This constant defines the port on which the pullupresistor is connected.
这个常量定义了上拉电阻是连接到哪个口的。
2
Omit resistor R3 if IC2 includes built-in pullup resistors, as do many modern microcontrollers.
如果IC2包括内置的上拉电阻,就可以忽略电阻r 3,许多现代微处理器都是这样的。
3
Because the positive side of C1 cannot instantly change its polarity, it pulls low and slowly charges up again through the internal pullupresistor of the input.