ATmega48/88/168的PB5是SPI时钟输出，接74HC595/74HC165的移位时钟输入端；PB4是SPI的MISO数据输人，接74HC165的数据输出；PB3是SPI的MOSI数据输出，接74HC595的串行数据输入端SER；PB2接74HC595/74HC165的锁存时钟输入端。-PB5 The ATmega48/88/168 SPI clock output connected to the input end of a 74HC595/74HC165 the shift c

After sending one byte over SPI, you have to clear one byte in the data register (DR) again. In SPI you always send and receive a byte You cannot just send OR receive a byte, it's always AND. The technical explanation is that there is only one clock