Tiny RTC I2C Module 24C32 Memory DS1307 Clock High Precision Clock Timer Module Compatible with Arduino by Atomic Market

I had to practice my soldering skills putting two five pin headers on this board. It comes with NO documentation. P1 is the i2c connection for the device, P2 is a pass through connection to the next i2c device in the chain. Since the board is for Arduino and I wanted to use it for Raspberry Pi I needed to destroy the two resistors at the base of the board to keep the voltage logic at 3V rather than 5V (I think they are R1 and R2 - near the bottom edge)http://www.hobbyist.co.nz/?q=real_time_clock

I tested this RTC with several different connectors, libraries and batteries - it doesn't work. When tested via I2c, serial port returns "communication error." It will display an arbitrary time, but the time can't be set. JUNK!

only bad thing i can say is i wish it came with the battery

I2C interface is not that hard. Item was delivered on time and worked as expected.

Great.