This document explains how to add the capability of generating a PPS signal on a GPIO pin of Raspberrypi3. It involves tasks similar to those described in README.md, namely patching configuring and compiling a new kernel for the Raspberrypi. Hence the thereby described preliminary operations and default kernel build procedures are assumed.
Again, refer to the official kernel building raspberrypi.org page for more details or troubleshooting informations.
GPIO PPS generator is implemented by a kernel module. Required patch can be
found in patches/. You can apply them with the patch command or
with a git workflow in a separate branch as follow. Run these commands from
within the linux kernel sources directory.
git checkout -b pps-generator-patches
git am <path-to-this-repo>/patches/0001-pps-add-gpio-PPS-signal-generator.patch
git am <path-to-this-repo>/patches/0002-add-DT-overlay-for-pps-gen-gpio-generator.patch
The PPS generator driver must be enabled within the kernel configuration. Here
make bcm2709_defconfig is assumed to be run at least once to bootstrap
default configuration. Run these commands from within the linux kernel sources
directory.
echo 'CONFIG_PPS_GENERATOR_GPIO=y' >> .config
It is now possible to build the kernel with the changes we made:
cd linux
KERNEL=kernel7
make bcm2709_defconfig
make olddefconfig
make -j4 zImage modules dtbs
The new kernel image, modules and dtbs must be installed over the current ones.
sudo make modules_install
sudo cp arch/arm/boot/dts/*.dtb /boot/
sudo cp arch/arm/boot/dts/overlays/*.dtb* /boot/overlays/
sudo cp arch/arm/boot/dts/overlays/README /boot/overlays/
sudo cp arch/arm/boot/zImage /boot/$KERNEL.img
The RaspberryPi is now ready to run with PPS generator enabled on GPIO pin 18.
Which pin is used for the PPS signal can be changed in
arch/arm/boot/dts/overlays/pps-gen-gpio-overlay.dts. If you need to change
this value, you then need to run make dtbs and install the new overlay as
explained above.