7220 Datasheet, PDF (232/303 Pages) Renesas Technology Corp – 8-BIT SINGLE-CHIP MICROCOMPUTER

6.4.2 Connection of a bypass capacitor across
VSS line and VCC line
Connect an approximately 0.1 µF bypass capacitor
across the VSS line and the VCC line as follows:
qConnect a bypass capacitor across the VSS pin
and the VCC pin at equal length.
qConnect a bypass capacitor across the VSS pin
and the VCC pin with the shortest possible wiring.
qUse lines with a larger diameter than other signal
lines for VSS line and VCC line.
APPENDIX
6.4 Countermeasures against noise
Chip
VCC
VSS
Fig.6.4.5 Bypass capacitor across VSS line and
VCC line
6.4.3 Wiring to analog input pins
qConnect 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.
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. 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/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.
Noise
Thermistor
(Note)
Microcomputer
Analog
input pin
N.G. O.K.
VSS
Note : The resistor is used for dividing
resistance with a thermistor.
Fig.6.4.6 Analog signal line and resistor and capacitor
7220 Group User’s Manual
6-13