
32
ATtiny40 [DATASHEET]
8263B–AVR–01/2013
Figure 8-7.
Watchdog Timer
The Wathdog Timer can also be configured to generate an interrupt instead of a reset. This can be very helpful
when using the Watchdog to wake-up from Power-down.
To prevent unintentional disabling of the Watchdog or unintentional change of time-out period, two different safety
8.4.1
Procedure for Changing the Watchdog Timer Configuration
The sequence for changing configuration differs between the two safety levels, as follows:
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 special sequence is needed when disabling an enabled Watchdog Timer. To disable an enabled
Watchdog Timer, the following procedure must be followed:
1.
Write the signature for change enable of protected I/O registers to register CCP
2.
Within four instruction cycles, in the same operation, write WDE and WDP bits
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 protected change
is needed when changing the Watchdog Time-out period. To change the Watchdog Time-out, the following proce-
dure must be followed:
1.
Write the signature for change enable of protected I/O registers to register CCP
2.
Within four instruction cycles, write the WDP bit. The value written to WDE 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
Protected change
sequence
No limitations
Programmed
2
Enabled
Always enabled
Protected change
sequence
OSC/2K
OSC/4K
OSC/
8
K
OSC/16K
OSC/32K
OSC/64K
OSC/12
8
K
OSC/256K
OSC/512K
OSC/1024K
MCU RESET
WATCHDOG
PRESCALER
128 kHz
OSCILLATOR
WATCHDOG
RESET
WDP0
WDP1
WDP2
WDP3
WDE
MUX