M38260E2FS datasheet(73/91 Pages) RENESAS

73 / 91 page

Rev.1.00

Sep 06, 2006

page 73 of 88

REJ03B0181-0100

3826 Group (One Time PROM version)

3.3.2 Notes on I/O port

(1)

Modifying output data with bit managing instruction

When the port latch of an I/O port is modified with the bit managing instruction (Note), the value

of the unspecified bit may be changed.

● Reason

I/O ports can be set to input or output mode in a bit unit. When reading or writing are performed

to the port Pi (i = 0–7) register, the microcomputer operates as follows.

•Port in input mode

-Read-access: reads pin’s level (The contents of port latch and pin’s level are unrelated.)

-Write-access: writes data to port latch (The contents of port latch and pin’s level are unrelated.)

•Port in output mode

-Read-access: reads port latch (The contents of port latch and pin’s level are unrelated.)

-Write-access: writes data to port latch (The contents of port latch are output from the pin.)

The bit managing instructions are read-modify-write form instructions (refer to “3.3.1 Notes on

programming (6)”) for reading and writing data by a byte unit.

Therefore, when the bit managing instructions are executed to the port set to input mode, the

instruction read the pin’s states, modify the specification bit, and then write data to the port latch.

At this time, if the contents of the original port latch are different from the pins’s level, the contents

of the port latch of bit which is not specified by instruction will change.

In addition to this, if the bit managing instructions are executed to the port Pi register in order to

setting output data when port Pi is configured as a mixed input and output port, the contents of

the port latch of bit in the input mode which is not specified by instruction may change.

Note: Bit managing instructions: SEB instruction, CLB instruction

(2)

The port direction registers are write-only registers. Therefore, the following instructions cannot be

used to this register:

•LDA instruction

•Memory operation instruction when T flag is “1”

•Instructions operating in addressing mode that modifies direction register

•Bit test instructions such as BBC and BBS

•Bit modification instructions such as CLB and SEB

•Arithmetic instructions using read-modify-write form instructions such as ROR

The LDM, STA instructions etc. are used for setting of the direction register.

(3)

Pull-up Operation

When using each port which built in pull-up resistor as an output port, the pull-up control bit of

corresponding port becomes invalid, and pull-up resistor is not connected.

● Reason

Pull-up control is effective only when each direction register is set to the input mode.

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