问片外存储器时，下降沿用于控制外接的地址锁存器锁存从P0口输出的低8位地址。在没有接外部存储器时，可以将该引脚的输出作为时钟信号使用-Q-chip memory, the falling edge is used to control an external address latch P0 port output latch from the lower 8-bit address. In the absence of access to external memory, you can copy the output pin to use as a clock signal