7549 datasheet(77/86 Pages) RENESAS

77 / 86 page

Rev.2.02

Mar 31, 2009

Page 75 of 81

REJ03B0202-0202

7549 Group

5. Read-Modify-Write Instruction

Do not execute any read-modify-write instruction to the read

invalid (address) SFR.

The read-modify-write instruction reads 1-byte of data from

memory, modifies the data, and writes 1-byte the data to the

original memory.

In the 740 Family, the read-modify-write instructions are the

following:

(1) Bit handling instructions:

CLB, SEB

(2) Shift and rotate instructions:

ASL, LSR, ROL, ROR, RRF

(3) Add and subtract instructions:

DEC, INC

(4) Logical operation instructions (1’s complement):

COM

Although not the read-modify-write instructions, add and

subtract/logical operation instructions (ADC, SBC, AND, EOR,

and ORA) when T flag = “1” operate in the way as the read-

modify-write instruction. Do not execute them to the read invalid

SFR.

When the read-modify-write instruction is executed to the read

invalid SFR, the following may result:

As reading is invalid, the read value is undefined. The instruction

modifies this undefined value and writes it back, so the written

value will be indeterminate.

Notes on Peripheral Functions

Notes on I/O Ports

1. Pull-up control register

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.

Pull-up control is effective only when each direction register is

set to the input mode.

2. Use in Stand-By State

When using the MCU in stand-by state*1 for low-power

consumption, do not leave the input level of an I/O port

undefined. Be especially careful to the I/O ports for the N-

channel open-drain.

In this case, pull-up (connect to Vcc) or pull-down (connect to

Vss) these ports through a resistor.

When determining a resistance value, note the following:

• External circuit

• Variation in the output level during ordinary operation

When using a built-in pull-up resistor, note variations in current

values:

• When setting as an input port: Fix the input level

• When setting as an output port: Prevent current from

flowing out externally.

Even if a port is set to output by the direction register, when the

content of the port latch is “1”, the transistor becomes the OFF

state, which allows the port to be in the high-impedance state.

This may cause the level to be undefined depending on external

circuits.

As described above, if the input level of an I/O port is left

undefined, the power source current may flow because the

potential applied to the input buffer in the MCU will be unstable.

*1 Stand-by state: Stop mode by executing the STP instruction

Wait mode by executing the WIT instruction

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