7549 datasheet(78/86 Pages) RENESAS

78 / 86 page

Rev.2.02

Mar 31, 2009

Page 76 of 81

REJ03B0202-0202

7549 Group

3. Modifying Output Data with Bit Handling

Instruction

When the port latch of an I/O port is modified with the bit

handling instruction*1, the value of an unspecified bit may

change.

I/O ports can be set to input mode or output mode in byte units.

When the port register is read or written, the following will be

operated:

• Port as input mode

Read: Read the pin level

Write: Write to the port latch

• Port as output mode

Read: Read the port latch or peripheral function output

(specifications vary depending on the port)

Write: Write to the port latch (output the content of the port

latch from the pin)

Meanwhile, the bit handling instructions are the read-modify-

write instructions*2. Executing the bit handling instruction to the

port register allows reading and writing a bit unspecified with the

instruction at the same time.

If an unspecified bit is set to input mode, the pin level is read and

the value is written to the port latch. At this time, if the original

content of the port latch and the pin level do not match, the

content of the port latch changes.

If an unspecified bit is set to output mode, the port latch is

normally read, but the peripheral function output is read in some

ports and the value is written to the port latch. At this time, if the

original content of the port latch and the peripheral function

output do not match, the content of the port latch changes.

*1 Bit handling instructions: CLB, SEB

*2 Read-modify-write instruction: Reads 1-byte of data from

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

the original memory.

4. Direction Registers

The values of the port direction registers cannot be read. This

means, it is impossible to use the LDA instruction, memory

operation instruction when the T flag is “1”, addressing mode

using direction register values as qualifiers, and bit test

instructions such as BBC and BBS. It is also impossible to use bit

operation instructions such as CLB and SEB, and read-modify-

write instructions to direction registers, including calculations

such as ROR. To set the direction registers, use instructions such

as LDM or STA.

Termination of Unused Pins

1. Terminate unused pins

Perform the following wiring at the shortest possible distance (20

mm or less) from microcomputer pins.

(1)

I/O ports

Set the I/O ports for the input mode and connect each pin to VCC

or VSS through each resistor of 1 k

Ω to 10 kΩ. The port which

can select a built-in pull-up resistor can also use the built-in pull-

up resistor.

When using the I/O ports as the output mode, open them at “L”

or “H”.

• When opening them in the output mode, the input mode of the

initial status remains until the mode of the ports is switched

over to the output mode by the program after reset. Thus, the

potential at these pins is undefined and the power source

current may increase in the input mode. With regard to an

effects on the system, thoroughly perform system evaluation

on the user side.

• Since the direction register setup may be changed because of a

program runaway or noise, set direction registers by program

periodically to increase the reliability of program.

2. Termination remarks

(1)

I/O ports setting as input mode

[1] Do not open in the input mode.

• The power source current may increase depending on the first

stage circuit.

• An effect due to noise may be easily produced as compared

with proper termination (1) shown on the above “1. Terminate

unused pins”.

[2] Do not connect to VCC or VSS directly.

If the direction register setup changes for the output mode

because of a program runaway or noise, a short circuit may

occur.

[3] Do not connect multiple ports in a lump to VCC or VSS

through a resistor.

If the direction register setup changes for the output mode

because of a program runaway or noise, a short circuit may occur

between ports.

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