[time-nuts] GPS DO Alternatives

Chris Albertson albertson.chris at gmail.com
Sun Dec 9 17:03:46 UTC 2012

On Sat, Dec 8, 2012 at 11:14 PM, Hal Murray <hmurray at megapathdsl.net> wrote:

> albertson.chris at gmail.com said:
> > What if I use a flip flop.  The PPS from the GPS connects to the "set"
> input
> > of the FF and also to the Arduino interrupt.   The output of the OCXO
> goes
> > to the reset pin of the same FF.
> I don't think that is going to do what you want.  The problem is what
> happens
> if set and reset are on at the same time?

This would be the best case.  The result will be random and we'd read an
average value of 0.5 from the flip flop.

> If I was doing that, I'm make the PPS from the OCXO 50% duty cycle and use
> the PPS from the GPS as a clock with the PPS from the OCXO as the data
> input.

I think you want everything to be edge triggered.  The flip flop should
look only at the leading edge of each pulse.

> Which brings up another worm for the can.  How are you going to get a PPS
> from the OCXO?

You don't get a PPS from the OXCO.  It runs at 10MHz (Or whatever you like)
so it resets the flip flop every 100 nanoseconds.

The way this works is the PPS from the GPS sets the FF to "1" once per
second and then some time later the OXCO resets it back to zero.   If we
sample the FF at some fixed time after the PPS and that fixed time is less
then the OXCO period then the FF will be either 1 or 0.   THe computer
tries to drive the OXCO so that the sampled value is 0.5 on average.   So
it looks at the last 1000 samples and tries to hit "500".

Basically what we have is a one bit counter but if we average enough
samples we get pretty good resolution.

> That's going to be hard without some soldering.  My straw man
> would be to use the OCXO to clock whatever uP you end up using and generate
> the PPS with a counter/timer.
> An alternative would be 7 TTL chips or a CPLD/FLGA.

If you used the CPLD then you have to agree to supply programmed chips for
life and to make any change any experimenter asks for with fast turn

> --
> These are my opinions.  I hate spam.
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Chris Albertson
Redondo Beach, California

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