M38002M4 Datasheet, PDF (128/173 Pages) Mitsubishi Electric Semiconductor – 8-BIT SINGLE-CHIP MICROCOMPUTER

APPENDIX
3.4 Countermeasures against noise
3.4.3. 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.
Mutual inductance
M
Microcomputer
Large
current
XIN
XOUT
VSS
GND
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.
Fig.3.4.5 Wiring for a large current signal line
(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.
Do not cross
CNTR
XIN
XOUT
VSS
Fig.3.4.6 Wiring to a signal line where potential levels
change frequently
3.4.4 Setup for I/O ports
Setup I/O ports using hardware and software as follows:
Noise
O.K.
<Hardware>
qConnect a resistor of 100 Ω or more to an I/O port
inseries.
<Software>
qAs for an input port, read data several times by a
program for checking whether input levels are
equal or not.
qAs for an output port, since the output data may
Data bus
Direction register
Port latch
Noise
N.G.
I/O port
pins
reverse because of noise, rewrite data to its port
latch at fixed periods.
qRewirte data to direction registers and pull-up
Fig. 3.4.7 Setup for I/O ports
control registers (only the product having it) at fixed
periods.
When a direction register is set for input port again at fixed periods, a several-nanosecond short pulse may be
output from this port. If this is undesirable, connect a capacitor to this port to remove the noise pulse.
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3800 GROUP USER’S MANUAL