When the bus is idle, both the SDA and SCL lines are pulled high by the pullup resistors. The main device generates a start condition followed by a valid serial byte containing high-speed (HS) main device code 00001XXX. This transmission is made in fast (400 kHz) or standard (100 kHz) (F/S) mode at no more than 400 kHz. The device does not acknowledge the HS main device code, but does recognize it and switches its internal filters to support 2.94-MHz operation.

The main device then generates a repeated start condition (a repeated start condition has the same timing as the start condition). After this repeated start condition, the protocol is the same as F/S mode, except that transmission speeds up to 2.94 MHz are allowed. Instead of using a stop condition, use repeated start conditions to maintain the bus in HS-mode. A stop condition ends the HS-mode and switches all the internal filters of the device to support the F/S mode.