3.2 Program Counter – Component 1.3

Purpose of the test: Detect stuck bits in the program counter.

A correctly functioning program counter is important for any software to successfully run on an MCU. The program counter cannot be tested directly for stuck bits since any such test will rely on the program counter to function correctly in the first place.

Acceptable measures for Fault detection are functional test, periodic self-test, independent time-slot monitoring, or logical monitoring of program sequence.

The preferred solution is to use the watchdog for indirect time-slot monitoring of the application. If a program counter Fault occurs, the watchdog timer will either be reset at the wrong time or not reset at all, eventually causing a device reset. Since it is an integral safety feature, the tinyAVR® 1-series watchdog must be tested for proper operation in both regular and window mode before it can be relied on for catching program counter faults.

After the watchdog has been tested, it should be enabled in window mode at all times. The closed period should be at least as long as the open period, that is, at least 50% of the total period.