Adding a GPS Module (hat/shield) on a Raspberry Pi

[Ralph Smith]

On Feb 3, 2018, at 8:01 PM, Shamim Shahriar <shamim.shahriar at gmail.com> wrote:
> Thanks Ralph for the extended instructions -- very much appreciated. I'll try to arrange a system with both crochet and poudriere next week and see how far I can get. But very details instruction, and I'm sure it will take me to the objective. 
> 
> Just one question, will I be correct in assuming that the instructions are same/similar for rpi-3 but instead of rpi.dts I need to work on rpi2.dts? Currently I have a rpi-3 which too is intended for similar task (trying to have a fallback in case the pi-1b decides to push the daisies 😋), and if you could please confirm, I'll start working on both at the same time.

It should be similar, and I have verified this on the Pi Zero, Pi Model B, Pi 2 Model B, Pi 2 Model B v 1.2. With the changes made in the ports and the source tree, building a crochet image just works.

NTP using the PPS and GPSD/memory works great:
root at russet:~ # ntpq -p
     remote           refid      st t when poll reach   delay   offset  jitter
==============================================================================
*SHM(0)          .SHM.            0 l    2   16  377    0.000    0.997   3.591
oPPS(0)          .PPS.            0 l    1   16  377    0.000    0.039   0.007

This is after running about 15 minutes. Left longer the jitter settles down to about 0.004 from the PPS. Not too bad for a $30 total cost stratum 1 NTP server.

I’ve run into a bit of a snag on the Pi 3: Modifying u-Boot works, and keeps u-Boot from messing with the serial port. However, crochet grabs the DTB for the Pi 3 from Github rather than building one from source, and I haven’t figured out yet how to roll my own DTB for the 3. This results in FreeBSD turning the serial port into the console and spewing its console messages to the port. I’ll update if I figure out how to fix that, or would appreciate any pointers. As of now, I don’t have a lot of time to pursue that.

> 
> Thanks again for your excellent instructions.

Hope this works for you.

Ralph