SPI is a [single-master] communication protocol, which means that only one central device on the bus can initiate communication. When the SPI master device wants to read/write [slave device], it first pulls down the SS line corresponding to [slave device] (SS is active low), and then starts to send working pulses to the clock line, at the corresponding pulse time [Master device] sends the signal to MOSI to realize “write”, and can sample MISO to realize “read”
Both the SPI master and the slave have a serial shift register, and the master initiates a transfer by writing a byte to its SPI serial register.