A master can start a bus transaction only if it has detected that the bus is idle. As the TWI bus is a multi-master bus, it is possible that two devices may initiate a transaction at the same time. This results in multiple masters owning the bus simultaneously. This is solved using an arbitration scheme where the master loses control of the bus if it is not able to transmit a high level on the SDA line. The masters who lose arbitration must then wait until the bus becomes idle (i.e., wait for a STOP condition) before attempting to reacquire bus ownership. Slave devices are not involved in the arbitration procedure.

Figure 1. TWI Arbitration

Figure 1 shows an example where two TWI masters are contending for bus ownership. Both devices are able to issue a START condition, but DEVICE1 loses arbitration when attempting to transmit a high level (bit 5) while DEVICE2 is transmitting a low level.

Arbitration between a repeated START condition and a data bit, a STOP condition and a data bit, or a repeated START condition and a STOP condition are not allowed and will require special handling by software.