[time-nuts] Question on crystal jumps

Jim Palfreyman jim77742 at gmail.com
Fri Oct 24 01:12:24 UTC 2008


Thanks Brooke,

Just letting you know I've written the code without interrupts.

Basically I set the internal 16 bit timer to count up and watch it in an
infinite loop. After it gets to a certain number then I reset the timer to
zero and count a "tick". I count these ticks and get my time and 1 PPS from
this.

I recognise there are some possible flaws in my method. I'd expect the flaws
to represent themselves as a slow drift in accuracy of the 1 PPS and/or some
sawtooth of the order of 100ns (1/10 MHz) about the true 1 PPS.

What I wasn't expecting was these jumps of the order of tens of
microseconds.

I suppose a solution is to take the 10MHz from my Thunderbolt and use that
to drive the AVR and see if the problem remains and the fault is in the
LPRO.

Jim


2008/10/24 Brooke Clarke <brooke at pacific.net>

> Hi Jim:
>
> I've made similar clocks using PIC micro controllers where the input is the
> 10
> MHz from the frequency standard.
> http://www.prc68.com/I/PRC68COM.shtml#07092006
>
> There's a subtle problem that has to do with if an interrupt occurs during
> a 3
> cycle long instruction or a 2 cycle long instruction.  This can cause
> erratic
> behavior if not properly accounted for.
>
> The 1 PPS from your clock should be spot on all the time, not off by a few
> hundred nano seconds.
>
> Have Fun,
>
> Brooke Clarke
> http://www.prc68.com/P/Prod.html  Products I make and sell
> http://www.prc68.com/Alpha.shtml  All my web pages listed based on html
> name
> http://www.PRC68.com
> http://www.precisionclock.com
> http://www.prc68.com/I/WebCam2.shtml 24/7 Sky-Weather-Astronomy Web Cam
>
> Jim Palfreyman wrote:
> > This discussion has come at an interesting time.
> >
> > I've hooked up an LPRO-101 from ebay to an AVR micro-controller -
> basically
> > using it to clock the processor directly.
> >
> > I've written code so the AVR is a clock and I've been comparing my
> clock's 1
> > PPS to the GPS.
> >
> > Over a few minutes it hops back and forward a few hundred nsec. All quite
> > acceptable given the clock speed.
> >
> > When I come back after a few hours it will suddenly be 10-20 usec slow. I
> > did see it jump over a few seconds from 200nsec to 17 usec.
> >
> > At first I put this down to interference because it coincide with me
> > switching off a light. So I put the whole lot into a metal box, shorter
> > wires and tried again.
> >
> > It still has jumps but this time it is *gaining*.
> >
> > Until this post started I'd assumed it was maybe my code and was about to
> > dive in and check it.
> >
> > Could these crystal "jumps" account for my issues?
> >
> > Regards,
> >
> > Jim
>
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