M38002M4 Datasheet, PDF (126/173 Pages) Mitsubishi Electric Semiconductor – 8-BIT SINGLE-CHIP MICROCOMPUTER

APPENDIX
3.4 Countermeasures against noise
3.4 Countermeasures against noise
Countermeasures against noise are described below. The following countermeasures are effective against noise in
theory, however, it is necessary not only to take measures as follows but to evaluate before actual use.
3.4.1 Shortest wiring length
The wiring on a printed circuit board can be as an antenna which feeds noise into the microcomputer.
The shorter the total wiring length (by mm unit), the less the possibility of noise insertion into a microcomputer.
(1) Wiring for the RESET pin
Make the length of wiring which is connected to the RESET pin as short as possible. Especially, connect a capacitor
across the RESET pin and the VSS pin with the shortest possible wiring (within 20mm).
Reason
The reset works to initialize a microcomputer.
The width of a pulse input into the RESET pin is determined by the timing necessary conditions. If noise having
a shorter pulse width than the standard is input to the RESET pin, the reset is released before the internal state
of the microcomputer is completely initialized. This may cause a program runaway.
Noise
Reset
circuit
VSS
RESET
VSS
N.G.
3800 group
Reset
circuit
VSS
RESET
VSS
O.K. ^{3800 group}
Fig. 3.4.1 Wiring for the RESET pin
(2) Wiring for clock input/output pins
qMake the length of wiring which is connected to clock I/O pins as short as possible.
qMake the length of wiring (within 20mm) across the grounding lead of a capacitor which is connected to an
oscillatorand the VSS pin of a microcomputer as short as possible.
qSeparate the VSS pattern only for oscillation from other VSS patterns.
Reason
A microcomputer's operation synchronizes with a clock generated by the oscillator (circuit). If noise enters clock
I/O pins, clock waveforms may be deformed. This may cause a malfunction or program runaway.
Also, if a potential difference is caused by the noise between the VSS level of a microcomputer and the VSS level
of an oscillator, the correct clock will not be input in the microcomputer.
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3800 GROUP USER’S MANUAL