ATmega88PA Automotive Atmel Corporation, ATmega88PA Automotive Datasheet - Page 52

• Manufacturer Part Number: ATmega88PA Automotive
• Manufacturer: Atmel Corporation

52
Atmel ATmega48PA/88PA/168PA [Preliminary]
Figure 11-7. Watchdog Timer
In Interrupt mode, the WDT gives an interrupt when the timer expires. This interrupt can be
used to wake the device from sleep-modes, and also as a general system timer. One example
is to limit the maximum time allowed for certain operations, giving an interrupt when the opera-
tion has run longer than expected. In System Reset mode, the WDT gives a reset when the
timer expires. This is typically used to prevent system hang-up in case of runaway code. The
third mode, Interrupt and System Reset mode, combines the other two modes by first giving
an interrupt and then switch to System Reset mode. This mode will for instance allow a safe
shutdown by saving critical parameters before a system reset.
The Watchdog always on (WDTON) fuse, if programmed, will force the Watchdog Timer to
System Reset mode. With the fuse programmed the System Reset mode bit (WDE) and Inter-
rupt mode bit (WDIE) are locked to 1 and 0 respectively. To further ensure program security,
alterations to the Watchdog set-up must follow timed sequences. The sequence for clearing
WDE and changing time-out configuration is as follows:
The following code example shows one assembly and one C function for turning off the
Watchdog Timer. The example assumes that interrupts are controlled (e.g. by disabling inter-
rupts globally) so that no interrupts will occur during the execution of these functions.
1. In the same operation, write a logic one to the Watchdog change enable bit (WDCE)
2. Within the next four clock cycles, write the WDE and Watchdog prescaler bits (WDP)
and WDE. A logic one must be written to WDE regardless of the previous value of the
WDE bit.
as desired, but with the WDCE bit cleared. This must be done in one operation.
WATCHDOG
RESET
OSCILLATOR
128kHz
WDIE
WDIF
WDE
WDP0
WDP1
WDP2
WDP3
MCU RESET
INTERRUPT
9223B–AVR–09/11