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3-50
APPENDIX
7531 Group User’s Manual
3.4.3 Wiring to analog input pins
Connect 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 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.
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. 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
3.4 Countermeasures against noise
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
M
Microcomputer
Mutual inductance
Large
current
GND