Atmega2560 power down mode

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If I put the device in power down and I am using the external oscillator. The data sheet says that the external oscillator will be switched off. In that case, then with which clock does the device run?

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It doesn't run.
It goes to sleep.
It is in power down mode.

Four legs good, two legs bad, three legs stable.

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If you look at Table 11-1 in the datasheet it seems to show that in "power down" there are NO active clock domains.

Having said that almost all AVRs have a 1MHz separate oscillator for watchdog and it will continue to "tick" even when everything else is stopped. That's why there's an X under the "WDT interrupt" column on the "Power down" line as that, INT7-0 and TWI address match are the only possible ways you can get the clocks started again.

A common power saving strategy on AVRs is to put them into a deep sleep mode, set the WDT prescaler for the slowest (longest) time possible and then let the thing sleep. When the WDT eventually fires the chips wakes up, has a quick look round to see if anything needs doing, maybe increments a counter variable in RAM and if that hasn't reached a target just starts the cycle again. You can run for a VERY long time on very little power doing this.

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ah! Thanks Clawson. That was really helpful..

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Quote:

almost all AVRs have a 1MHz separate oscillator for watchdog ... That's why there's an X under the "WDT interrupt" column on the "Power down" line as that, INT7-0 and TWI address match are the only possible ways you can get the clocks started again.

Just a couple of clarifications:

-- AFAIK all modern AVR models have a supplementary 128kHz oscillator used for watchdog, EEPROM, and ??? .
-- Note that external interrupts wake on low-level only, and repeatedly fire.
-- Pin-change interrupts also awaken the AVR. Generally, I lean towards those when used as a pure wakeup source versus the low-level.
-- OP mentioned "external oscillator". Hmmm--is that a "crystal"? As the AVR is driving the crystal, it will indeed be stopped. The AVR cannot stop an external clock source. It could, however, ignore it.
-- Note that a crystal clock source needs a few milliseconds to get going again and be stable.

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