1 safety level 1, Attiny43u – Rainbow Electronics ATtiny43U User Manual
Page 52
52
8048B–AVR–03/09
ATtiny43U
clock cycle periods can be selected to determine the reset period. If the reset period expires
without another Watchdog Reset, the ATtiny43U resets and executes from the Reset Vector.
For timing details on the Watchdog Reset, refer to
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 levels are selected by the fuse WDTON as shown in Table 9-1. See
Sequences for Changing the Configuration of the Watchdog Timer” on page 52
for details.
Figure 9-7.
Watchdog Timer
9.8.1
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.
9.8.1.1
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 Watch-
dog 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 writ-
ten 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.
Table 9-1.
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
O
S
C/2K
O
S
C/4K
O
S
C/
8
K
O
S
C/16K
O
S
C/
3
2K
O
S
C/64K
O
S
C/12
8
K
O
S
C/256K
O
S
C/512K
O
S
C/1024K
MCU RESET
WATCHDOG
PRESCALER
128 kHz
OSCILLATOR
WATCHDOG
RESET
WDP0
WDP1
WDP2
WDP3
WDE