This constant defines the port on which the pullup resistor is connected.
这个常量定义了上拉电阻是连接到哪个口的。
It is the responsibility of the host designer to connect external pullup resistors to those lines.
主机设计者有责任给这些连接线加上外接的上拉电阻。
Because the positive side of C1 cannot instantly change its polarity, it pulls low and slowly charges up again through the internal pullup resistor of the input.
而C1的正向不能立即改变极性，其被拉低并通过输入的内部上拉电阻，缓慢的再次充电。
Because the positive side of C1 cannot instantly change its polarity, it pulls low and slowly charges up again through the internal pullup resistor of the input.
而C1的正向不能立即改变极性，其被拉低并通过输入的内部上拉电阻，缓慢的再次充电。
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