I agree with duh (de unknown hombre?) that a supervisor chip is mandatory (perhaps except for toys), my issue is that a single bit restart is very easily accomplished by runaway code. Some of the internal puppies have become quite sophisticated e.g. the Philips P80C51Rx2 pulls the reset pin high to reset external circuitry when the puppy bites.

Peter, I do not see how an external WD can be better than an internal, in either case you have to perform a restart within a given timeframe and if you do not, you get bitten. Of course using the prescence of an internal WD as an excuse to skip the reset suprevisor and use RC is plain old silly.

Have fun,

Erik