[Tinyos-help] Time counting

[Cory Sharp]

Of course the device capable for some definition of capable.  But the
keyword isn't capability but calibration: ensuring the mote records accurate
time across varying conditions and manufacture.  Keep in mind that "varying
conditions" even includes things like turning on a high power sensor causing
a drop in system voltage.  A "causal relation" to wall clock time stands to
reason, though perhaps you meant "casual relation".  Beyond that, I'm not
sure of what precisely both your observations and expectations are.

Cory

On 1/3/07, Michael Newman <mnewman at dragonnorth.com> wrote:
>
>  I actually discovered this problem when I started to implement a time
> synchronization protocol. I am trying to log the time a sample was actually
> taken on the mote and when that mote comes back in radio range report the
> list of samples and times they were taken.
>
> The problem is that my logs of when the samples were taken bear only
> causal relation to wall clock time.
>
> The hardware is capable of much more than this. I think there is a watch
> crystal on the Mica2 and Mica2Dot motes and time could be accurately counted
> and reported.
>
>  ------------------------------
> *From:* cory.sharp at gmail.com [mailto:cory.sharp at gmail.com] *On Behalf Of *Cory
> Sharp
> *Sent:* Wednesday, January 03, 2007 5:27 PM
> *To:* Michael Newman
> *Cc:* tinyos-help at millennium.berkeley.edu
> *Subject:* Re: [Tinyos-help] Time counting
>
> The timer system in TinyOS does not guarantee any particular accuracy,
> only that "accuracy MUST remain reasonable", probably something like +/-10%
> of the declared units.  This is for various reasons including but not
> limited to temperature, voltage, and manufacturing variations that affect
> hardware clocks, particular features or lack thereof offered by a
> microcontroller or its hardware platform, and timer computation choices
> within TinyOS.
>
> Probably the time units are actually 1024 ticks per second not 1000.  Even
> so, as listed above, there are many reasons why it may not be for your
> particular platform in general or given its current environmental and
> electrical conditions.
>
> If you need more accuracy, it is best that you incorporate a time
> synchronization protocol into your application.
>
> Best,
> Cory
>
> On 1/3/07, Michael Newman <mnewman at dragonnorth.com> wrote:
> >
> >  I have a complex application with a half dozen timers. One of the
> > timers is used to count a clock once per second. The timer is set up with a
> > call that looks like this:
> >
> >         call TimerClock.start(TIMER_REPEAT,1000);
> >
> > The timer is never stopped or changed.
> >
> > I count a variable in the TimerClock.fired event. I expect this variable
> > to reflect the number of seconds since the node has booted. What I observe
> > is that the counter counts faster than the actual number of elapsed seconds.
> >
> > Given other activities and the possibility of interrupts being off for
> > some periods I do not expect the timer to fire at an exact interval, what I
> > do expect is that the frequency of the timer will be exact with some jitter
> > to the individual firing events. I also expect the timer to never fire
> > early.
> >
> > Has anyone verified that timers actually fire on an exact interval and
> > that time is neither lost nor gained?
> >
> >
> >
> >  ------------------------------
> >
> > Michael Newman
> >
> > Dragonnorth Group
> >
> > 319 Dean Rd.
> > Brookline, MA 02445
> >
> > Cell: 617 821-4608
> > Home: 617 566-7975
> > Fax: 617 566-7975
> >
> > www.dragonnorth.com
> >
> > mnewman at dragonnorth.com
> >
> >
> > _______________________________________________
> > Tinyos-help mailing list
> > Tinyos-help at Millennium.Berkeley.EDU
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> >
> >
> >
>
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