[time-nuts] First success with very simple, very low cost GPSDO, under $8

Chris Albertson albertson.chris at gmail.com
Wed Apr 9 23:35:17 UTC 2014

On Wed, Apr 9, 2014 at 1:04 PM, Tom Harris <celephicus at gmail.com> wrote:
> Another point with the software is that your handler for the PPS just reads
> the counter. This gives an offset between the PPS edge and the value read,
> as your software takes time to respond to the interrupt and read the
> counter. In your code, it doesn't matter as you only have one interrupt.

Actually there are two interrupts.  One is for PPS and the other is
for overflow of the 16-bit counter.   This over flow happens about 76
times per seconds.
> However, if you have another interrupt enabled, this could run after the
> PPS pulse but before the handler runs, giving you a very rare jitter.
> A better way would be to use the input capture feature to read the timer
> into a capture register. Then the interrupt handler has until the next edge
> to read the capture register.

Do you know how to do this.  I don't see any way to capture the timer
value other then reading it with software.  The timer capture register
is 16 bits and is set atomically after each timer increment but I
don't see a way to make an external interrupt pin capture a timer.

The two interrupts do bump into each other about roughly every 100
seconds but I can detect that.  I think I'll just ignore that second.

Chris Albertson
Redondo Beach, California

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