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‘Message’ is the basic unit of communication on I2C bus where a message comprises one or more 8-bit bytes. See Figure 1.6. The first 8-bit byte contains the destination address, i.e. the 7-bit address of the slave that the master wishes to communicate with. The first 8-bit byte also contains one bit specifying the direction of transfer, a read or a write. Write requests transfer bytes from master to slave, read requests transfer bytes from slave to master. In other words, the master can be message receiver or message transmitter. Master always transmits the first byte which determines address and direction of transfer.
Every message whether transmitted or received always begins with an 8-bit byte containing the 7-bit slave address and transfer direction bit. The corresponding slave with matching address answers by acknowledging. The slave matches its address then begins receiving further bytes or transmitting bytes depending on the direction bit. Possible combinations of message format include but not restricted to:
