129
8331B–AVR–03/12
Atmel AVR XMEGA AU
Figure 11-1.
Normal mode operation.
11.4
Window Mode Operation
In window mode operation, the WDT uses two different timeout periods, a "closed" window time-
out period (TO
WDTW
) and the normal timeout period (TO
WDT
). The closed window timeout period
defines a duration of from 8ms to 8s where the WDT cannot be reset. If the WDT is reset during
this period, the WDT will issue a system reset. The normal WDT timeout period, which is also
8ms to 8s, defines the duration of the "open" period duirng which the WDT can (and should) be
reset. The open period will always follow the closed period, and so the total duration of the time-
out period is the sum of the closed window and the open window timeout periods. The default
closed window timeout period is controlled by fuses (
both
open and closed periods are con-
trolled by fuses). The window mode operation is illustrated in
Figure 11-2.
Window mode operation.
11.5
Watchdog Timer Clock
The WDT is clocked from the 1kHz output from the 32kHz ultra low power (ULP) internal oscilla-
tor. Due to the ultra low power design, the oscillator is not very accurate, and so the exact
timeout period may vary from device to device. When designing software which uses the WDT,
this device-to-device variation must be kept in mind to ensure that the timeout periods used are
valid for all devices. For more information on ULP oscillator accuracy, consult the device
datasheet.
