I2C

http://www.maxbotix.com/articles/095.htm links to https://github.com/felias-fogg/SoftI2CMaster

https://variecose.wordpress.com/2011/09/24/i2c-rtc-ds1307-and-arduino/ Mega2560 has internal pullups on the sda, sdc lines.

https://github.com/sosandroid/FRAM_MB85RC_I2C