4553 datasheet(60/145 Pages) RENESAS

60 / 145 page

Rev.3.02

Dec 22, 2006

page 60 of 142

REJ03B0024-0302

4553 Group

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 fol-

lows but to evaluate before actual use.

1. Shortest wiring length

(1) Wiring for RESET pin

Make the length of wiring which is connected to the RESET pin as

short as possible. Especially, connect a capacitor across the

RESET pin and the VSS pin with the shortest possible wiring.

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 mi-

crocomputer is completely initialized.

This may cause a program runaway.

Fig. 54 Wiring for the RESET pin

(2) Wiring for clock input/output pins

• Make the length of wiring which is connected to clock I/O pins as

short as possible.

• Make the length of wiring across the grounding lead of a capaci-

tor which is connected to an oscillator and the VSS pin of a

microcomputer as short as possible.

• Separate the VSS pattern only for oscillation from other VSS pat-

terns.

If noise enters clock I/O pins, clock waveforms may be deformed.

This may cause a program failure or program runaway. Also, if a

potential difference is caused by the noise between the VSS level of

a microcomputer and the VSS level of an oscillator, the correct

clock will not be input in the microcomputer.

RESET

Reset

circuit

Noise

VSS

Reset

circuit

VSS

RESET

VSS

N.G.

O.K.

Noise

XIN

XOUT

VSS

XIN

XOUT

VSS

N.G.

O.K.

Fig. 55 Wiring for clock I/O pins

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