手册上貌似写得很简单，如下：
The ports 1, 2 and 3 of standard 8051 have internal pull-up resistor,and port 0 has open-drain outputs. Each I/O pin can
be used independently as an input or an output. For I/O ports to be used as an input pin, the port bit latch must contain a
‘1’ which turns off the output driver FET. Then for port 1, 2 and 3 port pin is pulled high by a weak internal pull-up, and can
be pulled low by an external source. The port 0 has open-drain outputs which means its pull-ups are not active during nor-
mal port operation. Writing ‘1’ to the port 0 bit latch will causing bit floating so that it can be used as a high-impedance
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