25.6.1 Principle of Operation

The SPI is a high-speed synchronous data transfer interface. It allows high-speed communication between the device and peripheral devices.

The SPI can operate as host or client. As host, the SPI initiates and controls all data transactions. The SPI is single buffered for transmitting and double buffered for receiving.

When transmitting data, the Data register can be loaded with the next character to be transmitted during the current transmission.

When receiving, the data is transferred to the two-level receive buffer, and the receiver is ready for a new character.

The SPI transaction format is shown in SPI Transaction Format. Each transaction can contain one or more characters. The character size is configurable, and can be either 8 or 9 bits.

The SPI host must pull the SPI select line (SS) of the desired client low to initiate a transaction if multiple clients are connected to the bus. The SPI select line can be wired low if there is only one SPI client on the bus. The host and client prepare data to send via their respective Shift registers, and the host generates the serial clock on the SCK line.

Data is always shifted from host to client on the Host Output Client Input line (MOSI); data is shifted from client to host on the Host Input Client Output line (MISO).

Each time character is shifted out from the host, a character will be shifted out from the client simultaneously. To signal the end of a transaction, the host will pull the SS line high.