46
Atmel ATmega16/32/64/M1/C1 [DATASHEET]
7647K–AVR–12/13
If WDE is set, the watchdog timer is in interrupt and system reset mode. The first time-out in the 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.
Note:
1.
For the WDTON fuse “1” means unprogrammed while “0” means programmed.
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 conditions 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 running. The different prescaling values
Table 7-5.
Watchdog Timer Configuration
WDE
WDIE
Mode
Action on Time-out
1
0
Stopped
None
1
0
1
Interrupt mode
Interrupt
1
0
System reset mode
Reset
1
Interrupt and system reset mode
Interrupt, then go to system reset
mode
0
x
System reset mode
Reset
Table 7-6.
Watchdog Timer Prescale Select
WDP3
WDP2
WDP1
WDP0
Number of WDT Oscillator Cycles
Typical Time-out at
V
CC = 5.0V
0
2K (2048) cycles
16ms
0
1
4K (4096) cycles
32ms
0
1
0
8K (8192) cycles
64ms
0
1
16K (16384) cycles
0.125s
0
1
0
32K (32768) cycles
0.25s
0
1
0
1
64K (65536) cycles
0.5s
0
1
0
128K (131072) cycles
1.0s
0
1
256K (262144) cycles
2.0s
1
0
512K (524288) cycles
4.0s
1
0
1
1024K (1048576) cycles
8.0s
1
0
1
0
Reserved
1
0
1
0
1
0
1
0
1