hi,

this is the long story. Many years ago I started using Atmel products. There were AT89C51. We has not used power supervisors these times, but simple RC net. The only problem we reached is that MCU did not start sometimes. Well, it was not a problem but just annoy.
After some time we came to AT89S8252. Then real problems pop upped. We stuck with some troubles I already posted about. After Atmel made new revisions of the chip the problems gone out, ahem.

Now the main story. After some time we started to use couple of Atmel/Temic products: T89C5115, T89C51CC01 and T89C51AC2. Then we have found very strange problem: after some power off/power on, MCUs lost parts of code programmed into Flash and parts of data placed into EEPROM. Moreover, it lost even protect bits and then someone might read rest of code and data from it! Sh*t!
We did some tests and found that if MCU is not at reset state when power fails below 3V, then the problem happens. So it was the point from which we have been starting to use power SV. Some time late Atmel published this problem in separate document which describes all trobles and bug fixes including huge schematic etc. But in fact, the simple external power supervisor fixes this problem very well.
Now Atmel say that they have power monitor built-in for new products. But I remember that story and cannot believe that it will work good.

Regards,
Oleg