When the bus is idle, the SDA and SCL lines are pulled high by the pullup devices.
The master generates a START condition followed by a valid serial byte containing HS master code 00001XXX. This transmission is made in F/S mode at no more than 400 kbps. No device is allowed to acknowledge the HS master code, but all devices must recognize it and switch their internal setting to support 3.4-Mbps operation.
The master then generates a REPEATED START condition (a REPEATED START condition has the same timing as the START condition). After the REPEATED START condition, the protocol is the same as F/S mode, except transmission speeds up to 3.4 Mbps are allowed. A STOP condition ends the HS mode and switches all the internal settings of the slave devices to support F/S mode. Instead of using a STOP condition, REPEATED START conditions are used to secure the bus in HS mode.
Attempting to read data from register addresses not listed in this section results in 0xFF being read out.