[ntp:hackers] u-blox reference clock driver

[Achim Gratz]

Udo van den Heuvel via devel writes:
> What else could we do with the timemark functionality?
> See what is suggested at
> https://lists.ntpsec.org/pipermail/devel/2018-August/006447.html ?

Well, actually this suggestion was highly rasPi specific: The hardware
PWM is clocked directly off the crystal, so by programming it to deliver
one pulse per second and controlling the start of that pulse by the PPS
you get both: start of pulse timemark gets you the interrupt
delay+jitter, and the trailing edge gets you a measure of how far off
the XTAL is from nominal.  As a bonus, you can modulate the pulse width
to identify which timemark corresponds to which PPS echo and that
modulation can be used to dither the quantization error on the XTAL
frequency.


Regards,
Achim.
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