ATMEGA640 datasheet(62/449 Pages) ATMEL

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ATmega640/1280/1281/2560/2561

2549K–AVR–01/07

Watchdog Timer

ATmega640/1280/1281/2560/2561 has an Enhanced Watchdog Timer (WDT). The

main features are:

• Clocked from separate On-chip Oscillator

• 3 Operating modes

– Interrupt

– System Reset

– Interrupt and System Reset

• Selectable Time-out period from 16ms to 8s

• Possible Hardware fuse Watchdog always on (WDTON) for fail-safe mode

Figure 30. Watchdog Timer

The Watchdog Timer (WDT) is a timer counting cycles of a separate on-chip 128 kHz

oscillator. The WDT gives an interrupt or a system reset when the counter reaches a

given time-out value. In normal operation mode, it is required that the system uses the

WDR - Watchdog Timer Reset - instruction to restart the counter before the time-out

value is reached. If the system doesn't restart the counter, an interrupt or system reset

will be issued.

In Interrupt mode, the WDT gives an interrupt when the timer expires. This interrupt can

be used to wake the device from sleep-modes, and also as a general system timer. One

example is to limit the maximum time allowed for certain operations, giving an interrupt

when the operation has run longer than expected. In System Reset mode, the WDT

gives a reset when the timer expires. This is typically used to prevent system hang-up in

case of runaway code. The third mode, Interrupt and System Reset mode, combines the

other two modes by first giving an interrupt and then switch to System Reset mode. This

mode will for instance allow a safe shutdown by saving critical parameters before a sys-

tem reset.

The Watchdog always on (WDTON) fuse, if programmed, will force the Watchdog Timer

to System Reset mode. With the fuse programmed the System Reset mode bit (WDE)

and Interrupt mode bit (WDIE) are locked to 1 and 0 respectively. To further ensure pro-

gram security, alterations to the Watchdog set-up must follow timed sequences. The

sequence for clearing WDE and changing time-out configuration is as follows:

128kHz

OSCILLATOR

WDP0

WDP1

WDP2

WDP3

WATCHDOG

RESET

WDE

WDIF

WDIE

MCU RESET

INTERRUPT

ATMEGA640 Datasheet(PDF) 62 Page - ATMEL Corporation

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