38K2 datasheet(293/358 Pages) RENESAS

293 / 358 page

3.3 Countermeasures against noise

Rev.2.00

Oct 15, 2006

page 32 of 99

38K2 Group

REJ09B0338-0200

APPENDIX

3.3 Countermeasures against noise

Countermeasures against noise are described below. The following countermeasures are effective against

noise in theory, however, it is necessary not only to take measures as follows but to evaluate before actual use.

3.3.1 Shortest wiring length

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

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

(1)

Package

Select the smallest possible package to make the total wiring length short.

q Reason

The wiring length depends on a microcomputer package. Use of a small package, for example

QFP and not DIP, makes the total wiring length short to reduce influence of noise.

Fig. 3.3.1 Selection of packages

(2)

Wiring for RESET pin

Make the length of wiring which is connected to the RESET pin as short as possible. Especially,

connect a capacitor across the RESET pin and the VSS pin with the shortest possible wiring (within

20mm).

q Reason

The width of a pulse input into the RESET pin is determined by the timing necessary conditions.

If noise having a shorter pulse width than the standard is input to the RESET pin, the reset is

released before the internal state of the microcomputer is completely initialized. This may cause

a program runaway.

Fig. 3.3.2 Wiring for the RESET pin

DIP

SDIP

SOP

QFP

RESET

Reset

circuit

Noise

VSS

Reset

circuit

VSS

RESET

VSS

N.G.

O.K.

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