7540 datasheet(296/366 Pages) RENESAS

296 / 366 page

3-74

REJ09B0018-0200Z

Rev.2.00

Jun 21, 2004

APPENDIX

7540 Group

3.3 Notes on use

3.3.2 Termination of unused pins

(1) Terminate unused pins

➀ I/O ports :

• Set the I/O ports for the input mode and connect them to VCC or VSS through each resistor of

1 k

Ω to 10 kΩ.

Ports that permit the selecting of a built-in pull-up resistor can also use this resistor. Set the I/

O ports for the output mode and 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

➀ Input ports and I/O ports :

Do not open in the input mode.

q Reason

• 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 ➁ and

➂ shown on the above.

➁ I/O ports :

When setting for the input mode, do not connect to VCC or VSS directly.

q Reason

If the direction register setup changes for the output mode because of a program runaway or

noise, a short circuit may occur between a port and VCC (or VSS).

➂ I/O ports :

When setting for the input mode, do not connect multiple ports in a lump to VCC or VSS through

a resistor.

q Reason

If the direction register setup changes for the output mode because of a program runaway or

noise, a short circuit may occur between ports.

• At the termination of unused pins, perform wiring at the shortest possible distance (20 mm or less)

from microcomputer pins.

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