Datasheet

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ATtiny25/45/85 [DATASHEET]

2586Q–AVR–08/2013

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 Refer to “Timed Sequences for Changing the Con-

figuration 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.

8.4.1.1 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 Watchdog Timer. To disable an enabled

Watchdog Timer, the following procedure must be followed:

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

less of the previous value of the WDE bit.

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

the WDCE bit cleared.

8.4.1.2 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 proce-

dure must be followed:

1. 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.

2. 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.

Table 8-1. 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 1 Disabled Timed sequence No limitations

Programmed 2 Enabled Always enabled Timed sequence

OSC/2K

OSC/4K

OSC/8K

OSC/16K

OSC/32K

OSC/64K

OSC/128K

OSC/256K

OSC/512K

OSC/1024K

MCU RESET

WATCHDOG

PRESCALER

128 kHz

OSCILLATOR

WATCHDOG

RESET

WDP0

WDP1

WDP2

WDP3

WDE