APPENDIX
3.2 Countermeasures against noise
3820 GROUP USER’S MANUAL
3–12
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 program failure or program runaway.
Also, if a potential difference is caused by the noise between the V
SS
level of a microcomputer and
the V
SS
level of an oscillator, the correct clock will not be input in the microcomputer.
(3) Wiring to the V
PP
pin of the One Time
PROM version and the EPROM version
<When the V
SS
pin is also used as any other pin
than the CNV
SS
8
1
>
G
Make the length of wiring which is connected
to the V
PP
pin as short as possible.
G
Connect an approximately 5 k
resistor to the
V
PP
pin in serial (refer to
Figure 3.2.3
).
8
1 When a microcomputer does not have the
CNV
SS
pin, the V
PP
pin is also as the input
pin adjacent to the RESET input pin.
Reason
The V
PP
pin of the One Time PROM and the
EPROM version is the power source input pin
for the built-in PROM. When programming in
the built-in PROM, the impedance of the V
PP
pin is low to allow the electric current for writ-
ing flow into the PROM. Because of this, noise
can enter easily. If noise enters the V
PP
pin,
abnormal instruction codes or data are read
from the built-in PROM, which may cause a
program runaway.
3.2.2 Connection of a bypass capacitor across
the V
SS
line and the V
CC
line
Connect an approximately 0.1
μ
F bypass capacitor
across the V
SS
line and the V
CC
line as follows:
G
Connect a bypass capacitor across the V
SS
pin
and the V
CC
pin at equal length.
G
Connect a bypass capacitor across the V
SS
pin
and the V
CC
pin with the shortest possible wiring.
G
Use lines with a larger diameter than other signal
lines for V
SS
line and V
CC
line.
Fig. 3.2.4 Bypass capacitor across the V
SS
line
and the V
CC
line
Fig. 3.2.3 Wiring for the V
PP
pin of the One Time
PROM and the EPROM version
V
SS
V
CC
Chip
When the microcomputer does
not have the CNV
SS
pin, the V
PP
pin is also used as the input pin
adjacent to the RESET pin.
3820
P4
0
/V
PP
RESET
Approximately
5 k