A processor system may operate outside of normal conditions or enter deadlock due to a bug in the program code, incorrect timer, or poor response of external devices, in addition to power supply transients. When the processor deviates from the expected behavior, it may not know it is operating abnormally or needs to be reset. This is when watchdog can solve the problem.
A watchdog is a low cost solution to ensuring the processor is operating correctly. The processor is required to “kick” the dog repeatedly within a designated short time interval to reset the watchdog timer. Otherwise, the supervisory circuit will trigger the system alarm. 1.6s is a common watchdog timeout period used. Other timeouts from 1ms to 1minute are also available. When choosing a timeout period, one should consider: “Before triggering reset, how long can I allow the system to operate in the faulty condition?” Some devices allow switching between longer startup timeout and a normal 1.6s short timeout mode (e.g MAX16056 has normal and extended mode) This allows the system to run long initialization processes at startup, and then reduce the timeout period when the system is under normal operation.