What is a watchdog?
A hardware device that is used to reset a microcontroller. The
watchdog consists of a timer, an input and an output. In your
application program you need to change the state of the watchdog input
pin to clear the timer. If the timer is not cleared before the timeout
period ends the timer output will reset the microcontroller. This is
an automatic "reboot" for an embedded system.
The watchdog can be an external or internal peripheral. The Atmel
microcontrollers (and most other microcontrollers) have internal
watchdogs. Maxim, Texas Instruments, Analog Devices and other
semiconductor manufacturers make external watchdog ICs that you can
connect to the reset pin. The external watchdogs usually integrate
other functions such as brownout voltage detection.