ATmega169P datasheet(55/393 Pages) ATMEL

55 / 393 page

8018N–AVR–08/09

ATmega169P

• Bit 3 – WDE: Watchdog Enable

When the WDE is written to logic one, the Watchdog Timer is enabled, and if the WDE is written

to logic zero, the Watchdog Timer function is disabled. WDE can only be cleared if the WDCE bit

has logic level one. To disable an enabled Watchdog Timer, the following procedure must be

followed:

In the same operation, write a logic one to WDCE and WDE. A logic one must be written

to WDE even though it is set to one before the disable operation starts.

Within the next four clock cycles, write a logic 0 to WDE. This disables the Watchdog.

In safety level 2, it is not possible to disable the Watchdog Timer, even with the algorithm

described above. See ”Timed Sequences for Changing the Configuration of the Watchdog

Timer” on page 52.

• Bits 2:0 – WDP2, WDP1, WDP0: Watchdog Timer Prescaler 2, 1, and 0

The WDP2, WDP1, and WDP0 bits determine the Watchdog Timer prescaling when the Watch-

dog Timer is enabled. The different prescaling values and their corresponding Timeout Periods

are shown in Table 10-2.

Note:

Also see Figure 29-54 on page 364.

The following code example shows one assembly and one C function for turning off the WDT.

The example assumes that interrupts are controlled (e.g. by disabling interrupts globally) so that

no interrupts will occur during execution of these functions.

Table 10-2.

Watchdog Timer Prescale Select

WDP2

WDP1

WDP0

Number of WDT

Oscillator Cycles

Typical Time-out at

VCC = 3.0V

Typical Time-out at

VCC = 5.0V

16K cycles

15.4 ms

14.7 ms

32K cycles

30.8 ms

29.3 ms

64K cycles

61.6 ms

58.7 ms

128K cycles

0.12 s

256K cycles

0.25 s

0.23 s

512K cycles

0.49 s

0.47 s

1,024K cycles

1.0 s

0.9 s

2,048K cycles

2.0 s

1.9 s

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