====== Enable PPS support in Raspian ======
DUNE has a task called ''%%Supervisors.ClockPPS%%'' that is used to dicipline the clock from both ''%%IMC::GpsFix%%'' messages and a PPS source. To get this to work, the kernel has to be compiled with two options not currently enabled by default:
  * CONFIG_PPS ("PPS support" in "make menuconfig") 
  * CONFIG_NTP_PPS ("PPS kernel consumer support" in "make menuconfig") 
==== Install dependencies ====
''%%sudo apt install git bc bison flex libssl-dev%%''
==== Download kernel ====
  mkdir ppskernel
  cd ppskernel
  git clone https://github.com/raspberrypi/linux.git
  cd linux
  make mrproper
  zcat /proc/config.gz > .config
  make oldconfig

==== Configure kernel build options ====

Then comes the hard part, setting the options. Normally this is straight forward, but due to some options only showing when others are disabled, it may be a little tricky.
  * Run ''%%make menuconfig%%''

  * Go to ''%%General setup ---> Timers subsystem --->%%'' and disable ''%%Old Idle dynticks config%%'', then enter ''%%Timer tick handling%%'' and change it to ''%%Periodic timer ticks (constant rate, no dynticks)%%''
  * Turn on PPS support: ''%%PPS kernel consumer support %%'' is the option that should have become visible after changing the timer settings. If not, save and reload ''%%make menuconfig%%''.
  * Enter Device Drivers ---> PPS support --->
\\
            <M> PPS support  
            [ ]    PPS debugging messages
            [*]    PPS kernel consumer support   
                   *** PPS clients support ***         
            < >    Kernel timer client (Testing client, use for debug)
            <M>    PPS line discipline
            <M>    PPS client using GPIO
                   *** PPS generators support *** 
\\
  * Save and exit. To verify, check the ''%%.config%%'' file that has been created in the same folder (nano .config). These options should be present somewhere in the file:
\\
  CONFIG_PPS=y
  CONFIG_NTP_PPS=y
  #
  # Timers subsystem
  #
  CONFIG_TICK_ONESHOT=y
  CONFIG_HZ_PERIODIC=y
  # CONFIG_NO_HZ_IDLE is not set
  # CONFIG_NO_HZ_FULL is not set
  # CONFIG_NO_HZ is not set
  CONFIG_HIGH_RES_TIMERS=y
  # CONFIG_PREEMPT_NONE is not set
  CONFIG_PREEMPT_VOLUNTARY=y
  # CONFIG_PREEMPT is not set
==== Compile the kernel ====
  make -j4
  make modules
\\
  * Gain superuser privileges and transfer a copy of the new kernel to the boot partition:
\\
  sudo -i
  cd ~pi/ppskernel/linux-rpi-[kernel version number].y
  make modules_install
  cp ./arch/arm/boot/zImage /boot/kernel_new.img
\\
  * Tell the system to use this new kernel by adding ''%%kernel=kernel_new.img%%'' to ''%%/boot/config.txt %%''.
  * Reboot
==== Add PPS source and load kernel module ====
  * Open ''%%/boot/config.txt %%'' again, and ''%%dtoverlay=pps-gpio,gpiopin=[GPIOPIN]%%'' where GPIOPIN is the PPS signal.
  * Load kernel module: ''%%sudo modprobe pps-gpio%%''
==== Test  and make permanent====
  * Test
\\
  apt-get install pps-tools
  sudo ppstest /dev/pps0
\\
  * Expected result is something like:
\\
  trying PPS source "/dev/pps0"
  found PPS source "/dev/pps0"
  ok, found 1 source(s), now start fetching data...
  source 0 - assert 1572772437.999997402, sequence: 6168 - clear  0.000000000, sequence: 0
  source 0 - assert 1572772438.999997803, sequence: 6169 - clear  0.000000000, sequence: 0
  source 0 - assert 1572772440.000000924, sequence: 6170 - clear  0.000000000, sequence: 0
  source 0 - assert 1572772440.999998834, sequence: 6171 - clear  0.000000000, sequence: 0
  source 0 - assert 1572772442.000001424, sequence: 6172 - clear  0.000000000, sequence: 0
  source 0 - assert 1572772442.999999478, sequence: 6173 - clear  0.000000000, sequence: 0
  source 0 - assert 1572772444.000000128, sequence: 6174 - clear  0.000000000, sequence: 0
  source 0 - assert 1572772444.999999537, sequence: 6175 - clear  0.000000000, sequence: 0
  source 0 - assert 1572772446.000000578, sequence: 6176 - clear  0.000000000, sequence: 0


\\
  * If successful, make permanent by opening ''%%/etc/modules%%'' and adding ''%%pps-gpio%%'' to the end.
  * Running a DUNE configuration with ''%%Supervisors.ClockPPS%%'' should now work

===== Links =====
  * [[https://support.ntp.org/bin/view/Sandbox/HowtoPpsOnRaspberryPi|How to PPS on Raspberry Pi]]
  * [[https://ava.upuaut.net/?p=951|5 minute guide to making a GPS Locked Stratum 1 NTP Server with a Raspberry Pi]]
  * [[http://brandon.penglase.net/index.php?title=NavSpark_GPS%2BGLONASS_Stratum-1_NTP_Time_Source_with_a_Raspberry_Pi#Configure_the_kernel|NavSpark GPS+GLONASS Stratum-1 NTP Time Source with a Raspberry Pi]]
  * [[https://www.satsignal.eu/ntp/Raspberry-Pi-NTP.html|The Raspberry Pi as a Stratum-1 NTP Server]]