Datasheet

DS1387 (http://adafru.it/aX0). Theres a lithium battery in there which is why it's so big.

The RTC we'll be using is the PCF8523 (http://adafru.it/reb) or the

DS1307 (http://adafru.it/rec).

If you have an Adafruit Datalogger Shield rev B, you will be using the PCF8523 - this

RTC is newer and better than the DS1307. Look on your shield to see if you see PCF8523

written above the chip.

If you have an older Datalogger shield, you will be using the DS1307 - there's no text

so you'll just need to remember that if it doesn't say PCF8523 it's the DS1307

Battery Backup

As long as it has a coin cell to run it, the RTC will merrily tick along for a long time, even

when the Arduino loses power, or is reprogrammed.

Use any CR1220 3V lithium metal coin cell battery: