Multi-Master Mode Bus Arbitration

When the bus is idle, any master device may attempt to take control of the bus. Two or more master devices may issue a Start condition within the minimum hold time (T_HD:STA), which triggers a valid Start on the bus. The master devices must then compete using bus arbitration to determine who takes control of the bus and completes their transaction.

Bus arbitration takes place bit by bit, and it may be possible for two masters who have identical messages to complete the entire transaction without either device losing arbitration.

During every bit period, while SCL is high each master device compares the actual signal level of SDA to the signal level the master actually transmitted. SDA sampling is performed during the SCL high period because the SDA data must be stable during this period; therefore, the first master to detect a low signal level on SDA while it expects a high signal level loses arbitration. In this case, the ‘losing’ master device detects a bus collision and sets the Bus Collision Detect Interrupt Flag (BCLIF), and if the Bus Collision Detect Interrupt Enable (BCLIE) bit is set, the generic I2CxEIF is also set.

Arbitration can be lost in any of the following states:

Address transfer
Data transfer
Start condition
Restart condition
Acknowledge sequence
Stop condition

If a collision occurs during the data transfer phase, the transmission is halted and both SCL and SDA are released by hardware. If a collision occurs during a Start, Restart, Acknowledge, or Stop, the operation is aborted and hardware releases SCL and SDA. If a collision occurs during the addressing phase, the master that ‘wins’ arbitration may be attempting to address the ‘losing’ master as a slave. In this case, the master that lost arbitration must switch to its Slave mode and check to see if an address matches.

Important: The I²C Specification states that a bus collision cannot occur during a Start condition. If a collision occurs during a Start, BCLIF will be set during the addressing phase.

User software must clear BCLIF to resume operation.

Figure 1. Bus Collision

Figure 2. Multi-Master Mode Transmission