Detailed explanation of SPI interface protocol

1. Introduction to SPI bus

The SPI bus is a high-speed, full-duplex, synchronous communication bus developed by Motorola, which usually consists of four lines:

• CS chip select signal
• SCLK clock signal
• MOSI master output slave input interface
• MISO master input slave output interface

SPI is divided into master and slave, and the master provides the clock signal required for communication.

2. Four working modes of SPI

The two most important parameters of SPI are clock polarity (CPOL) and phase (CPHA).

• CPOL : Clock Polarity determines whether the idle state level of the clock is high or low
  CPOL = 0: the clock is low when the clock is idle, and is valid when the clock is high
  CPOL = 1: the clock is high when the clock is idle, and is valid when the clock is low
• CPHA : Clock Phase determines the data transmission sampling and shifting method
  CPHA = 0: sampling on the first transition edge of the clock signal SCK
  CPHA = 1: sampling on the second transition edge of the clock signal SCK

Different combinations of clock and phase form four working modes of SPI:

model	CPOL	WASH
Mode 0	0	0
Mode 1	0	1
Mode 2	1	0
Mode 2	1	1

Let's take the spi bus of stm32 as an example:

Mode 0: when CPOL is 0 and CPHA is 0, data is sampled on the first transition edge (rising edge) of the clock; Mode 1: when CPOL is 0 and CPHA is 1, data is sampled on the first transition edge (falling edge); Mode 1: when CPOL is 1 and CPHA is 0, data is sampled on the first transition edge (falling edge); Mode 1: when CPOL is 1 and CPHA is 1, data is sampled on the first transition edge (
rising edge );

model	phase polarity	sampling	SCL idle level
Mode 0	CPOL = 0, CPHA = 0	rising edge	low level
Mode 1	CPOL = 0, CPHA = 1	falling edge	low level
Mode 2	CPOL = 1, CPHA = 0	falling edge	high level
Mode 2	CPOL = 1, CPHA = 1	rising edge	high level

Whether the SPI slave device SCL is high or low when idle determines whether CPOL is 0 or 1.

3. SPI full duplex and half duplex

When using mcu such as stm32, SPI usually has full-duplex and half-duplex options. The so-called full-duplex means that SPI can also receive data while sending data. The so-called half-duplex means that SPI sends and receives time-sharing, that is, it cannot receive data when sending, and cannot send data when receiving data.

4. Application circuit

One to one:

One-to-many: