Now is there any job left for my old ADM1232 supervisor. If I don't use a supervisor what device should be used to provide power on reset. Should I use the old RC style reset which I used in my early days with micros.

Dear Prahlad,

there's much written about what is better, to have the watchdog outside of micro or built-in. From a theoretical point-of-view the clear separation between watchdog and micro will show best results. But why to use an additional watchdog, when the micro has one built-in?

Away from this discussion, the use of an internal watchdog is mandatory, if you intend to put the micro into power-down. Why? While the external watchdog would all the time wait for the strobe pulse, which a micro put into power-down mode cannot deliver, of course, the internal watchdog will be inactive during power-down, means no need for reseting the watchdog during power-down!

So, whenever you want to put your P89V51RD2 into power-down mode, the use of internal watchdog is essential.

Your P89V51RD2 contains also a brown-out detector. But to be correct, a somewhat enhanced brown-out detector was implemented: If supply voltage of micro (Vcc) falls below 3.85V (typically 4.15V) a brown-out interrupt can be invoked. This interrupt can be used to store some critical data in a nonvolatile RAM, for instance, or doing something different useful...
If this brown-out interrupt routine is finished, the micro can be reseted. So, the built-in brown-out detector works like an 'early power fail detetcor'. But keep in mind, that with longer brown-out interrupt routines Vcc can fall much below this 3.85V...
Of course, the brown-out detector can also be configured to simply reset the micro.

But:
A threshold level of 3.85V for the brown-out detection is a bit low, when your circuit also contains digital chips, which are specified for a supply voltage of Vcc = 5V +-10% or even Vcc = 5V +-5%. As 3.85V is 23% under 5V, it's questionable whether such a brown-out detector is usefull at all.
If you take an ADM1232, on the other hand, brown-out detector can be configured for a threshold of 4.5V...4.74V (4.62V typically).
So, this built-in brown-out detector of P89V51RD2 makes only sense for stand-alone applications, where external circuitry needing exact 5V is not present.

P89V51RD2 does not contain a POR (power-on-reset). So, even when using built-in watchdog and built-in brown-out detector an external POR is needed. But please never use this stupid RC circuit 'recommended' in datasheet!!! It's a miracle to me, why this totally helpless RC stuff is recommended in the datasheets even today, where we know how susceptible the flash code memory is against information loss caused by Vcc bounces...

So, what can we conclude?
If you do not intend to put your micro into power-down, then there's no relevant reason to use the internal watchdog. And if you have no stand-alone application, then there's no need to use the internal brown-out detector. Then you can use the ADM1232 as you did it in your last applications, and by this profiting from the advantages of external watchdog, tighter brown-out threshold level and state-of-the-art POR.

Kai