On Mon, Nov 28, 2016 at 01:38:35PM -0600, Dan Drown wrote:
> People have done things like replacing the main oscillator on a system, but
> I wanted something more accessable.  I created a microcontroller with a TCXO
> and had it output 1PPS (via timer output compare hardware, controllable in
> 1ppb resolution, but dithered at 20ns steps).  I'm using a custom gpio-pps
> driver that captures CLOCK_MONOTONIC_RAW timestamps instead of
> CLOCK_REALTIME (so clock adjustments don't get fed back into the system).

Nice. That's an interesting approach.

> The question is: how do I feed that frequency data into chrony?  Right now
> I'm feeding the filtered data into chrony's tempcomp via "tempcomp /run/tcxo
> 16 0 0 1 0"  Is there a better way to do this?

Better as something that doesn't require a separate program to feed
chronyd? Probably not without modification of the code. I guess one
approach would be to add a new option for refclocks that would allow
them to work with MONOTONIC_RAW timestamps and modify the source
selection and combining algorithms to use the frequency estimate. I
suspect it wouldn't work very well as measurements of the other
sources would still be based on the unstable system time.

The tempcomp correction looks to me like a better place to do this.
The question is how do that without duplicating the code of the PPS
refclock and the refclock filter. I'm not sure if the current
interfaces allow that. They might need to be redesigned. I'll need to
think about that.

> Additional information:
> https://dan.drown.org/stm32/adev-mix.png
> ^ comparison of various adev (think "standard deviation of frequency's
> stability when sampled at the given interval")
> TCXO ADEV - TCXO vs GPS PPS, timer input compare measurement (20ns/2e-8
> resolution samples)

Do you have an adev plot from this system when the frequency
stabilization is disabled so we could see the difference?

Miroslav Lichvar

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