M38002M4 Datasheet, PDF (129/173 Pages) Mitsubishi Electric Semiconductor – 8-BIT SINGLE-CHIP MICROCOMPUTER

APPENDIX
3.4 Countermeasures against noise
3.4.5 Providing of watchdog timer function by
software
If a microcomputer runs away because of noise or
others, it can be detected by a software watchdog
timer and the microcomputer can be reset to normal
Main routine Interrupt processing routine
operation. This is equal to or more effective than
program runaway detection by a hardware watchdog
timer. The following shows an example of a watchdog
timer provided by software.
In the following example, to reset a microcomputer to
normal operation, the main routine detects errors of
the interrupt processing routine and the interrupt
processing routine detects errors of the main routine.
This example assumes that interrupt processing is
repeated multiple times in a single main routine
processing.
(SWDT)← N
CLI
Main processing
≠N _(SWDT)
=N?
=N
(SWDT) ← (SWDT)—1
Interrupt processing
(SWDT) ^>0
≤0?
RTI
≤0
Return
<The main routine>
qAssigns a single byte of RAM to a software watchdog
timer (SWDT) and writes the initial value N in the
Interrupt processing routine
errors
Main routine
errors
SWDT once at each execution of the main routine.
The initial value N should satisfy the following ^{Fig. 3.4.8 Watchdog timer by software}
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.
qWatches the operation of the interrupt processing routine by comparing the SWDT contents with counts of
interrupt processing count after the initial value N has been set.
qDetects that the interrupt processing routine has failed and determines to branch to the program initialization
routine for recovery processing in the following cases:
If the SWDT contents do not change after interrupt processing
<The interrupt processing routine>
qDecrements the SWDT contents by 1 at each interrupt processing.
qDetermins 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).
qDetects that the main routine has failed and determines to branch to the program initialization routine for recovery
processing in the following case:
When the contents of the SWDT reach 0 or less by continuative decrement without initializing to the initial value
N.
3800 GROUP USER’S MANUAL
3-27