7721 datasheet(522/577 Pages) RENESAS

522 / 577 page

7721 Group User’s Manual

17–59

APPENDIX

Appendix 8. Countermeasure against noise

General countermeasure examples against noise are described below. Although the effect of these

countermeasure depends on each system, refer to the following when an noise-related problem occurs.

1. Short wiring length

The wiring on a printed circuit board may function as an antenna which feeds noise into the microcomputer.

The shorter the total wiring length (by mm unit), the less possibility of noise insertion into the microcomputer.

(1)

______

Wiring for RESET pin

______

Make the length of wiring connected to the RESET pin as short as possible.

______

In particular, connect a capacitor between the RESET pin and the Vss pin with the shortest possible

wiring (within 20 mm).

Reason:

______

If noise is input to the RESET pin, the microcomputer restarts operation before the internal

state of the microcomputer is completely initialized. This may cause a program runaway.

Noise

XIN

XOUT

Vss

XIN

XOUT

Vss

M37721

Not acceptable

Acceptable

Fig. 4 Wiring for clock input/output pins

Reset

circuit

Vss

RESET

Vss

M37721

Acceptable

RESET

Reset

circuit

Noise

Vss

M37721

Not

acceptable

______

Fig. 3 Wiring for RESET pin

(2)

Wiring for clock input/output pins

q Make the length of wiring connected to the clock input/output pins as short as possible.

q Make the length of wiring between the grounding lead of the capacitor, which is connected to

the oscillator, and the Vss pin of the microcomputer, as short as possible (within 20 mm).

q Separate the Vss pattern for oscillation from all other Vss patterns. (Refer to “Figure 11.”)

Reason:

The microcomputer’s operation

synchronizes with a clock generated

by the oscillation circuit.

If noise enters clock I/O pins, clock

waveforms may be deformed. This

may cause a malfunction or a

program runaway.

Also, if the noise causes a potential

difference between the Vss level of

the microcomputer and the Vss level

of an oscillator, the correct clock

will

not

input

the

microcomputer.

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