4553 datasheet(63/145 Pages) RENESAS

63 / 145 page

Rev.3.02

Dec 22, 2006

page 63 of 142

REJ03B0024-0302

4553 Group

Fig. 61 Watchdog timer by software

• 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

≥ (Counts of interrupt processing executed in each main rou-

tine)

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 com-

paring 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 deter-

mines 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 process-

ing.

• 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.

Main routine

(SWDT)

← N

Main processing

(SWDT)

Interrupt processing

routine errors

Interrupt processing routine

(SWDT)

← (SWDT)—1

Interrupt processing

(SWDT)

Main routine

errors

≤0

RTI

Return

=N?

≤0?

≠N

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 re-

verse because of noise, rewrite data to its port 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 fol-

lowing shows an example of a watchdog timer provided by

software.

In the following example, to reset a microcomputer to normal op-

eration, 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 mul-

tiple times in a single main routine processing.

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