由8051组成的单片机系统通常情况下，P0口分时复用作为地址、数据总线，P2口提供A15-A8即高8位地址，P3口用作第二功能，只有P1口通常用作I/0口。P1口是8位准双向口，它的每一位都可独立地定义为输入或输出，因此既可作为8位的并行I/O口，也可作为8位的输入输出端。当工作在输入方式时，对应位的锁存器必须先置1,才能正确地读到引脚上的信号,否则,执行读引脚指令时,若对应位的锁存器的值为0，读的结果永远为0。-By 8051, composed of single-chip microcomputer system under normal circumstances, P0 timeshare I reuse as address, data bus, P2 I provide A15-A8 high eight address, P3 mouth for the second function, I usually only P1 for I/0 I. P1 I was eight quasi-two-way port, it can be independent of each and every defined as input or output, it can be used as 8-bit parallel I/O port, can also be used as 8-bit input and output side. When the work in the input, the corresponding bit latch must first buy one in order to correctly read the signal pin, or pin the implementation of reading instruction, if the corresponding bit latch the value of 0, Reading the results of Forever to 0.