Many microcontrollers have integrated watchdog timers, so an external watchdog timer may not be necessary. However, when the watchdog timer is integrated inside the processor, the watchdog timer shares the same power supply and system clock, and therefore is prone to being affected by the same fault conditions. In particular, when a program fault occurs, the internal clock may shut down, which will affect the watchdog timer's operation if it depends on this particular clock source. This will result in the program not being able to restart.
The external watchdog timer can automatically reset the microprocessor when a program error occurs, increasing system reliability. Many reset ICs currently available have integrated watchdog functionality. It consumes very little power and can provide higher system reliability. For example, MAX16056 is a low power (125nA) supervisory circuit with a capacitor-adjustable watchdog and reset timeouts. When the watchdog timer runs out or the monitored voltage drops below a predetermined threshold, the device will trigger a processor reset.