3800 GROUP USER’S MANUAL
3-26
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.
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.
(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.
3.4.4 Setup for I/O ports
Setup I/O ports using hardware and software as follows:
<Hardware>
G
Connect a resistor of 100
or more to an I/O port
inseries.
<Software>
G
As for an input port, read data several times by a
program for checking whether input levels are
equal or not.
G
As for an output port, since the output data may
reverse because of noise, rewrite data to its port
latch at fixed periods.
G
Rewirte data to direction registers and pull-up
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.
Fig.3.4.5 Wiring for a large current signal line
Fig.3.4.6 Wiring to a signal line where potential levels
change frequently
X
IN
X
OUT
V
SS
M
Microcomputer
Mutual inductance
Large
current
GND
X
IN
X
OUT
V
SS
Do not cross
Fig. 3.4.7 Setup for I/O ports
Direction register
Port latch
Data bus
I/O port
pins
Noise
Noise
N.G.
O.K.