To start the TWI as Master, write a '1' to the ENABLE bit in the Master Control A register (TWI.MCTRLA), followed by writing the slave address to the Master Address (TWI.MADDR) register. The TWI.MADDR register also has a R/W bit which indicates whether the Master is transmitting or receiving. The Master DATA register (TWI.MDATA) is written in case master is transmitting data.

To enable the TWI as Slave, write the Slave Address (ADDR) in TWI.SADDR, and write a '1' to the ENABLE bit in the Slave Control A register (TWI.SCTRLA). The TWI peripheral will wait to receive a byte addressed to it.