M38260ECFS datasheet(58/91 Pages) RENESAS

58 / 91 page

Rev.1.00

Sep 06, 2006

page 58 of 88

REJ03B0181-0100

3826 Group (One Time PROM version)

NOTES ON USE

Countermeasures Against Noise

(1) Shortest wiring length

➀ 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 20 mm).

● 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 com-

pletely initialized. This may cause a program runaway.

Fig. 64 Wiring for clock I/O pins

(2) Connection of bypass capacitor across VSS line and VCC line

In order to stabilize the system operation and avoid the latch-up,

connect an approximately 0.1 µF bypass capacitor across the VSS

line and the VCC line as follows:

• Connect a bypass capacitor across the VSS pin and the VCC pin

at equal length.

• Connect a bypass capacitor across the VSS pin and the VCC pin

with the shortest possible wiring.

• Use lines with a larger diameter than other signal lines for VSS

line and VCC line.

• Connect the power source wiring via a bypass capacitor to the

VSS pin and the VCC pin.

Fig. 63 Wiring for the RESET pin

➁ 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.

● Reason

If noise enters clock I/O pins, clock waveforms may be de-

formed. 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 oscil-

lator, the correct clock will not be input in the microcomputer.

Fig. 65 Bypass capacitor across the VSS line and the VCC line

RESET

Reset

circuit

Noise

VSS

Reset

circuit

VSS

RESET

VSS

N.G.

O.K.

Noise

XIN

XOUT

VSS

XIN

XOUT

VSS

N.G.

O.K.

VSS

VCC

VSS

VCC

N.G.

O.K.

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