Ben's notes

Configure NTPd with PPS

Based on Raspbian Stretch

• Disable hardware; Wifi, Bluetooth, HDMI.
• Disable services; avahi, triggerhappy, wpa_supplicant
• Add user, remove pi user.
• Set correct timezone.
• Update packages.

• Disable Bluetooth:

  echo "dtoverlay=pi3-disable-bt" >> /boot/config.txt

• Disable the HCI UART:

  sudo systemctl disable hciuart

• Disable getty:

  systemctl mask serial-getty@serial0.service

• Remove console=serial0,115200 from /boot/cmdline.txt
• Reboot
• Test GPS output: cat /dev/serial0

• Install PPS tools:

  apt install pps-tools

• Configure GPIO for PPS:

  echo "dtoverlay=pps-gpio,gpiopin=18" >> /boot/config.txt

• Reboot
• There should be a /dev/pps0 device
• Check for pulse

  # ppstest /dev/pps0
  trying PPS source "/dev/pps0"
  found PPS source "/dev/pps0"
  ok, found 1 source(s), now start fetching data...
  source 0 - assert 1552847111.000139273, sequence: 152 - clear  0.000000000, sequence: 0
  source 0 - assert 1552847112.000135096, sequence: 153 - clear  0.000000000, sequence: 0
  source 0 - assert 1552847113.000135337, sequence: 154 - clear  0.000000000, sequence: 0
  source 0 - assert 1552847114.000134886, sequence: 155 - clear  0.000000000, sequence: 0
  source 0 - assert 1552847115.000135615, sequence: 156 - clear  0.000000000, sequence: 0
  source 0 - assert 1552847116.000137416, sequence: 157 - clear  0.000000000, sequence: 0

• Install packages:

  apt install gpsd gpsd-clients

• Configure udev rules in /etc/udev/rules.d/gps.rules

  KERNEL=="ttyAMA0", SUBSYSTEM=="tty", DRIVER=="", SYMLINK+="gps0", MODE="0666"
  KERNEL=="pps0", SUBSYSTEM=="pps", DRIVER=="", SYMLINK+="gpspps0", MODE="0666"

• Configure devices in /etc/default/gpsd

  DEVICES="/dev/ttyAMA0 /dev/pps0"

• Reboot
• Test GPS output with cgps or gpsmon.

• Disable systemd timesyncd:

  systemctl stop systemd-timesyncd
  systemctl disable systemd-timesyncd

• Install NTP:

  apt install ntp

• Add to /etc/ntp.conf

  # GPS Serial
  server 127.127.28.0
  fudge 127.127.28.0 time1 refid GPS
  
  # GPS PPS
  server 127.127.28.1 prefer
  fudge 127.127.28.1 refid PPS

• Enable and start ntpd:

  systemctl enable ntp
  systemctl start ntp

• Comment ntp option in /etc/dhcpcd.conf

  #option ntp_servers

• Reboot