4571 datasheet(61/126 Pages) RENESAS

61 / 126 page

Rev.1.02

May 25, 2007

Page 61 of 124

REJ03B0179-0102

4571 Group

4. Oscillator concerns

Take care to prevent an oscillator that generates clocks for a

microcomputer operation from being affected by other signals.

(1) Keeping oscillator away from large current signal lines

Install a microcomputer (and especially an oscillator) as far as

possible from signal lines where a current larger than the

tolerance of current value flows.

In the system using a microcomputer, there are signal lines for

controlling motors, LEDs, and thermal heads or others. When a

large current flows through those signal lines, strong noise

occurs because of mutual inductance.

(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.

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 67. Wiring for a large current signal line

Fig 68. 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.

Fig 69. Vss pattern on the underside of an oscillator

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 port latch at fixed

periods.

• Rewrite data to pull-up control registers at fixed periods.

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.

XIN

XOUT

VSS

Microcomputer

Mutual inductance

Large

current

GND

XIN

XOUT

VSS

CNTR

Do not cross

N.G.

Oscillator wiring

pattern example

An example of VSS patterns on the

underside of a printed circuit board

Separate the VSS line for oscillation from other VSS lines

XIN

XOUT

VSS

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