ATtiny88 datasheet(43/266 Pages) ATMEL

43 / 266 page

8008C–AVR–03/09

ATtiny48/88

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 8-1. See “Timed

Sequences for Changing the Configuration of the Watchdog Timer” on page 43 for details.

Figure 8-7.

Watchdog Timer

8.4.1

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.

8.4.2

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 writ-

ten 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.

8.4.3

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 cleare d. The value written to the WDE bit is irrelevant.

Table 8-1.

WDT Configuration as a Function of the WDTON Fuse Setting

WDTON

Safety Level

WDT Initial State

Disable WDT

Change Time-out

Unprogrammed

Disabled

Timed sequence

No limitations

Programmed

Enabled

Always enabled

Timed sequence

128kHz

OSCILLATOR

WDP0

WDP1

WDP2

WDP3

WATCHDOG

RESET

WDE

WDIF

WDIE

MCU RESET

INTERRUPT

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