ATtiny261-ESMZ datasheet(45/227 Pages) ATMEL

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7753C–AVR–07/09

ATtiny261/ATtiny461/ATtiny861

Figure 9-7.

Watchdog Timer

9.3

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.

9.3.1

Safety Level 1

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

to one without any restriction. A timed sequence is needed when disabling an enabled Watch-

dog Timer. 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 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.

9.3.2

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.

9.4

9.4.1

MCUSR – MCU Status Register

The MCU Status Register provides information on which reset source caused an MCU Reset.

MCU RESET

WATCHDOG

PRESCALER

128 kHz

OSCILLATOR

WATCHDOG

RESET

WDP0

WDP1

WDP2

WDP3

WDE

Bit

4321

0x34 (0x54)

–

WDRF

BORF

EXTRF

PORF

MCUSR

Read/Write

R/W

Initial Value

See Bit Description

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