Watchdog timer, Atmega169v/l – Rainbow Electronics Atmega169L User Manual
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ATmega169V/L
2514A–AVR–08/02
Watchdog Timer
The Watchdog Timer is clocked from a separate On-chip Oscillator which runs at
1 MHz. This is the typical value at V
CC
= 5V. See characterization data for typical values
at other V
CC
levels. By controlling the Watchdog Timer prescaler, the Watchdog Reset
interval can be adjusted as shown in Table 21 on page 43. The WDR – Watchdog Reset
– instruction resets the Watchdog Timer. The Watchdog Timer is also reset when it is
disabled and when a Chip Reset occurs. Eight different clock cycle periods can be
selected to determine the reset period. If the reset period expires without another
Watchdog Reset, the ATmega169 resets and executes from the Reset Vector. For tim-
ing details on the Watchdog Reset, refer to Table 21 on page 43.
To prevent unintentional disabling of the Watchdog or unintentional change of time-out
period, two different safety levels are selected by the fuse WDTON as shown in Table
20. Refer to “Timed Sequences for Changing the Configuration of the Watchdog Timer”
on page 44 for details.
Figure 20. Watchdog Timer
Table 20. WDT Configuration as a Function of the Fuse Settings of WDTON
WDTON
Safety
Level
WDT Initial
State
How to Disable the
WDT
How to Change
Time-out
Unprogrammed
1
Disabled
Timed sequence
Timed sequence
Programmed
2
Enabled
Always enabled
Timed sequence
WATCHDOG
OSCILLATOR