On Thu, Nov 16, 2023 at 11:21:03AM -0500, Greg Troxel wrote:
> If within 1s is good enough, NMEA only is ok.
>
> If you have a system that is well-connected and you set up NMEA you will
> find out that there are delays and that these vary by type of receiver.
> You use 'time1' in ntp.conf to adjust this. I have 3 devices that need
> values of
> 0.116
> 0.0445
> 0.072
> to be close.
>
> That is very coarse compared to 1 ms for PPS. But indeed, if you have a
> machine that is not connected at all, it's really hard to tell it is a
> second off.
>
> So basically you might think
> NMEA only: 200 ms
> USB PPS: 1 ms
> gpio PPS: 100 us
>
> and to do better (or to know you are doing better) you need to really
> pay attention.
To do better I think I'd go with an implementation of ntpd on a microcontroller
(maybe an ESP32, which has native ethernet) and use the PPS signal with a
capture channel. Doing true real time on more complex CPUs is really hard.
I know there is an implementation on arduino, but I think a 32 bit MCU
would be a better choice.
--
Manuel Bouyer <bou...@antioche.eu.org>
NetBSD: 26 ans d'experience feront toujours la difference
--
