now and then, the entire thing would lock up for no apparent reason.
He had no tools except a fairly simple logic analyzer and the stare-at-the-code
method of debug. At that time, no good emulators were available for the pro-
cessor he was using. The entire project got so far behind schedule that the reseller
sent out its ace troubleshooter to get things going. The troubleshooter was aggres-
sively proactive and carried a custom-made wooden briefcase. He even went so far
as to order, on the company credit card via overnight delivery, an emulator that
was supposed to be the best there was. That was when we found out nothing was
suitable.
The consultant told me that if he could view the internal memory contents, he
could fix the problem. We went over an inventory of what he had available, which
was little. The board had no spare output ports but one unused interrupt input.
We solved the problem by connecting a push-button switch to the interrupt. When
the button was pressed, generating an interrupt, the processor produced as output
three sync bytes in a sequence that would never occur in normal operation. Then
the processor sequentially wrote the entire contents of the internal RAM to an
external register. The logic analyzer, connected to the register, waited for these

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