ATmega16HVB datasheet(46/273 Pages) ATMEL

46 / 273 page

8042A–AVR–07/09

ATmega16HVB/32HVB

11.4

Watchdog Timer

11.4.1

Features

• Clocked from separate On-chip Oscillator

• 3 Operating modes

–Interrupt

– System Reset

– Interrupt and System Reset

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

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

11.4.2

Overview

ATmega16HVB/32HVB has an Enhanced Watchdog Timer (WDT). The WDT counts cycles of

the Ultra Low Power RC 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.

Figure 11-6. Watchdog Timer

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 inter-

rupt and then switch to System Reset mode. This mode will for instance allow a safe shutdown

by saving critical parameters before a system reset.

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

tem 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 program security, altera-

tions to the Watchdog set-up must follow timed sequences. The sequence for clearing WDE and

changing time-out configuration is as follows:

Ultra Low Power RC

OSCILLATOR

WDP0

WDP1

WDP2

WDP3

WATCHDOG

RESET

WDE

WDIF

WDIE

MCU RESET

INTERRUPT

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