[time-nuts] GPS module recommendation for Pi timing

[Tim Shoppa]

If your product depends on monotonic times, there are a lot of good reasons
for using NTPD. It will slew the clock rather than jump it which can be
hugely important for many applications if there is some need for data to be
accumulated with monotonic timestamps. It will also apply good drift
correction for times when sky and satellites might not be visible (which in
terms of real world products is very common.

You prominently mention both milliseconds and TOD RTC. Note that all common
RTC chips (e.g. DS1307) only directly read out time to the previous second.
They do not read out directly for milliseconds although you can try to
interpolate based on seconds edge.

Tim N3QE


On Fri, Mar 13, 2020 at 10:07 AM 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)
> _______________________________________________
> time-nuts mailing list -- time-nuts at lists.febo.com
> To unsubscribe, go to
> http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com
> and follow the instructions there.
>