3802 GROUP USER’S MANUAL
3-29
APPENDIX
3.4 Countermeasures against noise
Fig. 3.4.2 Wiring for clock I/O pins
(3) Wiring for the V
PP
pin of the One Time PROM
version and the EPROM version
(In this microcomputer the V
PP
pin is also used
as the CNV
SS
pin)
Connect an approximately 5 k
resistor to theV
PP
pin the shortest possible in series and also to the V
SS
pin. When not connecting the resistor, make the
length of wiring between the V
PP
pin and the V
SS
pin
the shortest possible.
Note:Even when a circuit which inclued an
approximately 5 k
resistor is used in the Mask ROM
version, the maicrocomputer operates correctly.
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 wiring flow into the PROM. Be-
cause of this, noise can enter easily. If noise enters
the V
PP
pin, abnormal in struction codes or data are
read from the built-in PROM, which may cause a
program runaway.
3.4.2 Connection of a bypass capacitor across the
Vss line and the Vcc 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. 3.4.3 Wiring for the V
PP
pin of the One Time PROM
and the EPROM version
Fig. 3.4.4 Bypass capacitor across the V
SS
line and
the V
CC
line
V
SS
V
CC
V
SS
V
CC
Chip
X
IN
X
OUT
V
SS
An example of V
SS
patterns on the
underside of a printed circuit board
Oscillator wiring
pattern example
Separate the V
SS
line for oscillation from other V
SS
lines
Noise
X
IN
X
OUT
V
SS
X
IN
X
OUT
V
SS
N.G.
O.K.
CNV
SS
/V
PP
Approximately
5k
3802 group
V
SS
Make it the shortest possible