4571 datasheet(62/126 Pages) RENESAS

62 / 126 page

Rev.1.02

May 25, 2007

Page 62 of 124

REJ03B0179-0102

4571 Group

• 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

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

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 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 70. Watchdog timer by software

≠N

≤0

Main routine

(SWDT)

← N

Main processing

(SWDT)

=N?

Interrupt processing

routine errors

Interrupt processing routine

(SWDT)

← (SWDT)−1

Interrupt processing

(SWDT)

≤0?

RTI

Return

Main routine

errors

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