4513 datasheet(191/210 Pages) RENESAS

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3-24

APPENDIX

3.4 Notes on noise

4513/4514 Group User’s Manual

3.4 Notes on noise

Countermeasures against noise are described below.

The following countermeasures are effective against

noise in theory, however, it is necessary not only to

take measures as follows but to evaluate before actual

use.

3.4.1 Shortest wiring length

The wiring on a printed circuit board can function

as an antenna which feeds noise into the

microcomputer.

The shorter the total wiring length (by mm unit), the

less the possibility of noise insertion into a

microcomputer.

(1)

Package

Select the smallest possible package to make

the total wiring length short.

q Reason

The wiring length depends on a microcom-

puter package. Use of a small package,

for example QFP and not DIP, makes the

total wiring length short to reduce influence

of noise.

Fig. 3.4.1 Selection of packages

(2)

Wiring for RESET input pin

Make the length of wiring which is connected

to the RESET input pin as short as possible.

Especially, connect a capacitor across the

RESET input pin and the VSS pin with the

shortest possible wiring.

q Reason

In order to reset a microcomputer correctly,

1 machine cycle or more of the width of a

pulse input into the RESET pin is required.

If noise having a shorter pulse width than

this is input to the RESET input pin, the

reset is released before the internal state

of the microcomputer is completely initialized.

This may cause a program runaway.

DIP

SDIP

SOP

QFP

Fig. 3.4.2 Wiring for the RESET input pin

RESET

Reset

circuit

Noise

VSS

Reset

circuit

VSS

RESET

VSS

N.G.

O.K.

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