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APPENDIX
3.2 Countermeasures against noise
3822 GROUP USER’S MANUAL
3–12
3.2.3 Wiring to analog input pins
qConnect an approximately 100
to 1 k 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 possi-
ble.
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.
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.
3.2.4 Oscillator concerns
Take care to prevent an oscillator that generates
clocks for a microcomputer operation from being
affected by other signals.
(1) Installing an oscillator away from large cur-
rent 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.
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. 3.2.6 Wiring for a large current signal line
Fig. 3.2.5 Analog signal line and a resistor and
a capacitor
Analog
input pin
VSS
Sensor
Noise
Microcomputer
XIN
XOUT
VSS
M
Microcomputer
Mutual inductance
Large
current
GND