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。 -ATmega48/88/168 the PB5 is the SPI clock output, then 74HC595/74HC165 shift clock input　PB4 is the MISO SPI data input, data output connected 74HC165　PB3 SPI, MOSI is the data output, then the string of 74HC595 line data input SER　PB2 access 74HC595/74HC165 latch clock input.