ST6240B datasheet(23/76 Pages) STMICROELECTRONICS

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ST62T40B/E40B

3.3 DIGITAL WATCHDOG

The digital Watchdog consists of a reloadable

downcounter timer which can be used to provide

controlled recovery from software upsets.

The Watchdog circuit generates a Reset when the

downcounter reaches zero. User software can

prevent this reset by reloading the counter, and

should therefore be written so that the counter is

regularly reloaded while the user program runs

correctly. In the event of a software mishap (usual-

ly caused by externally generated interference),

the user program will no longer behave in its usual

fashion and the timer register will thus not be re-

loaded periodically. Consequently the timer will

decrement down to 00h and reset the MCU. In or-

der to maximise the effectiveness of the Watchdog

function, user software must be written with this

concept in mind.

Watchdog behaviour is governed by one option,

known

“WATCHDOG

ACTIVATION”

(i.e.

HARDWARE or SOFTWARE) (See Table 9).

In the SOFTWARE option, the Watchdog is disa-

bled until bit C of the DWDR register has been set.

When the Watchdog is disabled, low power Stop

mode is available. Once activated, the Watchdog

cannot be disabled, except by resetting the MCU.

In the HARDWARE option, the Watchdog is per-

manently enabled. Since the oscillator will run con-

tinuously, low power mode is not available. The

STOP instruction is interpreted as a WAIT instruc-

tion, and the Watchdog continues to countdown.

When the MCU exits STOP mode (i.e. when an in-

terrupt is generated), the Watchdog resumes its

activity.

Table 9. Recommended Option Choices

Functions Required

Recommended Options

Stop Mode

“SOFTWARE WATCHDOG”

Watchdog

“HARDWARE WATCHDOG”

595

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