See the DALLAS Clock Page for more details, quote as follows:
I wanted to add a realtime clock to my SAM other then the hard to get realtime clock chip used in the sambus and discovered the realtime clock chip from dallas semiconductors which has two big advantages: All clock components inclusive a backup battery are in a single package so implementation is easy, and this clock can be found on PC motherboards which makes it easy to get.
The Dallas clock can operate in INTEL mode and MOTOROLA mode. In order to reduce the chip count I choose to use the MOTOROLA
My original design of the DALLAS clock was based on integration with the SAM's Comms interface (In which I already build my EDDAC interface). But to make it function as a stand alone interface a 74LS138 address decoder was added.
The extra 114 bytes on the chip (DS12887, 50 bytes on the DS1287) can also be used as non-volatile storage such as Defender for the 'All time high score' value.
I recommend fitting the clock chip in a 24 pin DIL socket. IC 2 and the ceramic capacitor can be fitted inside the DIL socket.
|DS 12887 (or DS 1287)
|22 nF ceramic capacitor
|22 uF/16V electrolytic capacitor *
* Not required when clock is built inside Comms interface.