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6.4 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.
6.4.1 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 microcomputer.
(1) Wiring for 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 V
shortest possible wiring (within 20mm).
Reason
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.
(2) Wiring for clock input/output pins
Make the length of wiring which is connected
to clock
I/O pins as short as possible.
Make the length of wiring (within 20mm)
across the grounding lead of a capacitor
which is connected to an oscillator and the
V
pin of a microcomputer as short as
SS
possible.
Separate the V
from other V
SS
Reason
If noise enters clock I/O pins, clock waveforms
may be deformed. This may cause a program
failure or program runaway. Also, if a potential
difference is caused by the noise between
the V
level of a microcomputer and the V
SS
level of an oscillator, the correct clock will not
be input in the microcomputer.
pin with the
SS
pattern only for oscillation
SS
patterns.
7220 Group User's Manual
6.4 Countermeasures against noise
Reset
circuit
V
SS
Fig.6.4.1 Wiring for RESET input pin
Noise
N.G.
Fig.6.4.2 Wiring for clock I/O pin
SS
APPENDIX
Noise
RESET
V
SS
N.G.
Reset
RESET
circuit
V
V
SS
SS
O.K.
X
X
IN
IN
X
X
OUT
OUT
V
V
SS
SS
O.K.
6-11
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