ATMEGA640 datasheet(67/449 Pages) ATMEL

67 / 449 page

ATmega640/1280/1281/2560/2561

2549K–AVR–01/07

Timer is in Interrupt Mode, and the corresponding interrupt is executed if time-out in the

Watchdog Timer occurs.

If WDE is set, the Watchdog Timer is in Interrupt and System Reset Mode. The first

time-out in the Watchdog Timer will set WDIF. Executing the corresponding interrupt

vector will clear WDIE and WDIF automatically by hardware (the Watchdog goes to Sys-

tem Reset Mode). This is useful for keeping the Watchdog Timer security while using

the interrupt. To stay in Interrupt and System Reset Mode, WDIE must be set after each

interrupt. This should however not be done within the interrupt service routine itself, as

this might compromise the safety-function of the Watchdog System Reset mode. If the

interrupt is not executed before the next time-out, a System Reset will be applied.

Note:

1. WDTON Fuse set to “0“ means programmed and “1” means unprogrammed.

• Bit 4 - WDCE: Watchdog Change Enable

This bit is used in timed sequences for changing WDE and prescaler bits. To clear the

WDE bit, and/or change the prescaler bits, WDCE must be set.

Once written to one, hardware will clear WDCE after four clock cycles.

• Bit 3 - WDE: Watchdog System Reset Enable

WDE is overridden by WDRF in MCUSR. This means that WDE is always set when

WDRF is set. To clear WDE, WDRF must be cleared first. This feature ensures multiple

resets during conditions causing failure, and a safe start-up after the failure.

• Bit 5, 2:0 - WDP3:0: Watchdog Timer Prescaler 3, 2, 1 and 0

The WDP3:0 bits determine the Watchdog Timer prescaling when the Watchdog Timer

is running. The different prescaling values and their corresponding time-out periods are

shown in Table 31 on page 68.

Table 30. Watchdog Timer Configuration

WDTON(1)

WDE

WDIE

Mode

Action on Time-out

Stopped

None

Interrupt Mode

Interrupt

System Reset Mode

Reset

Interrupt and System

Reset Mode

Interrupt, then go to

System Reset Mode

Reset

ATMEGA640 Datasheet(PDF) 67 Page - ATMEL Corporation

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