4513_03 Datasheet, PDF (194/210 Pages) Renesas Technology Corp – 4-BIT SINGLE-CHIP MICROCOMPUTER

APPENDIX
3.4 Notes on noise
3.4.3 Wiring to analog input pins
• Connect 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.
• 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.
Noise
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.
Mutual inductance
M
Microcomputer
Thermistor
(Note)
Microcomputer
Analog
input pin
N.G.
O.K.
VSS
Note : The resistor is used for dividing
resistance with a thermistor.
Fig. 3.4.7 Analog signal line and a resistor and a
capacitor
Large
current
XIN
XOUT
VSS
GND
Fig. 3.4.8 Wiring for a large current signal line
(2) Installing oscillator away from signal lines
where potential levels change frequently
Install an oscillator and a connecting pattern
of an oscillator 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.
q Reason
Signal lines where potential levels change
frequently (such as the CNTR pin signal
line) may affect other lines at signal rising
edge 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.
4513/4514 Group User’s Manual
3-27