Enable PPS support in Raspian
- Links

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

Table of Contents