Re: [chrony-users] Synchronizing clock with GPS with PPS

[Hal Murray <hmurray@xxxxxxxxxxxxxxx>]

> But then I do not use something like a Rasberry.

On a Pi, the built in serial port is /dev/ttyAMA0
It doesn't have any pins for the modem control signals.
At least with the typical setup.

On a Pi and similar SoC chips, there are not enough pins for all the potential 
uses.  There is a layer of muxing between the pins and the built in I/O 
devices.  The normal setup assigns lots of pins to GPIO.  With some work, you 
could probably activate the modem control signals.  I think device-tree is the 
buzz word.

On a Pi, the usual way is to turn one of the GPIO pins into a PPS.  gpsd 
normally sets up a /dev/pps<n> to go with a serial port.  There should be a 
way to tell GPSD to use /dev/pps0 rather than set up a new one.  I don't know 
how to do that, but that's what you should be looking for what scanning the 
gpsd documentation.

If gpsd is setting up /dev/pps1, it won't work.  That internal signal doesn't 
go anywhere.

-------------

My notes from setting up a Pi, non gpsd:

# PPS via gpio
#  http://www.satsignal.eu/ntp/Raspberry-Pi-NTP.html#easy
  vi /boot/cmdline.txt
    remove the chunk(s) with ttyAMA0
  vi /boot/config.txt
    add new line: dtoverlay=pps-gpio,gpiopin=18
    pin 4 on Adafruit GPS HAT
  vi /etc/modules
    add new line: pps-gpio
  vi /etc/inittab
    run raspi-config Interfacing-Options/Serial
    comment out getty line for ttyAMA0 (at bottom)

The GPIO pin numbering is different from the chip/hardware pin numbering.

When this gets sorted out, somebody should write a HOWTO for using chrony and 
gpsd on a Pi.


-- 
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