The SPI bus is a full-duplex, single master, multi-slave, synchronous serial interface; as for the I2C bus, it is used for the on-board connection of sensor chips with the main CPU. This bus requires at least (apart from the GND signal) three wires plus one chip select signal per slave. This line is typically called Slave Select (SS) or Chip Select (CS), and usually, it's active low (that is, the master must set it to 0 to enable the desired slave chip).
Note
A full-duplex is a connection that can transmit and receive at the same time on the bus.
Synchronous means that the clock is sent along with the data (in this case, it is the master that provides the clock).
Single master and multi-slave mean that in the bus there is only one master, who directs the communication, while more than one slave can be connected to the bus.
The communication starts when the bus master configures the clock, using a frequency supported...