What is your confidence to internally built Watch Dog?
I have circuit with Philips P89C669 and DS1232. DS1232 function was only power Supply monitoring. Strobe Input of DS1232 was trigged by ALE because of possibility to program microcontroller via serial port.

And I use internal Watch Dog.
And microcontroller is hanging up :(

I made all prevention steps as it was frequently discussed on these form (capacitor over supply pins, short connections wires etc...).
Finally I give it up all hope and I connect Strobe Input to output pin of microcontroller.
Strobe Input of DS1232 is now trigged via software.
At last circuit works perfectly now.

What does it mean?

Obviously internal Watch Dog is not efficient.
My explanation is:
Watch Dog which must be started across software is not a good solution.
In critical situation maybe software has no time to start Watch Dog.
Possible it hangs immediately after reset, before Watch Dog activation.

For now on I trust to internal Watch Dog only if it is activated during firmware programming. Maybe with additional parameter (like security bits).