The Serial Peripheral Interface (SPI) is a high-speed synchronous data transfer interface using three or four pins. It allows fast communication between an AVR device and peripheral devices or between several microcontrollers. The SPI supports full-duplex communication.

The SPI peripheral can be configured as either Master, controlling all data transactions by pulling a Slave's Select (SS) signal low, or Slave, controlled by an SPI Master.

A device connected to the bus must act as a master or slave. The master initiates and controls all data transactions. The interconnection between master and slave devices with SPI is shown in the block diagram. The system consists of two shift registers and a master clock generator. The SPI master initiates the communication cycle by pulling the slave select (SS) signal low for the desired slave. Master and slave prepare the data to be sent in their respective shift registers, and the master generates the required clock pulses on the SCK line to exchange data. Data is always shifted from master to slave on the master output, slave input (MOSI) line, and from slave to master on the master input, slave output (MISO) line. After each data packet, the master can synchronize the slave by pulling the SS line high.

In SPI slave mode, the control logic will sample the incoming signal on the SCK pin. To ensure correct sampling of this clock signal, the minimum low and high periods must each be longer than two CPU clock cycles.

This device provides one instance of the SPI peripheral, SPI0.