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Watchdog Timer

A problem we once had with a Super-8 MCU and a watchdog timer:

We were developing a product based on a Zilog Super-8 MCU and the hardware designer had added a hardware watchdog timer that was triggered by one of the lower address lines, probably a0, assuming that even by executing code alone it would be triggered all of the time when the system was functioning properly.

I was the programmer for the system and after a while we noticed that the system constantly reset some time after booting up (but only after some compilations of the software). What was the case as we found out months (!) later: Some time after booting a loop was executed and some times this loop executed all it's instructions from a small hidden cache (as we assume) and the data used was such that the address line was never changed and therefore didn't toggle the watchdog timer that promptly reset the system after a while.

And if only I had known that there was a watchdog timer in the system I could easily have put a scope on it, but it was added at the last moment and it wasn't in the schematic that I had...


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