M38002M4 Datasheet, PDF (127/173 Pages) Mitsubishi Electric Semiconductor – 8-BIT SINGLE-CHIP MICROCOMPUTER

APPENDIX
3.4 Countermeasures against noise
Noise
N.G.
XIN
XOUT
VSS
XIN
XOUT
VSS
An example of VSS patterns on the
underside of a printed circuit board
Oscillator wiring
pattern example
XIN
XOUT
VSS
O.K.
Separate the VSS line for oscillation from other VSS lines
Fig. 3.4.2 Wiring for clock I/O pins
(3) Wiring for the VPP pin of the One Time PROM
version and the EPROM version
(In this microcomputer the VPP pin is also used
as the CNVSS pin)
Connect an approximately 5 kΩ resistor to the V P P
pin the shortest possible in series and also to the VSS
pin. When not connecting the resistor, make the
length of wiring between the VPP pin and the VSS pin
the shortest possible.
Note:Even when a circuit which inclued an
approximately 5 kΩ resistor is used in the Mask ROM
version, the maicrocomputer operates correctly.
Approximately
5kΩ
CNVSS/VPP
VSS
3800 group
Make it the shortest possible
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 wiring flow into the PROM. Be-
cause of this, noise can enter easily. If noise enters
the VPP pin, abnormal in struction codes or data are
read from the built-in PROM, which may cause a
program runaway.
Fig. 3.4.3 Wiring for the VPP pin of the One Time PROM
and the EPROM version
3.4.2 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.
VCC
VSS
Chip
VCC
VSS
Fig. 3.4.4 Bypass capacitor across the VSS line and
the VCC line
3800 GROUP USER’S MANUAL
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