4501 datasheet(208/212 Pages) RENESAS

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3.4 Notes on noise

3-39

4501 Group

APPENDIX

Rev.2.01

Feb 07, 2005

REJ09B0192-0201

Fig. 3.4.9 Wiring to a signal line where potential

levels change frequently

(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.

XIN

XOUT

VSS

CNTR

Do not cross

N.G.

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

3.4.5 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 or an I/O port, since the

output data may reverse because of noise, rewrite

data to its output latch at fixed periods.

• Rewrite data to pull-up control registers at fixed

periods.

3.4.6 Providing of watchdog timer function by

software

If a microcomputer runs away because of noise or

others, it can be detected by a software watchdog

timer and the microcomputer can be reset to normal

operation. This is equal to or more effective than

program runaway detection by a hardware watchdog

timer. The following shows an example of a watchdog

timer provided by software.

In the following example, to reset a microcomputer

to normal operation, the main routine detects errors

of the interrupt processing routine and the interrupt

processing routine detects errors of the main routine.

This example assumes that interrupt processing is

repeated multiple times in a single main routine

processing.

(2)

Installing oscillator away from signal lines

where potential levels change frequently

Install an oscillator and a connecting pattern

of an oscillator away from signal lines where

potential levels change frequently. Also, do

not cross such signal lines over the clock lines

or the signal lines which are sensitive to noise.

q Reason

Signal lines where potential levels change

frequently (such as the CNTR pin signal

line) may affect other lines at signal rising

edge or falling edge. If such lines cross

over a clock line, clock waveforms may be

deformed, which causes a microcomputer

failure or a program runaway.

Fig. 3.4.10 VSS pattern on the underside of an

oscillator

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