4571 datasheet(59/126 Pages) RENESAS

59 / 126 page

Rev.1.02

May 25, 2007

Page 59 of 124

REJ03B0179-0102

4571 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

(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 microcomputer is completely initialized.

This may cause a program runaway.

Fig 62. 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

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.

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.

Fig 63. Wiring for clock I/O pins

(3) Port K Wiring

Do not leave port K open. Always connect it to the VDD pin or

VSS pin using the thickest wire at the shortest distance.

When port K is used for key matrix, connect it to the VDD pin

through a pull-up resistor.

In that case too, place a pull-up resistor close to port K and

connect it to port K or the VDD pin using the thickest wire at the

shortest distance as above.

Port K is also used as the power source input pin (VPP pin) for

the built-in QzROM.

When programming to the QzROM, the impedance of port K is

low so that the electric writing current will flow into the

QzROM. This allows noise to enter easily. If noise enters from

port K, abnormal instruction codes or data are read from the

QzROM, which may cause a program runaway.

Fig 64.

Wiring for port K

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.

VDD

VSS

Shortest

distance

(Note)

Note: This indicates pin.

VDD

VSS

Shortest

distance

(Note)

VDD

VSS

Shortest

distance

Shortest

distance

A pull-up resistor

(Note)

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