ATtiny167 datasheet(54/282 Pages) ATMEL

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7728D–AVR–07/09

ATtiny87/ATtiny167

Figure 6-7.

Watchdog Timer

The WDT gives an interrupt or a system reset when the counter reaches a given time-out

value. In normal operation mode, it is required that the system uses the WDR - Watchdog

Timer Reset - instruction to restart the counter before the time-out value is reached. If the sys-

tem doesn't restart the counter, an interrupt or system reset will be issued.

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:

In the same operation, write a logic one to the Watchdog change enable bit (WDCE)

and WDE. A logic one must be written to WDE regardless of the previous value of the

WDE bit.

Within the next four clock cycles, write the WDE and Watchdog prescaler bits (WDP)

as desired, but with the WDCE bit cleared. This must be done in one operation.

MCU

RESET

WATCHDOG

RESET

CLOCK

MONITORING

INTERRUPT

WDE

WDIF

WDIE

WDP0

WDP1

WDP2

WDP3

WATCHDOG

PRESCALER

~128 KHz

OSCILLATOR

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