7549 datasheet(59/84 Pages) RENESAS

59 / 84 page

Rev.2.01

Oct 15, 2007

Page 59 of 81

REJ03B0202-0201

7549 Group

2. Connection of bypass capacitor across VSS line

and VCC line

Connect an approximately 0.1

µF bypass capacitor across the

VSS line and the VCC line as follows:

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

pin at equal length.

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

pin with the shortest possible wiring.

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

VSS line and VCC line.

• Connect the power source wiring via a bypass capacitor to

the VSS pin and the VCC pin.

Fig 76. Bypass capacitor across the VSS line and the

VCC line

3. Wiring to analog input pins

The analog input pin is connected to the capacitor of a voltage

comparator. Accordingly, sufficient accuracy may not be

obtained by the charge/discharge current at the time of A/D

conversion when the analog signal source of high-impedance is

connected to an analog input pin. In order to obtain the A/D

conversion result stabilized more, please lower the impedance of

an analog signal source, or add the smoothing capacitor to an

analog input pin.

4. Oscillator concerns

Take care to prevent an oscillator that generates clocks for a mi-

crocomputer 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 toler-

ance of current value flows.

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.

(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.

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 wave-

forms may be deformed, which causes a microcomputer failure

or a program runaway.

Fig 77.

Wiring for a large current signal line/Writing of signal

lines 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 78. VSS pattern on the underside of an oscillator

VSS

VCC

VSS

VCC

N.G.

O.K.

XIN

XOUT

VSS

Microcomputer

Mutual inductance

Large

current

GND

XIN

XOUT

VSS

CNTR

Do not cross

N.G.

(1) Keeping oscillator away from large current

signal lines

(2) Installing oscillator away from signal lines

where potential levels change frequently

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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