4509 Datasheet, PDF (70/142 Pages) Renesas Technology Corp – SINGLE-CHIP 4-BIT CMOS MICROCOMPUTER

4509 Group
<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 rou-
tine)
As the main routine execution cycle may change because of an in-
terrupt processing or others, the initial value N should have a
margin.
• Watches the operation of the interrupt processing routine by com-
paring 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.
<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?
≤0 ^RTI
Return
Interrupt processing
routine errors
Main routine
errors
Fig. 72 Watchdog timer by software
Rev.1.02 2006.12.22 page 70 of 140
REJ03B0147-0102