PPS GPIO with Buildroot image on Raspberry Pi 3

Question

I using Buildroot trying to build a Linux image for the Raspberry Pi 3 in which I have access to pulse-per-second (PPS) inputs on one of the GPIO pins.

First off, I have tried this with the standard Raspbian distribution and got it to work with the following changes:

• Add dtoverlay=pps-gpio,gpiopin=20 to /boot/config.txt.
• Add pps-gpio to /etc/modules.

I then get an entry /dev/pps0 and when connecting a wire with PPS signal to physical pin 38 on the RPi3 and running pps-test /dev/pps0 I get the expected one signal per second. So far so good.

Now I would like to recreate this with my own image built with Buildroot. I'm using the default configs/raspberrypi3_64_defconfig configuration, but with the following changes in make nconfig:

• Using kernel branch rpi-4.14.y-rt from github.com/raspberrypi/linux
• systemd as init system
• /dev management using udev (from systemd)
• Target packages -> Hardware handling -> pps-tools activated

(I think that is all the changes I made, but I might have forgotten something..)

In the sdcard.img which I get as output I see the file pps-gpio.dtbo in the boot partition. I add the line dtoverlay=pps-gpio,gpiopin=20 to the file config.txt. I also add the line pps-gpio to a file named pps.conf which I put in /etc/modules-load.d on the file system.

When I boot the system I get no entry /dev/ppsX but when I run lsmod I get (among others): pps_gpio 16384 0 pps_core 20480 1 pps_gpio

Does this mean the dtoverlay has been correctly loaded? What can I try in order to get an entry in /dev/ppsX?

Answer 1

Turns out what was missing was to in make nconfig also select:

• Kernel -> Build Device Tree with overlay support

Also, in the file <buildroot>/board/raspberrypi3-64/genimage-raspberrypi3-64.cfg add the pps-gpio.dtbo file so that the image boot.vfat section looks like this:

image boot.vfat {
  vfat {
    files = {
      "bcm2710-rpi-3-b.dtb",
      "bcm2710-rpi-3-b-plus.dtb",
      "bcm2837-rpi-3-b.dtb",
      "rpi-firmware/bootcode.bin",
      "rpi-firmware/cmdline.txt",
      "rpi-firmware/config.txt",
      "rpi-firmware/fixup.dat",
      "rpi-firmware/start.elf",
      "Image"
    }

    file overlays/pps-gpio.dtbo {
      image = "rpi-firmware/overlays/pps-gpio.dtbo"
    }
  }
  size = 32M
}

Putting the pps.conf file under etc/modules-load.d is not necessary.

With these changes I get a /dev/pps0 device automatically when booting the system.