7721 datasheet(93/577 Pages) RENESAS

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7721 Group User’s Manual

INPUT/OUTPUT PINS

6–4

6.2.2 Port register

Data is input from or output to the external by writing/reading data to/from a port register. A port register

consists of a port latch which holds the output data and a circuit which reads the pin state. Each bit of the

port register corresponds one for one to each pin of the microcomputer. Figure 6.2.3 shows the structure

of the port Pi (i = 4 to 10) register.

q When outputting data from programmable I/O port set to output mode

By writing data to the corresponding bit of the port register, the data is written into the port latch.

The data is output from the pin according to the contents of the port latch.

By reading the port register of a port set to the output mode, the contents of the port latch is read

out, instead of the pin state. Accordingly, the output data is correctly read without being affected by

an external load, etc. (Refer to “Figures 6.2.4 and 6.2.5.”)

q When inputting data from programmable I/O port set to input mode

A pin which is set to the input mode enters the floating state.

By reading the corresponding bit of the port register, the data which is input from the pin can be

read out.

By writing data to the port register of a programmable I/O port set to the input mode, the data is written

only into the port latch and is not output to the external (Note). The pin remains floating.

Note: When executing a read-modify-write instruction (CLB, SEB, INC, DEC, ASL, ASR, LSR, ROL,

ROR) to the port register of a programmable I/O port set to the input mode, the instruction is

executed to the data which is input from the pin and the result is written into the port register.

6.2 Programmable I/O ports

Fig. 6.2.3 Structure of port Pi (i = 4 to 10) register

Bit

Bit name

Functions

Port Pi0’s pin

Port Pi2’s pin

Port Pi3’s pin

Port Pi4’s pin

Port Pi6’s pin

Data is input from or output to a pin

by reading from or writing to the

corresponding bit.

Port Pi5’s pin

Port Pi register (i = 4 to 10)

(Addresses A16, B16, E16, F16, 1216, 1316, 1616)

Port Pi1’s pin

Port Pi7’s pin

At reset

Undefined

0 : “L” level

1 : “H” level

Note: For bits 0 to 2 of the port P4 register, nothing is assigned and these bits are fixed to “0” at reading.

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