ATMEGA644PV ATMEL [ATMEL Corporation], ATMEGA644PV Datasheet - Page 55

• Manufacturer Part Number: ATMEGA644PV
• Description: 8-bit Microcontroller with 16/32/64K Bytes In-System Programmable Flash
• Manufacturer: ATMEL [ATMEL Corporation]

9.2
9.2.1
9.2.2
8011D–AVR–02/07
Watchdog Timer
Features
Overview
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ATmega164P/324P/644P has an Enhanced Watchdog Timer (WDT). The WDT is a timer count-
ing 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.
Figure 9-7.
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, alter-
ations to the Watchdog set-up must follow timed sequences. The sequence for clearing WDE
and changing time-out configuration is as follows:
Clocked from separate On-chip Oscillator
3 Operating modes
Selectable Time-out period from 16ms to 8s
Possible Hardware fuse Watchdog always on (WDTON) for fail-safe mode
– Interrupt
– System Reset
– Interrupt and System Reset
WATCHDOG
RESET
Watchdog Timer
OSCILLATOR
128kHz
WDIE
WDIF
WDE
ATmega164P/324P/644P
WDP0
WDP1
WDP2
WDP3
MCU RESET
INTERRUPT
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