4513_03 Datasheet, PDF (196/210 Pages) Renesas Technology Corp – 4-BIT SINGLE-CHIP MICROCOMPUTER

<The main routine>
• 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 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.
<The interrupt processing routine>
• 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.
Main routine
Interrupt processing routine
(SWDT)← N
EI
(SWDT) ← (SWDT)—1
Interrupt processing
Main processing
≠N _(SWDT)
=N?
N
(SWDT) ^>0
≤0?
RTI
≤0
Return
Interrupt processing
routine errors
Main routine
errors
Fig. 3.4.11 Watchdog timer by software
4513/4514 Group User’s Manual
APPENDIX
3.4 Notes on noise
3-29