[time-nuts] GPS module recommendation for Pi timing

[shouldbe q931]

On Fri, Mar 13, 2020 at 2:07 PM Brian Lloyd <brian at lloyd.aero> wrote:
>
> I have an application where I need to synchronize the internal TOD RTC in a
> raspberry pi and need to pick a GPS module. We are building our own
> hardware but still using the Pi so interconnection will be via GPIO/serial.
> We won't try to use USB.
>
> This is not an NTP application. These units will be in the field and will
> most likely not have Internet access. I need their clocks to be pretty
> close. I am shooting for 1ms ... if possible.
>
> The new M9F and M9T modules from Ublox are a bit pricey. The M8T is a bit
> more reasonable. OTOH I realize there are limits to how tightly I can
> control the Pi's RTC and will run into diminishing returns, so even the M8T
> might be overkill.
>
> Has anyone here figured out what the reasonable limit is for timing on a
> Pi, and what makes sense for a timing module for an application like this?
>
> Thanks.
>
> --
>
>
>
> Brian Lloyd
> 706 Flightline
> Spring Branch, TX 78070
> brian at lloyd.aero
> +1.210.802-8FLY (1.210.802-8359)

If you are using a Pi, then you are limited to the Pi onboard crystal,
however as long as the GPS signal is "good", and the temperature is
stable, <1ms should be easily acheivable (my logs show ~20us is) while
GPS is running, however without GPS, the drift can be significant.

Although you might not need external NTP, the "simple" stack would be
ue gpsd to listen to the GPS and PPS, and then ntp/ntpsec/chrony to
set the time.

If you wanted a "plug and play" solution, the uptronics board is the
probably simplest.
https://store.uputronics.com/index.php?route=product/product&product_id=81,
otherwise for 1ms, there are many suppliers of uBlox modules.

Cheers

Arne