It is full-duplex synchronised interface usually used in embedded systems for data transfer. SPI has master or controller and slave or chip to which data is to be transfered. Refer the master and slave images for better understanding.
__________SPI Master ______________
-> newd : When there is new data, it is turned to active high.
-> cs ( chip_select ) : Enable slave device on active low.
-> MOSI ( Master_output_slave_input ) : port to send data from master top slave serially.
-> sclk ( Synchronised_clock ) : serial clock to synchronise slave.
__________SPI Slave ______________
-> cs
-> MOSI
-> sclk
-> done : Triggered on transfer completion.
-> Dout : output port to collect data
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