34.6.2.4.6 High-Speed Mode
High-speed transfers are a multi-step process, see High Speed Transfer.
First, a Host code (0b00001nnn, where 'nnn' is a unique Host code) is transmitted in Full-speed mode, followed by a NACK since no Client should acknowledge. Arbitration is performed only during the Full-speed Host Code phase. The Host code is transmitted by writing the Host code to the Address register (ADDR.ADDR) and writing the High-speed bit (ADDR.HS) to '0'.
After the Host code and NACK have been transmitted, the Host write interrupt will be asserted. In the meanwhile, the Client address can be written to the ADDR.ADDR register together with ADDR.HS=1. Now in High-speed mode, the Host will generate a repeated start, followed by the Client address with RW-direction. The bus will remain in High-speed mode until a stop is generated. If a repeated start is desired, the ADDR.HS bit must again be written to '1', along with the new address ADDR.ADDR to be transmitted.
Transmitting in High-speed mode requires the I2C Host to be configured in High-speed mode (CTRLA.SPEED=0x2) and the SCL Clock Stretch mode (CTRLA.SCLSM) bit set to '1'.