上拉就是将不确定的信号通过一个电阻钳位在高电平，电阻同时起限流作用。下拉同理，也是将不确定的信号通过一个电阻钳位在低电平。(The pull up is to make the uncertain signal through a resistance clamp at the high level, and the resistance at the same time acts as the current limiting. The drop in the same way, is also the undetermined signal through a resistance clamp at the low level.)