3802 GROUP USER’S MANUAL
3-30
APPENDIX
3.4 Countermeasures against noise
3.4.3 Wiring to analog input pins
G
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.
G
Connect an approximately 1000 pF capacitor across
the V
SS
pin and the analog input pin. Besides,
connect the capacitor to the V
SS
pin as close as
possible. Also, connect the capacitor across the
analog input pin and the V
SS
pin at equal length.
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.
3.4.4. Consideration for oscillator
Take care to prevent an oscillator that generates
clocks for a microcomputer operation from being
affected by other signals.
(1) Keeping an 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.
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.
(2) Keeping an oscillator away from signal lines
where potential levels change frequently
Install an oscillator and a connecting pattern of an
osillator away from signal lines where potential levels
change frequently. Also, do not cross such signal
lines over the clock lines or the signal lines which are
sensitive to noise.
Reason
Signal lines where potential levels change frequently
(such as the CNTR pin line) may affect other lines at
signal rising or falling edge. If such lines cross over
a clock line, clock waveforms may be deformed,
which causes a microcomputer failure or a program
runaway.
Fig.3.4.6 Wiring for a large current signal line
Fig.3.4.7 Wiring to a signal line where potential levels
change frequently
X
IN
X
OUT
V
SS
M
Microcomputer
Mutual inductance
Large
current
GND
X
IN
X
OUT
V
SS
CNTR
Do not cross
Fig.3.4.5 Analog signal line and a resistor and a
capacitor
N.G.
O.K.
V
SS
Analog
input pin
Microcomputer
(Note)
Thermistor
Noise
Note:The resistor is for dividing resistance
with a thermister.