3.2 Countermeasures against noise
APPENDIX
3825 GROUP USER’S MANUAL
3–13
Fig. 3.2.7 Wiring to a signal line where potential
levels change frequently
3.2.5 Installing an oscillator away from signal lines where
potential levels change frequently
Install an oscillator away from signal lines where
potential levels change frequently. Also, do not cross
such signal lines over the clock lines or the signal
lines which are sensitive to noise.
Reason
Signal lines where potential levels change frequently
(such as the CNTR pin signal line) may affect other
lines at signal rising edge or falling edge. If such
lines cross over a clock line, clock waveforms may
be deformed, which causes a microcomputer failure
or a program runaway.
3.2.6 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.
3.2.7 Setup for I/O ports
Setup I/O ports using hardware and software as
follows:
<Hardware>
qConnect a resistor of 100
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.
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.
Fig. 3.2.9 Setup for I/O ports
Fig. 3.2.8 VSS pattern on the underside of an
oscillator
XIN
XOUT
VSS
CNTR
Do not cross
XIN
XOUT
VSS
An example of VSS patterns on the
underside of a printed circuit board
Oscillator wiring
pattern example
Separate the VSS line for oscillation from other VSS lines
Direction register
Data register
Data bus
I/O port
pins
Noise