[time-nuts] GPSDO Alternatives

Bob Camp lists at rtty.us
Thu Dec 6 23:55:13 UTC 2012


Hi

To be useful, you need an input capture that:

1) Runs at a fast enough clock (1 GHz would be nice)
2) Has enough bits to get to 1 pps (say 32 bits)
3) Has a built in period set, so the hardware works without a lot of silly stuff

Often you find parts that will do some of the above, but not all. 16 bit captures running off of a "few MHz" clock are pretty common. Some (but not all) ARM's have 32 bit captures that run off of 10's of MHz clocks and have the ability to set the period.

Bob


On Dec 6, 2012, at 6:34 PM, Lizeth Norman <normanlizeth at gmail.com> wrote:

> Bob et al:
> Have been following this thread with interest.
> Re the input capture versus interrupt, I do believe (at least the 2560
> does this) that you can do both. It's been a while since I looked. a
> look at the hardware manual. Was interested in this feature to do
> hardware timing.
> Norm
> 
> On Thu, Dec 6, 2012 at 6:25 PM, Bob Camp <lists at rtty.us> wrote:
>> Hi
>> 
>> That would be an input capture rather than an interrupt.
>> 
>> Bob
>> 
>> On Dec 6, 2012, at 6:16 PM, Hal Murray <hmurray at megapathdsl.net> wrote:
>> 
>>> 
>>> albertson.chris at gmail.com said:
>>>> You'd have to seriously divide down the output from the 10MHz OCXO if you
>>>> were going to use it as an interrupt.  Maybe to divide by 10,000? and even
>>>> at the higher clock rate you'd still have poor resolution.
>>> 
>>>> I image each interrupt handler would sample some internal counter and the
>>>> background task would look at the delta between the two and adjust the DAC
>>>> to drive the OCXO to close the difference.   The resolution would be
>>>> (maybe?) a "handful" of clock cycles.   Given enough time, say a 1000 second
>>>> period it might wrk well enough.  I can't know without doing a more detailed
>>>> design
>>> 
>>> Do some of the counter/timer modules have an option to run the counter off an
>>> internal clock and copy the value into another register on an external signal?
>>> 
>>> That avoids any interrupt latency.
>>> 
>>> 
>>> 
>>> 
>>> --
>>> These are my opinions.  I hate spam.
>>> 
>>> 
>>> 
>>> 
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>> 
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