4513 datasheet(196/210 Pages) RENESAS

196 / 210 page

4513/4514 Group User’s Manual

APPENDIX

3-29

3.4 Notes on noise

• 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. 3.4.11 Watchdog timer by software

(Counts of interrupt processing executed in

each main routine)

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

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