C508_01 Datasheet, PDF(231/267 Page) Infineon Technologies AG – 8-Bit CMOS Microcontroller

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C508_01 Datasheet, PDF (231/267 Pages) Infineon Technologies AG – 8-Bit CMOS Microcontroller

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C508

Fail Save Mechanisms

8.1.3 Starting the Watchdog Timer

The Watchdog Timer can be started by software (bit SWDT in SFR IEN1), but it cannot

be stopped while the device is in active mode. An internal reset will be initiated if the

software fails to clear the Watchdog Timer. The cause of the reset (either an external

reset or a reset caused by the Watchdog) can be examined by software (status flag

WDTS in IP0 is set). A refresh of the Watchdog Timer is done by setting bits WDT (SFR

IEN0) and SWDT consecutively. This double instruction sequence has been

implemented to increase system security.

It must be noted, however, that the Watchdog Timer is halted during the idle mode and

power-down mode of the processor (see Chapter 9). It is not possible to use the idle

mode in combination with the Watchdog Timer function. Therefore, even the Watchdog

Timer cannot reset the device if one of the power saving modes has been entered

accidentally.

8.1.4 Refreshing the Watchdog Timer

At the same time the Watchdog Timer is started, the 7-bit register WDTH is preset by the

contents of WDTREL.0 to WDTREL.6. After the Watchdog has started, it cannot be

stopped by software; but, can only be refreshed to the reload value by first setting bit

WDT (IEN0.6) and by the next instruction setting SWDT (IEN1.6). Bit WDT will

automatically be cleared during the second machine cycle after having been set. For this

reason, setting SWDT bit must be a one cycle instruction (for example, SETB SWDT).

This double-instruction refresh of the Watchdog Timer is implemented to minimize the

chance of an unintentional reset of the Watchdog.

The reload register WDTREL can be written to at any time, as mentioned previously.

Therefore, a periodic refresh of WDTREL can be added to the above mentioned starting

procedure of the Watchdog Timer. Thus, a wrong reload value, caused by a possible

distortion during the write operation to the WDTREL, can be corrected by software.

Userâs Manual

8-5

2001-05

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