Ok it is not altogether a very crude idea to use the 555 as a WD. In fact long back I have used it with 8748 MCU !! But even then the dog was patted with a port pin and not out of any interrupt code for obvious reasons.

But in todays situation a 555 will be the last resort when there is absolutely no way to get a regular reset controller. These regulars come with lots of other thing like browount detectors and what not.

Or alternately a MCU with built WDT is a better bet.

Raghu