Page 175 - The Art of Designing Embedded Systems

162 THE ART OF DESIGNING EMBEDDED SYSTEMS

matically recovers from the watchdog reset, you surely need some
way, during debug, to see that the time-out occurred.
When your tools are not working well, or perhaps you’ve simply
lost faith in them, you can still track overall program flow by as-
signing an 8-bit number to each important function. Output this
number to the debug port when the function starts. Collect the data
in the logic analyzer and you’ll instantly see what executes when,
and for how long.
Connect one or more of the more YO bits to LEDs, and instrument
the code to signal system state. Most tools do a poor job of read-
ing out state; generally you’ll have to stop the code or something
similar. The LED bank instantly shows things like, “It’s doing
WHAT???!!!!!”
If your main debug strategy revolves around a full-blown emulator,
if at all possible go ahead and add the BDM or JTAG connector (if the
CPU supports it). The cost is vanishingly small, and the option of doing
BDM debugging when the ICE falls flat or fails may save a lot of money
and time.
Conversely, if a BDM will be the main tool, add a connector (like the
Mictor) so that you can connect a logic analyzer for tracking real-time
events. It’s so terribly difficult to use analyzers via their standard multitude
of clips that we leave it as a last resort; if it’s easy to connect, we’ll use the
tool at the appropriate times.

ROM Burnout

Remember that every tool affects system operation in some manner.
Never wait until the night before shipping to test the system from ROM.
Make burning a ROM or loading the Flash a regular part of the test proce-
dure.
Debugging tools invariably have a different size of emulation
RAM than your target system’s ROM space (this is true using an
ICE or a ROM emulator, or even if you relink your code to run
from your system RAM area). If the code grows to exceed target
ROM space, it may run just fine from the (probably bigger) emu-
lation RAM area.
The compiler’s runtime package or constants might be improperly
initialized. Many C compilers require a startup procedure that
copies some critical variables to RAM. When you’re debugging,
you’ll generally replace system ROM with RAM merely to support