ATMEGA64-16AI datasheet(60/393 Pages) ATMEL

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ATmega64(L)

2490L–AVR–10/06

Timed Sequences for

Changing the

Configuration of the

Watchdog Timer

The sequence for changing configuration differs slightly between the three safety levels.

Separate procedures are described for each level.

Safety Level 0

This mode is compatible with the Watchdog operation found in ATmega103. The Watch-

dog Timer is initially disabled, but can be enabled by writing the WDE bit to 1 without

any restriction. The Time-out period can be changed at any time without restriction. To

disable an enabled Watchdog Timer, the procedure described on page 58 (WDE bit

description) must be followed.

Safety Level 1

In this mode, the Watchdog Timer is initially disabled, but can be enabled by writing the

WDE bit to 1 without any restriction. A timed sequence is needed when changing the

Watchdog Time-out period or disabling an enabled Watchdog Timer. To disable an

enabled Watchdog Timer, and/or changing the Watchdog Time-out, the following proce-

dure must be followed:

In the same operation, write a logic one to WDCE and WDE. A logic one must be

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

Within the next four clock cycles, in the same operation, write the WDE and WDP

bits as desired, but with the WDCE bit cleared.

Safety Level 2

In this mode, the Watchdog Timer is always enabled, and the WDE bit will always read

as one. A timed sequence is needed when changing the Watchdog Time-out period. To

change the Watchdog Time-out, the following procedure must be followed:

In the same operation, write a logical one to WDCE and WDE. Even though the

WDE always is set, the WDE must be written to one to start the timed sequence.

Within the next four clock cycles, in the same operation, write the WDP bits as

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

irrelevant.

ATMEGA64-16AI Datasheet(PDF) 60 Page - ATMEL Corporation