There are many many small switch mode power supply solutions that have universal inputs (i.e. 90->245 VAC @ 45-65 Hz) that can supply power to your device. The output of these could easily provide power AND the indication of the power outage indication. Many of these power supply solutions are very low cost. You may even find them in the local electronics goods stores in the form of a cell phone adapter/charger. Here is a link to one such type device.

Here is a schematic block diagram of how I would approach the design problem. Select an AC mains supply that steps the power down to something like 9 volts. Then use a cheap linear regulator circuit such as a 7805 to provide the Vcc needed by your microcontroller system. This should be isolated from the 9 volt supply via a series diode. The 7805 supply should be equipped with high capacity electrolytic capacitors that can hold up the Vcc for the microcontroller system for say about half of a second (i.e. long enough that the microcontroller is able to detect the power fail signal going high at its port pin and then to capture the reading from the RTC chip and save all the information into a non-volatile memory such as a serial EEPROM chip). There is no need for expensive "super caps" in this type of design. Simple aluminum electrolytic devices in the several hundred microfarad size should do the trick nicely.



The diode in front of the 7805 regulator is needed so that the opto-coupler power fail detector is isolated from the large hold up supply capacitors. Upon a power outage the AC power converter output will power off this cutting the diode current in the opto coupler. This will make the output of the opto coupler pull high to show "power fail".

In the microcontroller design do not forget to provide a good quality brownout detector so that the processor is held in reset whenever the Vcc out of the 7805 regulator goes too low for proper operation of the microcontroller. I would simply power the RTC chip off a small battery on the circuit board. You may also want to consider providing power to the RTC chip similar to the diagram below. Thus whenever the normal Vcc is present then the RTC battery is saved.



In your design you could reduce to a single BAT54C diode package and eliminate the RTC power source from the VBAT source shown.

Michael Karas