ATmega128 datasheet(58/386 Pages) ATMEL

58 / 386 page

2467S–AVR–07/09

ATmega128

The following code example shows one assembly and one C function for turning off the WDT.

The example assumes that interrupts are controlled (e.g. by disabling interrupts globally) so that

no interrupts will occur during execution of these functions.

Timed Sequences for Changing the Configuration of the Watchdog Timer

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

rate procedures are described for each level.

Safety Level 0

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

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 56 (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 procedure 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:

Assembly Code Example

WDT_off:

; Reset WDT

wdr

r16, WDTCR

; Write logical one to WDCE and WDE

ori

r16, (1<<WDCE)|(1<<WDE)

out

WDTCR, r16

; Turn off WDT

ldi

r16, (0<<WDE)

out

WDTCR, r16

ret

C Code Example

void

WDT_off(void)

{

/* Reset WDT*/

__watchdog_reset();

/* Write logical one to WDCE and WDE */

WDTCR |= (1<<WDCE) | (1<<WDE);

/* Turn off WDT */

WDTCR = 0x00;

}

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