7220 Datasheet, PDF (230/303 Pages) Renesas Technology Corp – 8-BIT SINGLE-CHIP MICROCOMPUTER

APPENDIX
6.4 Countermeasures against noise
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 VSS pin with the
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.
Reset
circuit
VSS
Noise
N.G.
RESET
VSS
Reset
circuit
VSS
RESET
VSS
O.K.
Fig.6.4.1 Wiring for RESET input pin
(2) Wiring for clock input/output pins
qMake the length of wiring which is connected
to clock
qI/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 oscillator and the
VSS pin of a microcomputer as short as
possible.
qSeparate the VSS pattern only for oscillation
from other VSS patterns.
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 VSS level of a microcomputer and the VSS
level of an oscillator, the correct clock will not
be input in the microcomputer.
Noise
XIN
XOUT
VSS
XIN
XOUT
VSS
N.G.
O.K.
Fig.6.4.2 Wiring for clock I/O pin
7220 Group User’s Manual
6-11