參數資料
型號： M38003M4-XXXFS
廠商： Mitsubishi Electric Corporation
英文描述： Octal Gigabit Ethernet Transceiver 289-BGA 0 to 70
中文描述： 8位單片機
文件頁數： 129/173頁
文件大小： 4203K
代理商： M38003M4-XXXFS
3800 GROUP USER’S MANUAL
3-27
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
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.
<The main routine>
G
Assigns a single byte 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)
Fig. 3.4.8 Watchdog timer by software
As the main routine execution cycle may change because of an interrupt processing or others, the initial value N
should have a margin.
G
Watches 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.
G
Detects 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>
G
Decrements the SWDT contents by 1 at each interrupt processing.
G
Determins 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).
G
Detects 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 .
Main routine
(SWDT)
←
N
CLI
Main processing
(SWDT)
=N
Interrupt processing routine
errors
≠
N
=N
Interrupt processing routine
(SWDT)
←
(SWDT)—1
Interrupt processing
(SWDT)
≤
0
Main routine
errors
>
0
≤
0
RTI
Return
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