Detecting a power loss and then writing to EEprom memory is a risky buisness,especially if the AT89S8252 can take up to 2.5ms to effect a single write cycle.
You will have to ensure that your supply capacitor has enough reserve capacity to allow for the write when the power is lost(or battery backed up).
If you have lots of data to store then you should look at the Ramtron memory that has no delays in the write cycle (although it will be an external chip)