4559 datasheet(72/148 Pages) RENESAS

72 / 148 page

Rev.1.04

Aug 23, 2007

Page 72 of 146

REJ03B0188-0104

4559 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

follows but to evaluate before actual use.

(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) 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.

•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.

Fig 74. Wiring for the RESET input 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

capacitor 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 patterns.

Fig 75. Wiring for clock I/O pins

•Reason

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 inpu t in the

microcomputer.

(3) Wiring to CNVSS pin

Connect an approximately 5 k

Ω resistor to the VPP pin and

also to the GND pattern supplied to the VSS pin with

shortest possible wiring.

•Reason

The CNVSS pin is the power source input pin for the built-in

QzROM. When programming in the built-in QzROM, the

impedance of the CNVSS pin is low to allow the electric

current for writing flow into the QzROM. Because of this,

noise can enter easily. If noise enters the CNVSS pin, abnormal

instruction codes or data are read from the built-in QzROM,

which may cause a program runaway.

Fig 76. Wiring for CNVSS pin

RESET

Reset

circuit

Noise

VSS

N.G.

Reset

circuit

VSS

RESET

VSS

O.K.

Noise

XIN

XOUT

VSS

N.G.

XIN

XOUT

VSS

O.K.

CN V s s

V SS

T h e shorte st

about 5k

Ω

N ote : T h is indica te s pin.

(Note)

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