# SPI protocol ## Introduction * SPI = Serial Peripheral Interface , its a Master/Slave (Controller/Peripheral) protocol thats synchronous (having a clk) * popular and easy to implement (basically using a shift register to shift in/out data thats it!) * serial in natue and the max theoretical speed = clk/2 * used in device to device comminucation over short distaces * one master , multiple slave (Multimaster is possible and collision / arbitration is also required NSS as input is used for this in stm32 ) ## configuration / connection

* CS = Chip select (NSS) , SCLK = clk , MISO = Master In slave out (salve sends data and master recieves) , MOSI = Master Out Slave In (Master send data Slave receives data) * CS is pulled low by master of corresponding slave to start communication * different configuration : full duplex , half duplex simplex * full duplex : bidirectional communication at the same clock pulse connection same as above * half duplex : single data line to either tx or rx data , single directional communication as configured * simplex: unidirectional communication

* 8 bit register or 16/12 depending on the shift register used ex: stm32f407vgtx has 16bit shift register * Software slave management / hardware slave management is possible using SSI and SSM in stm32 ## Details ### CPOL * Clock polarity sets the clock format : **cpol=0** clock state is normally low and is pulled up/down when active **(active high)** * **cpol =1** , idle state is high , pulled low during the transmission **(active low)** ### CPHA * determines when to sample data , rising edge or falling edge and when to send the data ring or falling edge * data should be sampled when its not transitioning to prevent reading it wrongly * **cpha = 0 , data sampled at leading edge of clock** * **CPHA = 1 , data sampled at trailing edge of clock**

### Data frame format * 8 bit normally and 16 bit if configured ## Usage and limitations * Used widely everywhere due to the ease of implementation and since no complex data frame is present its simple * Max theoretical speed is clk/2 , max distance is low (intended for PCB and have no differential pair signal so detection of error is hard also usually high frequency) , max distance in few meters * High frequency high data transfer rate , Mbps ## Types of SPI * TI and Motorola SPI are slightly different SPI modes ## references: {% embed url="" %} {% embed url="" %} {% embed url="" %} ## video: {% embed url="" %}