The term “Three Wire Connection” is used in this context to describe a connection without slave select signal. This connection relies solely on counting bits for determining the end of the current frame and resetting the shift register state machine, instead of using the slave select signal for this purpose. This way of communication is less robust than communication with slave select, but it saves a pin on both master and slave device.

The name “three wire connection” is true only in case of full-duplex connection, where exactly three signals (clock, data-in, data-out) are needed. In case of half-duplex and simplex connections only two signals (clock, data-in/data-out common data line) are necessary.

Figure 1. Three wire connection