36.4.3 I2C Multi-Host Mode Operation

In Multi-Host mode, multiple host devices reside on the same bus. A single device, or all devices, may act as both a host and a client. Control of the bus is achieved through clock synchronization and bus arbitration.

The Bus Free (BFRE) bit is used to determine if the bus is free. When BFRE is set (BFRE = 1), the bus is in an Idle state, allowing a host device to take control of the bus.
In Multi-Host mode, the Address Interrupt and Hold Enable (ADRIE) bit must be set (ADRIE = 1) and the Clock Stretching Disable (CSD) bit must be clear (CSD = 0), in order for a host device to be addressed as a client.
When a matching address is received into the receive shift register, the SMA and the Address Interrupt Flag (ADRIF) bits are set. Since ADRIE is also set, hardware sets the Client Clock Stretching (CSTR) bit, and hardware stretches the clock to allow time for software to respond to the host device being addressed as a client. Once the address has been processed, software must clear CSTR to resume communication.
Important: Client hardware has priority over host hardware in Multi-Host mode. Host mode communication can only be initiated when SMA = 0.