M38203M4 Datasheet, PDF (295/344 Pages) Mitsubishi Electric Semiconductor – 8-BIT SINGLE-CHIP MICROCOMPUTER

APPENDIX
3.2 Countermeasures against noise
3.2 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.
3.2.1 Shortest wiring length
The wiring on a printed circuit board can function 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 microcom-
puter.
(1) Wiring for the reset input pin
Make the length of wiring which is connected to the RESET input pin as short as possible. Especially,
connect a capacitor across the RESET input pin and the VSS pin with the shortest possible wiring
(within 20 mm).
Reason
The reset works to initialize the internal state of 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 input pin, the reset is
released before the internal state of the microcomputer is completely initialized. This may cause a
program runaway.
Reset
circuit
VSS
Noise
RESET
VSS
Reset
circuit
VSS
RESET
VSS
Fig. 3.2.1 Wiring for the RESET input 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 20 mm) across
the grouding lead of a capacitor which is con-
nected to an oscillator and the VSS pin of a
microcomputer as short as possible.
qSeparate the VSS pattern only for oscillation
from other VSS patterns.
Noise
XIN
XOUT
VSS
XIN
XOUT
VSS
Fig. 3.2.2 Wiring for clock I/O pins
3820 GROUP USER’S MANUAL
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