28.6.2.5.4 High-Speed Mode

When the I²C client is configured in High-speed mode (Hs, CTRLA.SPEED=0x2) and CTRLA.SCLSM=1, switching between Full-speed and High-speed modes is automatic. When the client recognizes a START followed by a host code transmission and a NACK, it automatically switches to High-speed mode and sets the High-speed status bit (STATUS.HS). The client will then remain in High-speed mode until a STOP is received.