7477 datasheet(284/354 Pages) RENESAS

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2-49

7470/7471/7477/7478 GROUP USER’S MANUAL

APPLICATION

2.9 Countermeasures against noise

7470

7471

7477

7478

Fig. 2.9.2 Wiring for clock I/O pins

(2) Wiring for clock input/output pins

q Make the length of wiring which is connected to clock I/O pins as short as possible.

q Make the length of wiring (within 20mm) across the grounding lead of a capacitor which is connected

to an oscillator and the VSS pin of a microcomputer as short as possible.

q Separate the VSS pattern only for oscillation from other VSS patterns.

Reason

A microcomputer’s operation synchronizes with a clock generated by the oscillator (circuit). If noise

enters clock I/O pins, clock waveforms may be deformed. This may cause a malfunction or program

runaway.

Also, if a potential difference is caused by the noise between the VSS level of a microcomputer and

the VSS level of an oscillator, the correct clock will not be input in the microcomputer.

,,,,

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

Noise

XIN

XOUT

VSS

XIN

XOUT

VSS

O.K.

N.G.

lines

7477 Datasheet(PDF) 284 Page - Renesas Technology Corp