38K2 datasheet(298/358 Pages) RENESAS

298 / 358 page

3.3 Countermeasures against noise

Rev.2.00

Oct 15, 2006

page 37 of 99

38K2 Group

REJ09B0338-0200

APPENDIX

Direction register

Port latch

Data bus

I/O port

pins

Noise

N.G.

O.K.

(3)

Oscillator protection using VSS pattern

As for a two-sided printed circuit board, print a VSS pattern on the underside (soldering side) of the

position (on the component side) where an oscillator is mounted.

Connect the VSS pattern to the microcomputer VSS pin with the shortest possible wiring. Besides,

separate this VSS pattern from other VSS patterns.

Fig. 3.3.10 VSS pattern on the underside of an oscillator

Fig. 3.3.11 Setup for I/O ports

3.3.5 Setup for I/O ports

Setup I/O ports using hardware and software as follows:

• Connect a resistor of 100

Ω or more to an I/O port in series.

• As for an input port, read data several times by a program for checking whether input levels are

equal or not.

• As for an output port, since the output data may reverse because of noise, rewrite data to its port

latch at fixed periods.

• Rewrite data to direction registers at fixed periods.

Note: When a direction register is set for input port again at fixed periods, a several-nanosecond short pulse

may be output from this port. If this is undesirable, connect a capacitor to this port to remove the noise

pulse.

XIN

XOUT

VSS

An example of VSS patterns on the

underside of a printed circuit board

Oscillator wiring

pattern example

Separate the VSS line for oscillation from other VSS lines

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