7477 datasheet(286/354 Pages) RENESAS

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2-51

7470/7471/7477/7478 GROUP USER’S MANUAL

APPLICATION

2.9 Countermeasures against noise

7470

7471

7477

7478

2.9.2 Connection of a bypass capacitor across the Vss line and

the Vcc line

Connect an approximately 0.1

µF bypass capacitor across

the VSS line and the VCC line as follows:

q Connect a bypass capacitor across the VSS pin and

the VCC pin at equal length.

q Connect a bypass capacitor across the VSS pin and

the VCC pin with the shortest possible wiring.

q Use lines with a larger diameter than other signal lines

for VSS line and VCC line.

2.9.3 Wiring to analog input pins

q Connect an approximately 100

Ω to 1 kΩ resistor to an

analog signal line which is connected to an analog

input pin in series. Besides, connect the resistor to the

microcomputer as close as possible.

q Connect an approximately 1000 pF capacitor across

the VSS pin and the analog input pin. Besides, connect

the capacitor to the VSS pin as close as possible. Also,

connect the capacitor across the analog input pin and

the VSS pin at equal length.

Reason

Signals which is input in an analog input pin (such as an

A-D converter input pin) are usually output signals from

sensor. The sensor which detects a change of event is

installed far from the printed circuit board with a

microcomputer, the wiring to an analog input pin is longer

necessarily.

This long wiring functions as an antenna which feeds

noise into the microcomputer, which causes noise to an

analog input pin.

If a capacitor between an analog input pin and the VSS

pin is grounded at a position far away from the VSS pin,

noise on the GND line may enter a microcomputer through

the capacitor.

Fig. 2.9.5 Analog signal line and a resistor

and a capacitor

Fig. 2.9.4 Bypass capacitor across the VSS

line and the VCC line

VSS

VCC

VSS

VCC

Chip

VSS

VCC

Thermistor

(Note)

Microcomputer

Analog

input pin

Note : The resistor is for dividing resistance

with a thermister.

VSS

Noise

N.G.

O.K.

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