I2C

buffers
https://www.nxp.com/docs/en/data-sheet/P82B96.pdf P82B96 Voltage Range bidirectional datatransfer between an I2C bus 2 V to 15 V range.

maxbotix
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