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7470/7471/7477/7478 GROUP USER’S MANUAL

HARDWARE

1.10 I/O pins

1-45

1.10.3 Notes on use

When using I/O ports, note the following.

(1) Modify of the content of I/O port latch

When the content of the port latch of an I/O port is modified with the bit managing instruction*, the

value of the unspecified bit may be changed.

Reason

The bit managing instruction is read-modify-write instruction for reading and writing data by a byte

unit. Accordingly, when this instruction is executed on one bit of the port latch of an I/O port, the

following is executed to all bits of the port latch.

q As for a bit which is set as an input port: The pin state is read in the CPU, and is written to this

bit after bit managing.

q As for a bit which is set as an output port: The bit value is read in the CPU, and is written to this

bit after bit managing.

Make sure the following:

q Even when a port which is set as an output port is changed for an input port, its port latch holds

the output data.

q Even when a bit of a port latch which is set as an input port is not specified with a bit managing

instruction, its value may be changed in case where content of the pin differs from a content of

the port latch.

∗ bit managing instructions: SEB and CLB instruction

(2) Pull-up control

To pull-up ports by software, note the following.

q When P1 is used in the serial I/O mode, the pull-up settings corresponding to P14 to P17 are

invalidated (pull-up is impossible).

Refer to the port block diagram for details.

q When a port is set in the output mode, the pull-up setting corresponding to the port is invalidated

(pull-up is impossible).

q Ports other than P0 cannot be controlled in one-bit units. For example, when P10 is pulled up at

P1 (pull-up control in units of 4 bits), P11 to P13 are also pulled up.

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