Hello Benny,

A 7805 powered from a small mains transformer is quite reliable, all my commercial products are powered like this, though layout and placing adequate decoupling and electrolytic capacitors in the appropriate palaces is very important.To assure you one of my custormers products (5KVA to 30KVA UPS) uses the same chip P89C51RD2HBP(and 7805) with no complaints so far (after manufacturing about 200 units), they have now begun to use the P89C51RD2BN hopefully this should also work reliably.

Have you provided brown out protection ?? etc, if not it MUST be provided especially to protect the 24C512.

There is an excellent application note on the crystals for RTCs by Dallas I dont remember the application note number, I had the printed book (a couple of years ago) I will have to search it out if required. You MUST read this if you want to design reliable RTC circuits ( your queries are answered there), there is also one application note by INTEL but the DALLAS one is specifically for RTCs.

If other traces or tracks run accros the the crystal pins, cross talk could occur and the RTC may not keep correct time, hence the ground ring around the crystal pins, this is explained in the DALLAS application note.

Regards,

John.