7540 datasheet(317/366 Pages) RENESAS

317 / 366 page

3-95

REJ09B0018-0200Z

Rev.2.00

Jun 21, 2004

7540 Group

APPENDIX

3.4 Countermeasures against noise

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.

q 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

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

Analog

input pin

VSS

Noise

Thermistor

Microcomputer

N.G.

O.K.

(Note)

Note : The resistor is used for dividing

resistance with a thermistor.

XIN

XOUT

VSS

Microcomputer

Mutual inductance

Large

current

GND

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