M38048FDHKP datasheet(334/387 Pages) RENESAS

334 / 387 page

3-44

REJ09B0212-0100Z

Rev.1.00

Jan 14, 2005

3804 Group (Spec.H)

APPENDIX

3.4 Countermeasures against noise

(2)

Wiring for clock input/output pins

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

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

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

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

q Reason

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

failure 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.

Noise

XIN

XOUT

VSS

XIN

XOUT

VSS

N.G.

O.K.

Fig. 3.4.2 Wiring for clock I/O pins

(3)

Wiring to CNVss pin

Connect the CNVss pin to the Vss pin with the shortest possible wiring.

q Reason

The processor mode of a microcomputer is influenced by a potential at the CNVss pin. If a

potential difference is caused by the noise between pins CNVss and Vss, the processor mode may

become unstable. This may cause a microcomputer malfunction or a program runaway.

Fig. 3.4.3 Wiring for CNVss pin

Noise

CNVSS

VSS

CNVSS

VSS

N.G.

O.K.

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