4559 datasheet(74/148 Pages) RENESAS

74 / 148 page

Rev.1.04

Aug 23, 2007

Page 74 of 146

REJ03B0188-0104

4559 Group

(4) 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.

(5) 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.

• Assigns a single word of RAM to a software watchdog timer

(SWDT) and writes the initial value N in the SWDT once at

each execution of the main routine. The initial value N should

satisfy the following condition:

N + 1

≥

As the main routine execution cycle may change because of

an interrupt processing or others, the initial value N should

have a margin.

• Watches the operation of the interrupt processing routine by

comparing the SWDT contents with counts of interrupt

processing after the initial value N has been set.

• Detects that the interrupt processing routine has failed and

determines to branch to the program initialization routine for

recovery processing in the following case:

If the SWDT contents do not change after interrupt processing.

• Decrements the SWDT contents by 1 at each interrupt

processing.

• Determines that the main routine operates normally when the

SWDT contents are reset to the initial value N at almost fixed

cycles (at the fixed interrupt processing count).

• Detects that the main routine has failed and determines to

branch to the program initialization routine for recovery

processing in the following case:

If the SWDT contents are not initialized to the initial value N

but continued to decrement and if they reach 0 or less.

Fig 81. Watchdog timer by software

Main routine

Interrupt processing

routine errors

(SWDT)

← N

Main processing

(SWDT)

= N?

≠ N

Interrupt processing routine

(SWDT)

← (SWDT)−1

Interrupt processing

(SWDT)

≤ 0?

≤ 0

> 0

RTI

Return

Main routine

errors

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