Moreover we have that PCA WDT feature for those who are allergic to noise/glitch/ESD. Now, Except for the fact that the internal watchdog is 'software programmed' and supposedly not as good as a hardware one* is there any other reason not to replace the supervisor IC in an AT89C51RD2 system with an RC?(Tantalium C)

1) the Intel kluge watchdog in PCA4 is worthless. As opposed to other internal watchdogs clearing one bit (which a runaway programm easily can do) will render it dead. NO watchdog should be able to be disabled once enabled. The only internal watchdog type I consider acceptable is the one that require two consequtive and different writes to reset. Such an internal WD is superior to an external (a runaway may toggle the bit that the external WDS requires.

2) if you do a RC reset you shoot yourself in the foot. Some uCs have an actual "supervisor" and do not require the RC, for those the supervisor is superfluous.

re the above: I laughed out loaud when Cygnal (as it was called in those days) told someone to add a RC to the internal supervisor as "noise suppression" effectively changing the internal supervisor to an external RC reset and even slowing down the brownout detect to rendering it worthless.

Erik