[time-nuts] iPhone keeping better time?

Colby Gutierrez-Kraybill colby at astro.berkeley.edu
Wed Nov 16 15:26:38 EST 2011


> 
> Message: 5
> Date: Wed, 16 Nov 2011 10:35:01 -0800
> From: Chris Albertson <albertson.chris at gmail.com>
> To: Discussion of precise time and frequency measurement
> 	<time-nuts at febo.com>
> Subject: Re: [time-nuts] iPhone keeping better time?
> Message-ID:
> 	<CABbxVHs6FLTsFE_miYSVxb0JOWrzDacwGY7XJMOV34sjHAduQw at mail.gmail.com>
> Content-Type: text/plain; charset=ISO-8859-1
> 
> I doubt we will ever see good time keeping on an IOS type device.  The
> problem is battery life.  Good time keeping requires a  stable local
> oscillator of some kind that must remain powered up 24x7.   But to get
> the long battery life they must power off everything they possibly
> can.  No mater how far technology advances it will always require less
> power to not ruin an oscillator then to run one.
> 
> I doubt Apple would run NTP in an iPhone.  They don't want to multi
> task the CPU and there is no stable  local oscillator to be
> disciplined.
> 


Ugh.  Yes, iOS devices sip power by turning as much of itself off whenever it can.  However, iOS is still running a version of the XNU kernel:
http://en.wikipedia.org/wiki/XNU

That kernel *does* multitask.  The multitasking you're referring to is what is allowed by apps that are run at the high layers of the system.  There are system daemons that do run, though ntpd is not one of them, probably for the exact reasons you mention about saving on battery usage.

The times you see on phone network based iOS devices come from the cell phone time keeping (on older devices) and then GPS (on later devices with actual GPS parts) (i.e. if the time is off, it could be the cell provider's fault).

See also:
[1] http://code.google.com/p/ios-ntp/wiki/WhatsItAllAbout
[2] http://www.quora.com/Will-iOS-5-Support-NTP-so-that-the-iPad-can-keep-time

Under iOS 5, you can tell an iOS device to automatically set its time and it appears to use a very simple timed to do it, see the second reference.


> On Wed, Nov 16, 2011 at 10:21 AM, David J Taylor
> <david-taylor at blueyonder.co.uk> wrote:
>>> It's even worse on the WiFi iPad - there is no way to automatically set
>>> the
>>> time. ?You can only do it via the "Settings" page and that only gives you
>>> minute resolution.
>>> 
>>> The Emerald-Sequoia app is nice, but since it can't actually "fix" the
>>> time, every app that has time constraints has to do its own NTP.
>> 
>> Criminal, really! ?Even SNTP would be better. ?I had hoped it would be fixed
>> in iOS 5.0 as I'd seen it discussed in the beta groups.
>> 
>> David
>> --
>> SatSignal software - quality software written to your requirements
>> Web: ?http://www.satsignal.eu
>> Email: ?david-taylor at blueyonder.co.uk
> 
> 
> -- 
> 
> Chris Albertson
> Redondo Beach, California
> 
> 




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