A watchdog timer is a special kind of timer commonly found on embedded systems that is used to detect when the running software is hung up on some task. The watchdog timer is basically a countdown timer that counts from some initial value down to zero. When zero is reached, the watchdog timer understands that the system is hung up and resets it. Therefore, the running software must periodically update the watchdog timer with a new value to stop it from reaching zero and causing a reset. When the running software is locked up doing a certain task and cannot update the watchdog timer, the timer will inevitably reach zero and a reset will occur.

Cameri’s Workbench: Auto-reboot a hung Raspberry Pi using the on-board watchdog timer.