7480 Group and 7481 Group User's Manual
2-40
2.8 Countermeasures against Noise
APPLICATIONS
2.8.2 Connection of Bypass Capacitor across VSS
Line and VCC Line
Connect an approximately 0.1
F bypass capacitor
across the VSS line and the VCC line as follows:
Connect a bypass capacitor across the VSS pin
and the VCC pin at equal length.
Connect a bypass capacitor across the VSS pin
and the VCC pin with the shortest possible wiring.
Use lines with a larger diameter than other signal
lines for VSS line and VCC line.
Connect the power source line to VSS pin and
VCC pin through a bypass capacitor.
2.8.3 Connection of Bypass Capacitor across VSS
Line and VREF Line
Connect an approximately 0.01
F bypass capacitor
across the VSS line and the VREF line as follows:
Connect a bypass capacitor across the VSS pin
and the VREF pin at equal length.
Connect a bypass capacitor across the VSS pin
and the VREF pin with the shortest possible wiring.
Use lines with a larger diameter than other signal
lines for VSS line and VREF line.
2.8.4 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 0.1 to 1
F 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 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.
Figure 2.8.4 Bypass Capacitor across VSS Line and
VCC Line
Figure 2.8.5 Bypass Capacitor across VSS Line and
VREF Line
Figure 2.8.6 Analog Signal Line and Resistor and
Capacitor
N.G.
O.K.
VSS
Analog
input pin
Microcomputer
(Note)
Thermistor
Noise
Note:The resistor is used for dividing
resistance with a thermistor.
VSS
VCC
VSS
VCC
N.G.
O.K.
VSS
VREF
VSS
VREF
N.G.
O.K.