SPI Bus Connection

In Figure 1, the connection between one master and one slave is shown.

Figure 1. SPI Bus Connection

The different lines are as follows:

MISO Master Input Slave Output. The line where the data is shifted out from the slave and into the master.
MOSI Master Output Slave Input. The line where the data is shifted out from the master and into the slave.
SCK Serial Clock. Generated by the master device.
SS Slave Select. To initiate a transaction, the master must pull this line low.

If the bus consists of several SPI slaves, they can be connected in parallel and the SPI master can use general I/O pins to control separate SS lines to each slave on the bus.

It is also possible to connect all slaves in series. In this configuration, a common SS is provided to N slaves, enabling them simultaneously. The MISO from the N-1 slaves is connected to the MOSI on the next slave. The Nth slave connects its MISO back to the master. For a complete transaction, the master must shift N+1 characters.