ATmega48PA/88PA/168PA/328P
Watchdog Timer will set WDIF. Executing the corresponding interrupt vector will clear WDIE and
WDIF automatically by hardware (the Watchdog goes to System Reset Mode). This is useful for
keeping the Watchdog Timer security while using the interrupt. To stay in Interrupt and System
Reset Mode, WDIE must be set after each interrupt. This should however not be done within the
interrupt service routine itself, as this might compromise the safety-function of the Watchdog
System Reset mode. If the interrupt is not executed before the next time-out, a System Reset
will be applied.
Table 10-1. Watchdog Timer Configuration
WDTON(1)
WDE
WDIE
Mode
Action on Time-out
None
1
1
1
0
0
1
0
1
0
Stopped
Interrupt Mode
System Reset Mode
Interrupt
Reset
Interrupt and System Reset
Mode
Interrupt, then go to System
Reset Mode
1
0
1
x
1
x
System Reset Mode
Reset
Note:
1. WDTON Fuse set to “0” means programmed and “1” means unprogrammed.
• Bit 4 - WDCE: Watchdog Change Enable
This bit is used in timed sequences for changing WDE and prescaler bits. To clear the WDE bit,
and/or change the prescaler bits, WDCE must be set.
Once written to one, hardware will clear WDCE after four clock cycles.
• Bit 3 - WDE: Watchdog System Reset Enable
WDE is overridden by WDRF in MCUSR. This means that WDE is always set when WDRF is
set. To clear WDE, WDRF must be cleared first. This feature ensures multiple resets during con-
ditions causing failure, and a safe start-up after the failure.
• Bit 5, 2..0 - WDP3..0: Watchdog Timer Prescaler 3, 2, 1 and 0
The WDP3..0 bits determine the Watchdog Timer prescaling when the Watchdog Timer is run-
ning. The different prescaling values and their corresponding time-out periods are shown in
Table 10-2 on page 55.
Table 10-2. Watchdog Timer Prescale Select
Number of WDT Oscillator
Cycles
Typical Time-out at
VCC = 5.0V
WDP3
WDP2
WDP1
WDP0
0
0
0
0
0
0
0
0
0
0
0
1
1
1
0
0
1
1
0
0
1
0
1
0
1
0
1
0
2K (2048) cycles
4K (4096) cycles
16 ms
32 ms
64 ms
0.125 s
0.25 s
0.5 s
8K (8192) cycles
16K (16384) cycles
32K (32768) cycles
64K (65536) cycles
128K (131072) cycles
1.0 s
55
8161D–AVR–10/09
ATMEL [ ATMEL ]
