Datasheet

45
7701F–AVR–10/12
Atmel ATtiny24/44/84 [Preliminary]
To prevent unintentional disabling of the watchdog or unintentional change of time-out period,
two different safety levels are selected by the WDTON fuse, as shown in Table 9-2. See
“Timed Sequences for Changing the Configuration of the Watchdog Timer” on page 45 for
details.
Figure 9-7. Watchdog Timer
9.9 Timed Sequences for Changing the Configuration of the Watchdog Timer
The sequence for changing configuration differs slightly between the two safety levels. Sepa-
rate procedures are described for each level.
9.9.1 Safety Level 1
In this mode, the watchdog timer is initially disabled, but can be enabled by writing the WDE
bit to logical 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 logical one to WDCE and WDE. A logical one must be
written to WDE regardless 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.
9.9.2 Safety Level 2
In this mode, the watchdog timer is always enabled, and the WDE bit will always read as logi-
cal 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:
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 9-2. 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