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On the feature phones I used to have, I could set the alarm clock, then turn the phone "off": the alarm clock would trigger at the set time anyway. Can I do this on iPhone?

By "off" (in quotes, because, we know that from electrical point of view, some parts of the phone stay powered) I mean the state of the phone you get when it is turned on, then you press the power button for a couple of seconds, then a red slider appears saying "slide to power off" and then you slide it.

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    This is definitely a feature I miss from my old Nokia "dumb" phones. I don't understand why Mac's have been able to do this for years, but the iPhone can't.
    – newenglander
    Commented Sep 30, 2014 at 14:42
  • @newenglander Exactly what can Macs do? (I never had one). I would continue this conversation in chat, if I knew how to.
    – Nickolai Leschov
    Commented Oct 7, 2014 at 15:30
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    You can set a specific time every day for the mac to be turned on, which is kind of like what we're looking for on the iPhone. See this image: techraptor.net/wp-content/uploads/2013/04/…
    – newenglander
    Commented Oct 7, 2014 at 16:12

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Playing the Clock alarm sound requires the main application ARM processor/CPU to be running the OS and Clock app. When you "slide to power off", power to the main CPU, radios and audio amplifier is removed. So, although the iOS device keeps time (as long as the battery isn't completely dead yet), it can't run the Clock app to play any sound.

When you just "lock" the device (quickly tap the top button and the display turns off), the CPU still occasionally gets power (something like a few times per second), and the audio circuits can be powered and active (to allow background music, etc.), so alarms can go off in that state.

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    There is no technical reason why they couldn't make this work though. Desktop PCs can both wake the system and play an alarm at a designated time. Windows also allows you to wake the PC to run any event in the task scheduler. The system RTC (which doesn't require the CPU) can wake the system a few minutes prior to a scheduled event such that the phone could play any alarm sound as normal.
    – NickG
    Commented Feb 29, 2016 at 11:49
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The alarm will not ring since that code runs on hardware that is powered off and a software stack that is shut down at that point.

The only two items that will start an iOS device that has gone through the slide to power off "power down" process is plugging in to power and pressing the lock/wake button.

Nothing functions on the hardware in this state except the real time clock component and the hardware responsible for powering up the device / responding to USB power being applied to initiate charging and power up.

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Nop, it can't. The simple reason is because you turn off the power. So there is no way to count time and to know when it should ring

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    You never actually completely turn off the power in a phone. If you did, there would really be no way to count time, then your phone's clock would be off by the amount of time you kept it turned off the next time you turn it on. At least some part of the device called real-time clock (RTC) should keep working and count time. RTC can be a separate chip (like this) or a part of the main system-on-chip, work at miniscule frerquencies like 32 KHz consuming negligible amounts of power and last for decades on a single charge.
    – Nickolai Leschov
    Commented Jun 24, 2014 at 20:43
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    So at least the RTC is always awake, maybe more. RTC should be running at all times, so it could have a hardware alarm clock, too, and it could send a signal to wake up the main processor, so that the alarm app could wake me up. In fact, that's how it worked in every feature phone I owned. I understand that modern smartphone operating systems are more complicated than that, so it's harder to pull such tricks off, but it would be nice if a modern smartphone could work like the featurephones before them could.
    – Nickolai Leschov
    Commented Jun 24, 2014 at 20:51
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To answer your question in a short manner...

No, when you shutdown you iPhone by holding the power button and sliding to power off you are completely disabling your iPhone. Same with the battery being dead.

It will not turn on to wake you up. It will stay off nothing will happen.

It will only wake you up if it has battery power and is turned on.

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