The slave (Slave) also returns the contents of its own serial shift register (0~7) to the host through the MISO signal line. At the same time, the data sent by the host is received through the MOSI signal line, so that the contents of the two shift registers are exchanged.
SPI is only divided into master mode and slave mode. There is no reading and writing. The write and read operations of peripherals are completed synchronously. If only a write operation is performed, the master simply ignores the received byte; conversely, if the master wants to read a byte from the slave, it must send a null byte to initiate a transfer from the slave. That is to say, if you send a piece of data, you will definitely receive a piece of data; if you want to receive a piece of data, you must also send a piece of data first.