A watchdog timer is a computer hardware or software timer that triggers a system reset, a fail-safe, or other corrective action if the main program, due to some fault condition. The intention is to reset the system back from the unresponsive state into normal operation. The most common use of watchdog timers is in embedded systems, where this specialized timer is often a built-in unit of a microcontroller.
Watchdog timers are used as a trigger fail-safe control system to move into a safety state, such as turning off motors, preventing high-voltage electrical short-circuits, and other dangerous subsystems until the fault is cleared or resolved. These timers improve system reliability. The watchdog timer is a chip external to the processor. However, it could also be included within the same chip as the CPU; this is done in many micro-controllers. In either case, the watchdog timer is tied directly to the processor's reset signal. Expansion card based watchdog timers exist and can be fitted to computers without an on-board watchdog.
These timers increases safety and reliability by monitoring for and alerting the system of software code execution errors. Within an AED, a watchdog (MAX6814 4 pin timer) input detects a transitional edge, the internal watchdog timer clears and restarts, then begins counting again. If the watchdog timer exceeds the watchdog timeout period (1.6s typ), the active-low, push-pull watchdog output asserts for the watchdog pulse period (140ms min) to alert the system of the fault. Another example, is AED shutting down during its self-test when it checks its internal Joules discharge and its processor detects the device is no within tolerance thus disabling the device as "out of service".