Re: [chrony-dev] [RFC] Transparent fallback from NTP reference clock to RTC

[Ahmad Fatoum <a.fatoum@xxxxxxxxxxxxxx>]

Hello Dan,

On 4/1/25 19:38, Dan Drown wrote:
> On 4/1/2025 8:08 AM, Ahmad Fatoum wrote:
> :
>> We have measured the accuracy we get out of our RTC as mentioned before
>> and it's sufficient for our use case. Epson sells this RTC as "High-
>> Stability",
>> so I do not refuse the claim that cheaper RTCs can be much worse.
> :
>> This is a wrong summary of my proposal. The RTC is only used as reference
>> clock while the system has no network connectivity. Once online,
>> system time
>> is synchronized against NTP and the RTC is reset to UTC to be better
>> equipped
>> for the next offline period.
> :
> 
> Does your RTC output a PPS and is that PPS connected to a CPU GPIO? If
> so, you can use the "local" option to the PPS refclock:
> https://chrony-project.org/doc/4.6.1/chrony.conf.html#refclock

Yes, the RTC interrupt is connected to a CPU GPIO.

> Chrony can use that to correct the system's frequency and improve the
> timekeeping while offline. I have some more details on: https://
> blog.dan.drown.org/strat-2-tcxo/

Thank you. This was insightful. I prefer not to go the PPS route though,
but use local as you do for the RTC reference clock instead.
That way the kernel driver can be used as-is to set up the periodic
interrupt.

Thanks,
Ahmad

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