4501 datasheet(207/212 Pages) RENESAS

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3.4 Notes on noise

3-38

Rev.2.01

Feb 07, 2005

4501 Group

APPENDIX

REJ09B0192-0201

3.4.3 Wiring to analog input pins

• 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.

• 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/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.

Fig. 3.4.7 Analog signal line and a resistor and a

capacitor

Analog

input pin

VSS

Noise

Thermistor

Microcomputer

N.G.

O.K.

(Note)

Note : The resistor is used for dividing

resistance with a thermistor.

3.4.4 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.

q 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. 3.4.8 Wiring for a large current signal line

XIN

XOUT

VSS

Microcomputer

Mutual inductance

Large

current

GND

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