89S8252 and RTC 72421
I'm using a 89S8252 system as a 'timekeeper'.
All timing is done with a software RTC under control of timer0.
A hardware RTC (72421) with a backup battery is used to keep
the time and date when the power is off.
The software RTC is loaded with the time/date of the 72421 at poweron reset.
Most of the time everything works OKay, but sometimes there is a
little problem. The hour or the month is incorrect. (Resets to zero.)
I hope you can help me with the following questions:

Is this a fault in the RTC chip?

Is it possible there are write's to the 72421 during shutdown or power up.
If so, when must I expect most problems:
At power down or at power up?

Are the simple solutions?

I already did a search for this problem (Brownout, backup and suff like that)
but I have not found a simple solution.

I hope you can help me

Robert Henneke (NL)