Introduction
Suppose there is a hungry dog guarding a house (Figure 12.1), and someone wishes to break in. If the burglar’s accomplice repeatedly throws the guard dog small pieces of meat at 2-minute intervals, then the dog will be so busy concentrating on the food that he will ignore his guard duties and will not bark. However, if the accom- plice run out of meat or forgets to feed the dog for some other reason, the animal will start barking, thereby alerting the neighbours, property occupants or police.
This same basic approach is followed in computerized ‘watchdog timers’. Very simply, these are timers which, if not refreshed at regular intervals, will overflow. In most cases, overflow of the timer will reset the system. Such watchdogs are intended to deal with the fact that, even with meticulous planning and careful design, embed- ded systems can ‘hang’ due to unexpected problems. The use of a watchdog can be used to recover from this situation, in certain circumstances.
The pattern HARDW ARE W A TCHDOG [page 217] considers how to apply these tech- niques to good effect in your embedded application.