[time-nuts] x86 CPU Timekeeping and clock generation

[Magnus Danielson]

Tom,

The RTC typically has a wristwatch type 32,768 kHz resonator. This is
independent to the oscillator setting bus and processor speed. They have
common factors in temperature and other environment, but nothing
steering them together.

You can have more independent oscillators, so things can drift away in
"interesting" ways. A fun exercise is to take timestamps of different
timestampers and locking one oscillators in reference to another. Then
one can follow the variations in frequency error in easy form. Otherwise
just plot the phase-difference and toss into TimeLab.

Cheers,
Magnus

On 2021-01-06 14:16, Tom Holmes wrote:
> Am I missing something or maybe I don't understand
> the situation , but I am under the impression that
> the RTC has it's own battery and crystal unrelated
> to the processor clock. Seems like in that case,
> the 24 MHz won't have any effect on the
> timekeeping drift. 
>
> Tom Holmes, N8ZM
>
> -----Original Message-----
> From: time-nuts <time-nuts-bounces at lists.febo.com>
> On Behalf Of Trent Piepho
> Sent: Wednesday, January 06, 2021 6:03 AM
> To: Discussion of precise time and frequency
> measurement <time-nuts at lists.febo.com>
> Subject: Re: [time-nuts] x86 CPU Timekeeping and
> clock generation
>
> 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.
>
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