7220 Datasheet, PDF (233/303 Pages) Renesas Technology Corp – 8-BIT SINGLE-CHIP MICROCOMPUTER

APPENDIX
6.4 Countermeasures against noise
6.4.4 Oscillator concerns
Take care to prevent an oscillator that generates clocks for a microcomputer operation from being affected
by other signals.
(1) Keeping an 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.
Mutual inductance
M
Microcomputer
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.
Large
current
XIN
XOUT
VSS
GND
Fig.6.4.7 Wiring for large current signal line
(2) Installing an 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 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.
N.G.
Do not cross
CNTR
XIN
XOUT
VSS
Fig.6.4.8 Wiring for signal line where potential
levels charge frequently
(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.
An example of V SS patterns on the
underside of a printed circuit board
Oscillator wiring
pattern example
XIN
XOUT
VSS
Separate the VSS line for oscillation from other V SS lines
Fig.6.4.9 VSS pattern on underside of an oscillator
6-14
7220 Group User’s Manual