Rainbow-electronics ATmega8515L Instrukcja Użytkownika Pobierz pdf (Strona 50)

ATmega8515(L)

2512A–AVR–04/02

Timed Sequences for

Changing the

Configuration of the

Watchdog Timer

The sequence forchanging configuration differs slightly between thethree safetylevels.

Separate procedures are describedfor each level.

Safety Level 0 This modeiscompatible with the Watchdog operation found in AT90S4414/8515. The

Watchdog Timer is initially disabled, but can beenabledbywriting the WDE bit to 1 with-

out anyrestriction. Thetime-out periodcan be changed at any time without restriction.

To disableanenabledWatchdog Timer, the procedure described on page 48 (WDE bit

description) must be followed.

Safety Level 1 Inthis mode, the Watchdog Timer is initially disabled, but can be enabledbywriting the

WDE bit to 1 without anyrestriction. A timedsequenceis neededwhen changing the

Watchdog Time-out period ordisablinganenabledWatchdog Timer.Todisablean

enabledWatchdog Timer, and/orchanging the Watchdog Time-out, the following proce-

duremust be followed:

1. Inthe same operation,write a logic one to WDCE andWDE.Alogic one must be

written to WDE regardless of the previous value of the WDE bit.

2. Within thenext fourclock cycles, in the same operation,write the WDE andWDP

bits asdesired, but with the WDCE bit cleared.

Safety Level 2 Inthis mode, the Watchdog Timer is always enabled, and the WDE bit will always read

as one. A timedsequenceis neededwhen changing the Watchdog Time-out period.To

change the Watchdog Time-out, the following proceduremust be followed:

1. Inthe same operation,write a logical one to WDCE andWDE. Even though the

WDE always isset, the WDE must be written to one to startthetimedsequence.

2. Within thenext fourclock cycles, in the same operation,write the WDP bits as

desired, but with the WDCE bit cleared.Thevalue written to the WDE bit is

irrelevant.

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Rainbow-electronics ATmega8515L Instrukcja Użytkownika Strona 50