38D2 datasheet(120/136 Pages) RENESAS

120 / 136 page

Rev.3.02

Apr 10, 2008

Page 120 of 131

REJ03B0177-0302

38D2 Group

3. 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.

4. Pull-Up Control

Only for the pin set to input mode, pull-up is controlled by the

PULL register and the segment output disable register.

Notes on Termination of Unused Pins

1. Termination of Unused Pins

Perform the following at the shortest possible distance (20 mm or

less) from the MCU pins.

(1) I/O ports

Set the ports to input mode and connect each pin to VCC or VSS

through a resistor of 1 k to 10 k

Ω. An internal pull-up resistor can

also be used for the port where the internal pull-up resister is

selectable.

To set the ports to output mode, leave open at “L” or “H” output.

• When setting the ports to output mode and leave open,

input mode in the initial state remains until the mode of the

ports are switched to output mode by a program after a

reset. This may cause the voltage level of the pins to be

undefined and the power source current to increase while

the ports remains in input mode. For any effects on the

system, careful system evaluations should be implemented

on the user side.

• The direction registers may be changed due to a program

runaway or noise, so reset the registers periodically by a

program to increase the program reliability.

2. Termination Concerns

(1) When setting I/O ports to input mode

[1] Do not leave open

• The power source current may increase depending on the

first-stage circuit.

• The ports are more likely affected by noise when compared

with the termination shown on the above “1. (1) I/O ports”

[2] Do not connect to VCC or VSS directly

If the direction registers are changed to output mode due to 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 registers are changed to output mode due to a

program runaway or noise, a short circuit may occur between the

ports.

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