Does anyone have a working I2C sketch for the 8475A input? I have output working. In my case I need two chips to scan a 4x4 keypad. I could trudge through it, but I feel sure it's been done before.
The ATmega32U4 also supports I2C (TWI) and SPI communication. The Arduino software includes a Wire library to simplify use of the I2C bus; see the documentation for details. For SPI communication, use the SPI library.