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7542 Group
NOTES ON USE
Countermeasures against noise
1. Shortest wiring length
(1) Package
Select the smallest possible package to make the total wiring
length short.
<Reason>
The wiring length depends on a microcomputer package. Use of a
small package, for example QFP and not DIP, makes the total wir-
ing length short to reduce influence of noise.
DIP
SDIP
Fig. 84 Selection of packages
(2) Wiring for 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 V
pin with the shortest possible wiring
SS
(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 pin, the reset is released
before the internal state of the microcomputer is completely initial-
ized. This may cause a program runaway.
Reset
circuit
V
SS
Reset
circuit
Fig. 85 Wiring for the RESET pin
Rev.3.02
Oct 31, 2006
REJ03B0006-0302
SOP
QFP
Noise
RESET
V
SS
N.G.
RESET
V
V
SS
SS
O.K.
Page 68 of 134
(3) 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 20 mm) across the grounding
lead of a capacitor which is connected to an oscillator and the
V
pin of a microcomputer as short as possible.
SS
• Separate the V
pattern only for oscillation from other V
SS
terns.
<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
of a microcomputer and the V
clock will not be input in the microcomputer.
Noise
X
X
V
N.G.
Fig. 86 Wiring for clock I/O pins
(4) Wiring to CNVss pin
Connect the CNVss pin to the Vss pin with the shortest possible
wiring.
In the normal microcomputer mode, disconnect a wiring of a serial
rewrite circuit, which is for the flash memory version, from the
MCU by a jumper switch.
<Reason>
The processor mode of a microcomputer is influenced by a poten-
tial at the CNVss pin. If a potential difference is caused by the
noise between pins CNVss and Vss, the processor mode may be-
come unstable. This may cause a microcomputer malfunction or a
program runaway.
A wiring of a serial rewrite circuit may function as an antenna
which feeds noise into the microcomputer.
Noise
CNV
SS
V
SS
N.
Fig. 87 Wiring for CNVss pin
level of an oscillator, the correct
SS
X
IN
IN
X
OUT
OUT
V
SS
SS
O.K.
Flash memory version
Jumper switch
CNV
CNV
SS
SS
V
V
SS
SS
O.
O.
pat-
SS
level
SS
Serial
rewrite
circuit
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