Renesas 4514 - Manual (Page 196)

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