[time-nuts] current crop of GPS receivers for Rpi/Beaglebone for NTP server/etc

[Pete Stephenson]

On Mon, Oct 16, 2017, at 03:45 PM, jimlux wrote:
> I have beagles, but others have pis.
> 
> There seem to be dozens of GPS receivers out there in a variety of form 
> factors.
> 
> What's the current "best" inexpensive choice for run of the mill 
> time-setting/1pps  that's a "catalog" item
> 
> Plenty of online "how-to" from 2013 and 2014, but we here on the list 
> know that the "cheap GPS" receiver business is a very moving target - 4 
> years is a long time.

It's a bit old-school, but I have an Oncore UT+ attached to my Pi 2
Model B timing server at home. A simple MOSFET-based level shifter[1]
converts the serial and PPS signal from the 5V the Oncore outputs to the
3.3V the Pi requires and is sufficiently fast to not make a significant
difference in timing (I can't see any delay, even with my oscilloscope
turned down to 5ns per div). I also built a little interface board with
a supercapacitor-based backup for the Oncore's RAM that holds the level
shifter. 

The Oncore driver for NTP is a bit chatty, so I have it save logs to a
small ramdisk[2][3][4] with the log being rotated every two days. Works
well.

I suppose using a 3.3V unit would be nice, but I had a bunch of Oncores
lying around that needed use, so why not? :)

Cheers!
-Pete

[1] <https://www.adafruit.com/product/757>
[2] The line in fstab is, omitting quotes, "tmpfs /tmp  tmpfs
defaults,noatime,nosuid,size=100M 0 0"
[3] The entry in the ntp.conf file is "logfile /tmp/ntp.log", which was
done after making the change detailed at [4]
[4] <http://lists.ntp.org/pipermail/hackers/2010-November/005004.html>

-- 
Pete Stephenson