7643 datasheet(366/424 Pages) RENESAS

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7643 Group

REJ09B0133-0200

APPENDIX

3.4 Countermeasure against noise

Rev.2.00

Aug 28, 2006

page 43 of 98

3.4.5 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 byte 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 routine)

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. 3.4.8 Watchdog timer by software

Main routine

(SWDT)

← N

CLI

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

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