[time-nuts] x86 CPU Timekeeping and clock generation

[Trent Piepho]

On Tue, Jan 5, 2021 at 9:42 PM Luiz Paulo Damaceno
<luizpauloeletrico42 at gmail.com> wrote:
> The 24 MHz comes from an synthesizer that is locked to an atomic clock, the
> clock of NTP server (also 24 MHz, but an embedded board (Tinkerboard)) also
> comes from the same Atomic clock that is feeding other synthesizer for
> generates 24 MHz to this board.

The RK3288 has some PWM generators.  These are of course also fed from
PLLs derived from the same 24 MHz input.

So, why not produce a signal on the PWM that can be compared to your
reference?  This would tell you if the error is in the clock
generation on the SoC or something in software that happens afterward.
Or at least as far as the PWM clock tree overlaps the kernel
timesource clock tree, which could be the CPU clock but it can be
other things too.

> The experiment is the following: 1- synchronize the computer's clock to NTP
> server then leave it running free (no periodic synchronization), 2 -

NTP will set the frequency skew too, so even if it is not doing
periodic synchronization, there may still be a programmed frequency
skew.