ATmega325P datasheet(49/364 Pages) ATMEL

49 / 364 page

8023F–AVR–07/09

ATmega325P/3250P

ADC is used. To reduce power consumption in Power-down mode, the user can avoid the three

conditions above to ensure that the reference is turned off before entering Power-down mode.

10.4

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 VCC levels. By

controlling the Watchdog Timer prescaler, the Watchdog Reset interval can be adjusted as

shown in Table 10-2 on page 51. The WDR – Watchdog Reset – instruction resets the Watch-

dog 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 ATmega325P/3250P resets and executes

from the Reset Vector. For timing details on the Watchdog Reset, refer to Table 10-2 on page

51.

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 Figure 10-1 Refer to

”Timed Sequences for Changing the Configuration of the Watchdog Timer” on page 49 for

details.

Figure 10-7. Watchdog Timer

10.4.1

Timed Sequences for Changing the Configuration of the Watchdog Timer

The sequence for changing configuration differs slightly between the two safety levels. Separate

procedures are described for each level.

10.4.2

Safety Level 1

In this mode, the Watchdog Timer is initially disabled, but can be enabled by writing the WDE bit

to 1 without any restriction. A timed sequence is needed when changing the Watchdog Time-out

period or disabling an enabled Watchdog Timer. To disable an enabled Watchdog Timer, and/or

changing the Watchdog Time-out, the following procedure must be followed:

Table 10-1.

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

Disabled

Timed sequence

Programmed

Enabled

Always enabled

Timed sequence

WATCHDOG

OSCILLATOR

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