The 555 was only very awkward to use as watchdog. You need tons of additional circuits to do all these what was included inside a real watchdog (transition sensitive retriggerable monoflop if triggered successful and switching to astable flipflop, if time expired, low voltage sensing and so on).

The 555 has also no defined characteristics on power up and low voltages, but real watchdogs deliver valid reset signals already near 1V VCC.

So, if the watchdog was intended to get more reliability, the 555 must be avoided !

E.g. the MAX813L was ideally suited for using with 8051 derivates.


Peter