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XMEGA A [MANUAL]
8077I–AVR–11/2012
11.4
Window Mode Operation
In window mode operation, the WDT uses two different timeout periods, a "closed" window timeout period (TOWDTW) and
the normal timeout period (TOWDT). 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 during which the WDT can (and should) be
reset. The open period will always follow the closed period, and so the total duration of the timeout 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 controlled by fuses). The window mode operation is illustrated in Figure 11-2.
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 oscillator. 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.
11.6
Configuration Protection and Lock
The WDT is designed with two security mechanisms to avoid unintentional changes to the WDT settings.
The first mechanism is the configuration change protection mechanism, employing a timed write procedure for changing
the WDT control registers. In addition, for the new configuration to be written to the control registers, the register’s
change enable bit must be written at the same time.
The second mechanism locks the configuration by setting the WDT lock fuse. When this fuse is set, the watchdog time
control register cannot be changed; hence, the WDT cannot be disabled from software. After system reset, the WDT will
resume at the configured operation. When the WDT lock fuse is programmed, the window mode timeout period cannot
be changed, but the window mode itself can still be enabled or disabled.
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