SNLS477B October 2014 – November 2018 DS90UB948-Q1
The bidirectional control channel in the FPD-Link III devices implements I2C-compatible bus arbitration in the proxy I2C master implementation. When sending a data bit, each I2C master senses the value on the SDA line. If the master sends a logic 1 but senses a logic 0, the master loses arbitration. The master will stop driving SDA and retry the transaction when the bus becomes idle. Thus, multiple I2C masters may be implemented in the system.
For example, there might also be a local I2C master at each camera. The local I2C master could access the image sensor and EEPROM. The only restriction would be that the remote I2C master at the camera should not attempt to access a remote slave through the BCC that is located at the host controller side of the link. In other words, the control channel should only operate in camera mode for accessing remote slave devices to avoid issues with arbitration across the link. The remote I2C master should also not attempt to access the deserializer registers to avoid a conflict in register access with the Host controller.
If the system does require master-slave operation in both directions across the BCC, some method of communication must be used to ensure only one direction of operation occurs at any time. The communication method could include using available R/W registers in the deserializer to allow masters to communicate with each other to pass control between the two masters. An example would be to use register 0x18 or 0x19 in the deserializer as a mailbox register to pass control of the channel from one master to another.