In a multi-processor operation when the SPI interface is used, the microcontrollers operate in the master and slave modes. In master mode the microcontroller is in full control of the communication since it initiates and ends the communication session and generates the SPI clock signal. In slave mode the microcontroller listens when the master initiates and ends the communication session and uses the SPI clock signal generated by the master. SPI master/slave connection is shown in Fig. 9-2. The figure shows the 4-pin SPI interface even though this connection can also operate with the 3-pin SPI interface.
Fig. 9-2 Master/slave connection using SPI interface