7220 datasheet(233/303 Pages) RENESAS

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APPENDIX

6.4 Countermeasures against noise

7220 Group User’s Manual

6-14

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.

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)

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.

(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

Microcomputer

Mutual inductance

Large

current

GND

Fig.6.4.7 Wiring for large current signal line

XIN

XOUT

VSS

CNTR

Do not cross

N.G.

Fig.6.4.8 Wiring for signal line where potential

levels charge frequently

XIN

XOUT

VSS

An example of V SS patterns on the

underside of a printed circuit board

Oscillator wiring

pattern example

Separate the VSS line for oscillation from other V SS lines

Fig.6.4.9 VSS pattern on underside of an oscillator

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