M30620FCAFP datasheet(611/615 Pages) RENESAS

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Appendix 5

Mitsubishi microcomputers

M16C / 62A Group

SINGLE-CHIP 16-BIT CMOS MICROCOMPUTER

2-294

Main routine

(SWDT)

← N

FSET I

Main processing

(SWDT)

Interrupt processing

routine errors

≠N

Interrupt processing routine

(SWDT)

← (SWDT) – 1

Interrupt processing

(SWDT)

Main routine

errors

≤0

REIT

Return

=N?

≤0?

1.6 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 process-

ing.

• 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 condi-

tion:

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 initial-

ization routine for recovery processing in the following case:

If the SWDT contents do not change after interrupt processing.

• 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.

Figure 1.6.1. Watchdog timer by software

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