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Atmel AVR132, Примечание к применению

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AVR 132 Application Note

2551A–AVR–11/03

Using the WDT Interrupt 
Mode

As described above, the WDT has its own internal oscillator running independently from
the main CPU clock. This makes it possible to use the WDT Interrupt as a wakeup
source from all sleep modes. By enabling only the WDT Interrupt Mode, a timeout will
generate an interrupt request, but not cause any system resets on further timeouts.

Having a wakeup source without running the main CPU clock is an excellent way of sav-
ing power. Using power-down sleep mode with the WDT as a wakeup source draws
approx 3mA when running at 3V supply voltage. An example on how to use the WDT as
a wakeup source is shown in Figure 4.

Figure 4.  Using the WDT as a Wakeup Timer

If periodic wakeups are preferred, the disabling of the WDT Interrupt Mode can be left
out. The WDT will then generate an interrupt on every timeout, waking up the CPU if it is
in sleep mode.

Note that the WDT System Reset Mode must not be enabled when using the WDT
solely as a wakeup timer. If it is enabled, a system reset will occur on the next timeout.
Using the WDT both as a wakeup timer and system reset source is described in the fol-
lowing section.

Ready to sleep

Enable sleep mode if

not already enabled

Enable interrupts if not

already enabled

Enable WDT Interrupt

Mode

SLEEP

WDT timeout wakeup

Disable WDT Interrupt

Mode

Continue

Set WDT timeout

period

Содержание AVR132

Страница 1: ...ous damage to its surroundings Automatic handling and recovery of such cases is the job of a watchdog timer The Enhanced Watchdog Timer WDT runs independent of the rest of the system causing system re...

Страница 2: ...age 3 shows what happens when a WDT timeout occurs The dotted boxes describe actions performed by the system The solid lined boxes describe actions to be performed by the application When using the En...

Страница 3: ...tion path through the main loop of your application This includes expected interrupt handlers as well If your main loop is very large several checkpoints could be inserted inside the loop to allow a s...

Страница 4: ...period The Everything ok check at the end of the loop is the part of the loop deciding whether the application is operating correctly or not One solution is to use flags that are set in different part...

Страница 5: ...to WDT System Reset The Write Complete Flag could be a byte in EEPROM indicating whether the backup operation was finished before the system reset This flag is checked in the startup code if the WDT...

Страница 6: ...3V supply voltage An example on how to use the WDT as a wakeup source is shown in Figure 4 Figure 4 Using the WDT as a Wakeup Timer If periodic wakeups are preferred the disabling of the WDT Interrupt...

Страница 7: ...pt or a timeout warning interrupt An example interrupt handler is shown in Figure 5 Figure 5 Dual purpose WDT Interrupt Handler Note that the wakeup flag must be set manually prior to entering sleep m...

Страница 8: ...safe state or at least be able to supply some debugging information to the operator Changing the WDT Configuration To prevent accidental changes to the WDT configuration special timed sequences are n...

Страница 9: ...igure 7 Figure 7 Circuit Diagram for Application Example The examples demonstrate the following concepts Using the WDT as a system reset source Using the WDT as a Wakeup Timer Using the WDT as a combi...

Страница 10: ...dashed frames are only used in the Combined Operation code example and are described later Figure 8 Initialization Routine When Using the WDT as a System Reset Source Any res et f lags s et Initializa...

Страница 11: ...uman operator to manually reset an application that has been halted by too many WDT System Resets The opera tor must of course try to find the source of the WDT System Resets before resetting Blindly...

Страница 12: ...in the switch block Execute Command In this routine the command code decides which action to perform Command 1 has no particular action but it keeps the main loop running healthy by being a valid comm...

Страница 13: ...loop of this example flashes the LED connected to PB0 10 times to show that it is awake It then resets the WDT enables the WDT Interrupt Mode and enters sleep mode When the WDT times out it wakes up t...

Страница 14: ...decides what action to take If it is already cleared an error has occurred and the failure LED is lit The rest of the interrupt handler implementation complies with the flowchart The rest of the code...

Страница 15: ...9F Tonetsu Shinkawa Bldg 1 24 8 Shinkawa Chuo ku Tokyo 104 0033 Japan Tel 81 3 3523 3551 Fax 81 3 3523 7581 Memory 2325 Orchard Parkway San Jose CA 95131 USA Tel 1 408 441 0311 Fax 1 408 436 4314 Mic...

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