7220 datasheet(232/303 Pages) RENESAS

232 / 303 page

7220 Group User’s Manual

APPENDIX

6.4 Countermeasures against noise

6-13

6.4.2 Connection of a 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:

qConnect a bypass capacitor across the VSS pin

and the VCC pin at equal length.

qConnect a bypass capacitor across the VSS pin

and the VCC pin with the shortest possible wiring.

qUse lines with a larger diameter than other signal

lines for VSS line and VCC line.

6.4.3 Wiring to analog input pins

qConnect 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.

qConnect 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/comparator 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.6.4.5 Bypass capacitor across VSS line and

VCC line

VSS

VCC

Chip

Fig.6.4.6 Analog signal line and resistor and capacitor

Analog

input pin

VSS

Noise

Thermistor

Microcomputer

N.G.

O.K.

(Note)

Note : The resistor is used for dividing

resistance with a thermistor.

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