首先来了解一下上拉/下拉电阻的概念及作用
Usually, pull-ups are relatively weak (low amounts of resistance) so the processor can
drive a pulled-up I/O line to be low. The pull-up means that the signal on that line is
defined to be high even when the processor isn't driving it. A processor may have internal
pull-ups so that inputs to the processor have a default state even when unconnected.
Note that there are also pull-downs, which means resistors to ground. All of this applies to
them except that their default logic level is low instead of high.
通常，上拉电阻相对较弱（低阻值，弱上拉），处理器可以将上拉电阻的 I/O线拉低。“上拉”意味着即使处理
器在没有驱动该 I/O线时（比如开机默认为高就是这么来的！），此 I/O线上的信号表现为（定义为）高电位。某些处理器自带内部的上拉电阻是为了在外部即使没有器件输入连接到处理器时，能使该I/O线有一个缺省的默认值。理解了“上拉电阻”，“下拉电阻”自然就一目了然了。
twjiang: staff of Nuvoton Corporation
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