ATmega169P datasheet(52/393 Pages) ATMEL

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8018N–AVR–08/09

ATmega169P

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.

10.4.1.1

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:

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

to WDE regardless of the previous value of the WDE bit.

Within the next four clock cycles, in the same operation, write the WDE and WDP bits as

desired, but with the WDCE bit cleared.

10.4.1.2

Safety Level 2

In this mode, the Watchdog Timer is always enabled, and the WDE bit will always read as one. A

timed sequence is needed when changing the Watchdog Time-out period. To change the

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

In the same operation, write a logical one to WDCE and WDE. Even though the WDE

always is set, the WDE must be written to one to start the timed sequence.

Within the next four clock cycles, in the same operation, write the WDP bits as desired, but with

the WDCE bit cleared. The value written to the WDE bit is irrelevant.

WATCHDOG

OSCILLATOR

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