
3-96
APPENDIX
7540 Group User’s Manual
(2) Installing oscillator away from signal lines where potential levels change frequently
Install an oscillator and a connecting pattern of 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.
G
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) Oscillator protection using V
SS
pattern
As for a two-sided printed circuit board, print a V
SS
pattern on the underside (soldering side) of the
position (on the component side) where an oscillator is mounted.
Connect the V
SS
pattern to the microcomputer V
SS
pin with the shortest possible wiring. Besides,
separate this V
SS
pattern from other V
SS
patterns.
3.4 Countermeasures against noise
X
IN
X
OUT
V
SS
CNTR
Do not cross
N.G.
AAA
AAA
AAA
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
3.4.5 Setup for I/O ports
Setup I/O ports using hardware and software as
follows:
<Hardware>
Connect a resistor of 100
or more to an
I/O port in series.
<Software>
As for an input port, read data several times
by a program for checking whether input
levels are equal or not.
As for an output port, since the output data
may reverse because of noise, rewrite data
to its port latch at fixed periods.
Rewrite data to direction registers and pull-
up control registers at fixed periods.
Note:
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. 3.4.10 V
SS
pattern on the underside of an oscillator
Fig. 3.4.9 Wiring of signal lines where potential
levels change frequently
Fig. 3.4.11 Setup for I/O ports
Direction register
Port latch
Data bus
I/O port
pins
Noise
Noise
N.G.
O.K.