当P0作为I/O口使用时，特别是作为输出时，输出级属于开漏电路，必须外接上拉电阻才会有高电平输出；如果作为输入，必须先向相应的锁存器写“1”，才不会影响输入电平。-When P0 as the I/O port to use, especially as the output, open drain output stage circuit is to be an external pull-up resistor will have a high output if, as input, t