Datasheet

38
ATtiny2313/V
2543H–AVR–02/05
Watchdog Timer ATtiny2313 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 20. 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 program security, alterations to the Watchdog set-up must follow
timed sequences. The sequence for clearing WDE and changing time-out configuration
is as follows:
1. In the same operation, write a logic one to the Watchdog change enable bit
(WDCE) and WDE. A logic one must be written to WDE regardless of the previ-
ous value of the WDE bit.
2. Within the next four clock cycles, write the WDE and Watchdog prescaler bits
(WDP) as desired, but with the WDCE bit cleared. This must be done in one
operation.
128kHz
OSCILLATOR
OSC/2K OSC/4K OSC/8K
OSC/16K OSC/32K OSC/64K
OSC/128K OSC/256K
OSC/512K
OSC/1024K
WDP0
WDP1
WDP2
WDP3
WATCHDOG
RESET
WDE
WDIF
WDIE
MCU RESET
INTERRUPT