Multi-Host Communication, Bus Collision and Bus Arbitration

Multi-Host mode support is achieved by bus arbitration. When the host outputs address/data bits onto the SDA pin, arbitration takes place when the host outputs a ‘1’ on SDA, by letting SDA float high and another host asserts a ‘0’. When the SCL pin floats high, data may be stable. If the expected data on SDA is a ‘1’ and the data sampled on the SDA pin is ‘0’, then a bus collision has taken place. The host will set the Bus Collision Interrupt Flag (BCLxIF) and reset the I²C port to its Idle state (see Figure 1).

If a transmit was in progress when the bus collision occurred, the transmission is halted, the BF flag is cleared, the SDA and SCL lines are deasserted, and the SSPxBUF can be written to. When software services the bus collision Interrupt Service Routine, and if the I²C bus is free, software can resume communication by asserting a Start condition.

If a Start, Repeated Start, Stop or Acknowledge condition was in progress when the bus collision occurred, the condition is aborted, the SDA and SCL lines are deasserted, and the respective control bits in the SSPxCON2 register are cleared. When software services the bus collision Interrupt Service Routine, and if the I²C bus is free, software can resume communication by asserting a Start condition.

The host will continue to monitor the SDA and SCL pins. If a Stop condition occurs, the SSPxIF bit will be set.

A write to the SSPxBUF will start the transmission of data at the first data bit, regardless of where the transmitter left off when the bus collision occurred.

In Multi-Host mode, the interrupt generation on the detection of Start and Stop conditions allows the determination of when the bus is free. Control of the I²C bus can be taken when the P bit is set, or the bus is Idle and the S and P bits are cleared.

Figure 1. Bus Collision Timing for Transmit and Acknowledge