7480 datasheet(256/337 Pages) RENESAS

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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

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

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