7542 Group
Rev.3.02
Oct 31, 2006
Page 69 of 134
REJ03B0006-0302
Fig. 88 Bypass capacitor across the VSS line and the VCC line
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 wiring via a bypass capacitor to the
VSS pin and the VCC pin.
VSS
VCC
VSS
VCC
N.G.
O.K.
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.
<Reason>
Signals which is input in an analog input pin (such as an A/D con-
verter/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 func-
tions as an antenna which feeds noise into the microcomputer,
which causes noise to an analog input pin.
Fig. 89 Analog signal line and a resistor and a capacitor
Analog
input pin
VSS
Noise
Thermistor
Microcomputer
N.G.
O.K.
(Note)
Note : The resistor is used for dividing
resistance with a thermistor.
The analog input pin is connected to the capacitor of a voltage
comparator. Accordingly, sufficient accuracy may not be ob-
tained by the charge/discharge current at the time of A/D
conversion when the analog signal source of high-impedance is
connected to an analog input pin. In order to obtain the A/D con-
version result stabilized more, please lower the impedance of an
analog signal source, or add the smoothing capacitor to an ana-
log input pin.