The SPI bus requires four bus signals namely:

i) SCLK (Serial Clock): It is also called as SCK or CLK.  It works as a synchronizing signal between the transmitter and receiver.

ii) MOSI (Master Out Slave In): It is also called as SIMO, SDO, DO, DOUT, SO or MTSR. This pin is used to give out the data from the transmitter or the master and taken to the receiver or the slave.

iii) MISO (Master In Slave Out): It is also called as SOMI, SDI, SI or MRST. It is used to take back the data from the slave into the master to check the correctness of the data.

iv) SS (Slave Select): It is also called as nCS, CS, CSB, CSN, nSS, STE or SYNC. it is used to select the slave device for a transfer of data.

The connection of the master and the slave in the SPI configuration is as shown in the figure below: