It is from a long time back so things are fuzzy now but here is my recollection. I wanted to determine the source of the corrupted temperature readings but every pin on the MCU were utilized so no easy way to read it out. I noticed one pin was used just for an activity LED so I modified that routine to pump out a serial bit pattern read from a variable. Then I instrumented various parts of the code related to the temperature measurement algorithm to see the earliest point where it was corrupted. I traced it to the interrupt routine that reads the temperature sensor. In investigating that code I modified it slightly which caused the problem to disappear. Experimenting showed that a particular instruction would be the source of the problem. At that point I consulted with the ASIC designer and the confirmed the bug and identified the scope of the issue. Also should add that various debugging hardware like ICE didn't work maybe due to timing differences.
