26.3.2.5 Transaction

A transaction is the complete transfer from a Start to a Stop condition, including any repeated Start conditions in between. The TWI standard defines three fundamental transaction modes: Master write, master read, and a combined transaction.

Figure 26-6 illustrates the master write transaction. The master initiates the transaction by issuing a Start condition (S) followed by an address packet with the direction bit set to '0' (ADDRESS+W).

Assuming the slave acknowledges the address, the master can start transmitting data (DATA) and the slave will ACK or NACK (A/A) each byte. If no data packets are to be transmitted, the master terminates the transaction by issuing a Stop condition (P) directly after the address packet. There are no limitations to the number of data packets that can be transferred. If the slave signals a NACK to the data, the master must assume that the slave cannot receive any more data and terminate the transaction.

Figure 26-7 illustrates the master read transaction. The master initiates the transaction by issuing a Start condition followed by an address packet with the direction bit set to '1' (ADDRESS+R). The addressed slave must acknowledge the address for the master to be allowed to continue the transaction.

Assuming the slave acknowledges the address, the master can start receiving data from the slave. There are no limitations to the number of data packets that can be transferred. The slave transmits the data while the master signals ACK or NACK after each data byte. The master terminates the transfer with a NACK before issuing a Stop condition.

Figure 26-8 illustrates a combined transaction. A combined transaction consists of several read and write transactions separated by repeated Start conditions (Sr).