26.5.7 Master Address

When this bit field is written, a Start condition and slave address protocol sequence is initiated dependent on the bus state.

If the bus state is unknown the Master Write Interrupt Flag (WIF) and Bus Error flag (BUSERR) in the Master Status register (TWIn.MSTATUS) are set and the operation is terminated.

If the bus is busy the master awaits further operation until the bus becomes idle. When the bus is or becomes idle, the master generates a Start condition on the bus, copies the ADDR value into the Data Shift register (TWIn.MDATA) and performs a byte transmit operation by sending the contents of the Data register onto the bus. The master then receives the response (i.e., the Acknowledge bit from the slave). After completing the operation the bus clock (SCL) is forced and held low only if arbitration was not lost. The CLKHOLD bit in the Master Setup register (TWIn.MSETUP) is set accordingly. Completing the operation sets the WIF in the Master Status register (TWIn.MSTATUS).

If the bus is already owned, a repeated Start (Sr) sequence is performed. In two ways the repeated Start (Sr) sequence deviates from the Start sequence. Firstly, since the bus is already owned by the master, no wait for idle bus state is necessary. Secondly, if the previous transaction was a read, the acknowledge action is sent before the Repeated Start bus condition is issued on the bus.

The master receives one data byte from the slave before the master sets the Master Read Interrupt Flag (RIF) in the Master Status register (TWIn.MSTATUS). All TWI Master flags are cleared automatically when this bit field is written. This includes bus error, arbitration lost, and both master interrupt flags.

This register can be read at any time without interfering with ongoing bus activity, since a read access does not trigger the master logic to perform any bus protocol related operations.