17.2.5.1 Arbitration

The serial clock line is always driven by a master device. However, all devices connected to the bus are allowed stretch the low period of the clock to slow down the overall clock frequency or to insert wait states while processing data. Both master and slave can randomly stretch the clock, which will force the other device into a wait-state until the clock line goes high again.

If two masters start transmitting at the same time, they will both transmit until one master detects that the other master is pulling the data line low. When this is detected, the master not pulling the line low, will stop the transmission and wait until the bus is idle. As it is the master trying to contact the slave with the lowest address that will get the bus ownership, this will create an arbitration scheme always prioritizing the slaves with the lowest address in case of a bus collision.