M38203M4 Datasheet, PDF (296/344 Pages) Mitsubishi Electric Semiconductor – 8-BIT SINGLE-CHIP MICROCOMPUTER

APPENDIX
3.2 Countermeasures against noise
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 VSS level of a microcomputer and
the VSS level of an oscillator, the correct clock will not be input in the microcomputer.
(3) Wiring to the VPP pin of the One Time
PROM version and the EPROM version
<When the VSS pin is also used as any other pin
than the CNVSS^V1 >
qMake the length of wiring which is connected
to the VPP pin as short as possible.
qConnect an approximately 5 kΩ resistor to the
VPP pin in serial (refer to Figure 3.2.3).
V1 When a microcomputer does not have the
CNVSS pin, the VPP pin is also as the input
pin adjacent to the RESET input pin.
Reason
The VPP 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 VPP
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 VPP pin,
abnormal instruction codes or data are read
from the built-in PROM, which may cause a
program runaway.
3820
Approximately
5 kΩ
P40/VPP
RESET
When the microcomputer does
not have the CNVSS pin, the VPP
pin is also used as the input pin
adjacent to the RESET pin.
Fig. 3.2.3 Wiring for the VPP pin of the One Time
PROM and the EPROM version
3.2.2 Connection of a bypass capacitor across
the VSS line and the VCC line
Connect an approximately 0.1 µF bypass capacitor
across the VSS line and the VCC line as follows:
qConnect a bypass capacitor across the VSS pin
and the VCC pin at equal length.
qConnect a bypass capacitor across the VSS pin
and the VCC pin with the shortest possible wiring.
qUse lines with a larger diameter than other signal
lines for VSS line and VCC line.
Chip
VC C
VSS
Fig. 3.2.4 Bypass capacitor across the VSS line
and the VCC line
3–12
3820 GROUP USER’S MANUAL