7721 datasheet(526/577 Pages) RENESAS

526 / 577 page

7721 Group User’s Manual

17–63

APPENDIX

Appendix 8. Countermeasure against noise

4. Oscillator protection

The oscillator, which generates the basic clock for the microcomputer operations, must be protected from

the affect of other signals.

(1)

Distance oscillator from signal lines with large current flows

Install the microcomputer, especially the oscillator, as far as possible from signal lines which handle

currents larger than the microcomputer current value tolerance.

Reason:

The microcomputer is used in

systems which contain signal lines

for controlling motors, LEDs,

thermal heads, etc. Noise occurs

due to mutual inductance when a

large current flows through the signal

lines.

(2)

Distance oscillator from signal lines with frequent potential level changes

q Install an oscillator and its wiring pattern away from signal lines where potential levels change

frequently.

q Do not cross these signal lines over the clock-related or noise-sensitive signal lines.

Reason:

Signal lines with frequently changing

potential levels may affect other

signal lines at a rising or falling edge.

In particular, if the lines cross over

a clock-related signal line, clock

waveforms may be deformed, which

causes a microcomputer malfunction

or a program runaway.

XIN

XOUT

Vss

M37721

Mutual inductance

Large

current

XIN

XOUT

Vss

Do not cross.

M377 1

V I/O pin for signal with frequently

changing potential levels

Fig. 10 Wiring for signal lines where potential levels

frequently change

(3)

Oscillator protection using Vss pattern

Print a Vss pattern on the bottom (soldering

side) of a double-sided printed circuit board,

under the oscillator mount position.

Connect the Vss pattern to the Vss pin of the

microcomputer with the shortest possible wiring,

separating it from other Vss patterns.

XIN

XOUT

Vss

An example of Vss pattern on the

underside of an oscillator.

Mounted pattern

example of

oscillator unit.

Separate Vss lines for oscillation and supply.

M37721

Fig. 11 Vss pattern underneath mounted oscillator

Fig. 9 Wiring for signal lines where large current

flows

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