
7480 Group and 7481 Group User's Manual
2-41
2.8 Countermeasures against Noise
APPLICATIONS
2.8.5 Consideration for Oscillator
Take care to prevent an oscillator that generates
clocks for a microcomputer operation from being
affected by other signals.
(1)
Keeping Oscillator Away from Large Current
Signal Lines
Install a microcomputer (and especially an
oscillator) as far as possible from signal lines
where a current larger than the tolerance of
current value flows.
REASON
In the system using a microcomputer, there
are signal lines for controlling motors, LEDs,
and thermal heads or others. When a large
current flows through those signal lines, strong
noise occurs because of mutual inductance.
(2)
Keeping 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.
REASON
Signal lines where potential levels change
frequently (such as the CNTR pin line) may
affect other lines at signal rising 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 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 oscillator is mounted.
Connect the VSS pattern to the microcomputer
VSS pin with the shortest possible wiring.
Besides, separate this VSS pattern from other
VSS patterns.
XIN
XOUT
VSS
M
7480 Group
7481 Group
Mutual inductance
Large
current
GND
Figure 2.8.7 Wiring for Large Current Signal Line
Figure 2.8.8 Wiring to Signal Line Where Potential
Levels Change Frequently
XIN
XOUT
7480 Group
7481Group
VSS
CNTR
Do not cross
Figure 2.8.9 VSS Patterns on Underside of Oscillator
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
7480 Group
7481 Group