175
3. APPE NDIX
3819 Group USER’S MANUAL
MITSUBISHI MICROCOMPUTER
3819 Group
3.2 Countermeasures against noise
Fig. 3.2.2 Wiring for clock I/O pins
(3) Wiring for the V
PP
pin of the One Time PROM
version and the eprom version
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.
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. Because 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.
3.2.2 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. 3.2.3 Wiring for the V
PP
pin of the One Time PROM
and the EPROM version
Fig. 3.2.4 Bypass capacitor across the V
SS
line and
the V
CC
line
AAA
AAA
AAA
A
A
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
AA
AA
V
Noise
X
IN
X
OUT
V
SS
X
IN
X
OUT
V
SS
3819 group
P4
0
/V
PP
Approximately
5k