Fujitsu F2MC-16LX Hardware Manual page 157

Note that the operation of port 5 R/W differs from that of another port.
Port 5 (P55 to P50) is a general-purpose I/O port of the open-drain output format.
When port 5 is used as an input port, to turn the open-drain output transistor off, the output data
register must be set to "1" and an external pull-up resistor must be installed.
Moreover, port 5 operates as follows depending on the reading instruction:
❍ Reading by a read modify write instruction
The description of the output data register is read. Even if a pin is set to "0" externally, the
contents of the bit not specified by the instruction do not change.
❍ Reading by an instruction other than the above instruction
The level of a pin can be read.
If port 5 is used as an output port, the value of a pin can be changed by writing a desired value
in the corresponding output data register.
"0" is read from the pin corresponding to a bit set to "1" in the ADER register.
7.3 I/O Port Registers
141