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4. Oscillator protection
The oscillator which generates the basic clock for the microcomputer operations must be protected from
the affect of other signals.
(1) Distance oscillator from signal lines with large current flows
Install the microcomputer, especially the oscillator, as far as possible from signal lines which handle
currents larger than the microcomputer current value tolerance.
Reason: A microcomputer is used in systems
which contain signal lines for
controlling motors, LEDs, thermal
heads, etc. Noise occurs due to
mutual inductance when a large
current flows through the signal
lines.
(2) Distance oscillator from signal lines with frequent potential level changes
Install an oscillator and a connecting pattern of an oscillator away from signal lines in which
potential levels change frequently.
Do not cross the signal lines over the clock-related or noise-sensitive signal lines.
Reason: S i g n a l s l i n e s w i t h f r e q u e n t l y
changing potential levels may affect
other signal lines at a rising or falling
edge. In particular, if the lines cross
over a clock-related signal line, clock
waveforms may be deformed, which
causes a microcomputer malfunction
or a program runaway.
Appendix 8. Examples of noise immunity improvement
Fig. 12. Connection of signal wires where a large
Fig. 13. Wiring of rapidly level changing signal wire
7751 Group User's Manual
Mutual inductance
M
Large
current
current flows
M37751
Do not cross.
X
IN
X
OUT
Vss
I/O pin for signal with frequently
changing potential levels.
APPENDIX
M37751
X
IN
X
OUT
Vss
20–67
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