7220 datasheet(235/303 Pages) RENESAS

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APPENDIX

6.4 Countermeasures against noise

7220 Group User’s Manual

6-16

6.4.6 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 processing.

qAssigns 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)

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 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 case:

If the SWDT contents do not change after interrupt processing.

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:

If the SWDT contents are not initialized to the initial value N but continued to decrement and if they reach

0 or less.

Fig. 6.4.11 Watchidog timer by software

Main routine

(SWDT)

← N

CLI

Main processing

(SWDT)

Interrupt processing

routine errors

Interrupt processing routine

(SWDT)

← (SWDT)—1

Interrupt processing

(SWDT)

Main routine

errors

≤0

RTI

Return

=N?

≤0?

≠N

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