M38203M4 Datasheet, PDF (298/344 Pages) Mitsubishi Electric Semiconductor – 8-BIT SINGLE-CHIP MICROCOMPUTER

APPENDIX
3.2 Countermeasures against noise
3.2.5 Oscillator protection using VSS pattern
As for a two-sided printed circuit board, print a VSS
pattern on the underside (soldering side) of the
position (on the component side) where an oscilla-
tor is mounted.
Connect the VSS pattern to the microcomputer VSS
pin with the shortest possible wiring. Besides, sepa-
rate this VSS pattern from other VSS patterns.
An example of VSS patterns on the
underside of a printed circuit board
Oscillator wiring
pattern example
XIN
XOUT
VSS
Separate the VSS line for oscillation from other VSS lines
3.2.6 Setup for I/O ports
Setup I/O ports using hardware and software as
follows:
<Hardware>
qConnect a resistor of 100 Ω or more to an I/O
port in series.
<Software>
qAs for an input port, read data several times by
a program for checking whether input levels are
equal or not.
qAs for an output port, since the output data may
reverse because of noise, rewrite data to its data
register at fixed periods.
qRewirte data to direction registers and pull-up con-
trol registers (only the product having it) at fixed
periods.
Fig. 3.2.7 VSS pattern on the underside of an
oscillator
Data bus
Direction register
Data register
Noise
Noise
I/O port
pins
Fig. 3.2.8 Setup for I/O ports
When a direction register is set for input port again at
fixed periods, a several-nanosecond short pulse may
be output from this port. If this is undesirable, connect
a capacitor to this port to remove the noise pulse.
3–14
3820 GROUP USER’S MANUAL