4559 datasheet(73/148 Pages) RENESAS

73 / 148 page

Rev.1.04

Aug 23, 2007

Page 73 of 146

REJ03B0188-0104

4559 Group

(2) Connection of bypass capacitor across VSS line

and VDD line

Connect an approximately 0.1

µF bypass capacitor across the

VSS line and the VDD line as follows:

• Connect a bypass capacitor across the VSS pin and the VDD pin

at equal length.

• Connect a bypass capacitor across the VSS pin and the VDD pin

with the shortest possible wiring.

• Use lines with a larger diameter than other signal lines for VSS

line and VDD line.

• Connect the power source wiring via a bypass capacitor to the

VSS pin and the VDD pin.

Fig 77. Bypass capacitor across the VSS line and the

VDD line

(3) Oscillator concerns

Take care to prevent an oscillator that generates clocks for a

microcomputer operation from being affected by other signals.

(1) Keeping oscillator away from large current signal lines

Install a microcomputer (and especially an oscillator) as far

as possible from signal lines where a current larger than the

tolerance of current value flows.

•Reason

In the system using a microcomputer, there are signal lines for

controlling motors, LEDs, and thermal heads or others. When

a large current flows through those signal lines, strong noise

occurs because of mutual inductance.

Fig 78. Wiring for a large current signal line

(2) Installing oscillator away from signal lines where potential

levels change frequently

Install an oscillator and a connecting pattern of an oscillator

away from signal lines where potential levels change

frequently. Also, do not cross such signal lines over the

clock lines or the signal lines which are sensitive to noise.

•Reason

Signal lines where potential levels change frequently (such as

the CNTR pin signal line) may affect other lines at signal

rising edge or falling edge. If such lines cross over a clock line,

clock waveforms may be deformed, which causes a

microcomputer failure or a program runaway.

Fig 79. Wiring to a signal line where potential levels

change frequently

(3) Oscillator protection using VSS pattern

As for a two-sided printed circuit board, print a VSS pattern

on the underside (soldering side) of the position (on the

component side) where an oscillator is mounted.

Connect the VSS pattern to the microcomputer VSS pin with

the shortest possible wiring.

Besides, separate this VSS pattern from other VSS patterns.

Fig 80. VSS pattern on the underside of an oscillator

VSS

VDD

VSS

VDD

N.G.

O.K.

XIN

XOUT

VSS

Microcomputer

Mutual inductance

Large

current

GND

XIN

XOUT

VSS

CNTR

Do not cross

N.G.

Oscillator wiring

pattern example

An example of VSS patterns on the

underside of a printed circuit board

Separate the VSS line for oscillation from other VSS lines

XIN

XOUT

VSS

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