AtomRTC

AtomRTC is a small RTC board based on the DS3234 from MAXIM-IC. This RTC chip is one of the best when it comes to accuracy and voltage ranges. It can be used with 3.3v or 5v micros as the ABSOLUTE MAX voltage is 6v. The nominal voltage range is 3.3v to 5.5v. This board holds a CR1220 battery and can last for a long time, the DS3234 uses a max of 2.5 µA to keep time. With a CR1220 you have about 48 mAH. So about 19200 hrs or 800 Days (2+ years) without being powered from any other source.

The pin out is as basic as it gets.

-   = GND
+   = 3.3v to 5v
SQW = INT/SQW (Square Wave Output)
SI  = MOSI
SO  = MISO
CLK = SCK
SS  = CS

Datasheet: DS3234

Schematic: