📈SPI protocol

Notes on 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

master slave

• 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

    

    haf duplex mode

    

    simplex mode
• 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

SPI mode 0

SPI mode 1

SPI mode 2

SPI mode 3

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:

SPI communication basics (modes)TI Training

Introduction to SPI Interface | Analog Devices

Serial Peripheral InterfaceWikipedia

video: