pps-gen-gpio

Linux kernel PPS generator using GPIO pins.

In kernel 5.10.162 there is no support for using a GPIO pin as a PPS generator, only a GPIO PPS client is available. This driver is derived from the current parallel port PPS generator and provides a PPS signal through a GPIO pin specified in the device tree. The PPS signal is synchronized to the tv_sec increment of the wall clock.

I have tested the driver with kernel 5.10.162 on a Beaglebone Black (P9.16) and Beaglebone AI64 (P9.16) where the specified pin is used as PPS output. On the Beaglebone Black I have encounters some errors when the pss-gen-gpio module was loaded. For example whole system halted, when I restarted a systemd service (chrony.service). Whenever I encountered this problems removing the module (rmmod), executing the desired command and then reinstering the module (insmod) worked for me. Additionally the pps-gen-gpio driver seems to not always create a pps signal. The behavior is better on the BeagleBone AI64. But some PPS-Signals are lost here aswell.

Usage

• You need to add the Kconfig, Makefile, pps_gen_gpio.c and pps-gen-gpio.txt file to the correct places when building your custom kernel.

  • See cross compile guide for bbb on how to build the kernel on another system
  • See cross compile guide for bbai64 on how to build the kernel on another system.

• Additionally, you need to compile the device tree overlay for the specified system.

  • See building the device trees on how to build the device tree on the system.

  • If you use a Beaglebone Black and want to change the PPS output pin, you have to change value from the "BB-BONE-P9_16-PPS-GEN.dts" file. Add the compiled overlay to your "/boot/uEnv.txt":

     	#skipping lines ...
     #Make sure overlays are enabled
     enable_uboot_overlays=1
     	#skipping lines ...
     uboot_overlay_addr4=/lib/firmware/BB-BONE-P9_16-PPS-GEN.dtbo
     	#skipping lines ...
    

  • If you use a Beaglebone AI64, you need to edit the "BBAI64-P9_16-PPS-GEN.dts" file. Add the compiled overlay to "/boot/firmware/extlinux/extlinux.conf":

   	#skipping lines ...
   # Multiple overlays can be added in the following line!
   fdtoverlays /overlays/BBAI64-P9_16-PPS-GEN.dtbo
   #fdtoverlays /overlays/BONE-UART1.dtbo /overlays/BBAI64-P9_18-GPS-PPS.dtbo /overlays/BBAI64-P9_16-PPS-GEN.dtbo
   	#skipping lines ...
  

• Make sure you have the right pin multiplexing setting:

  • Beaglebone Black: Check the BBB System Reference Manual and the am335x Technical Reference Manual to find out the correct pinmux table offset of the chosen pin.
  • BBAI64: You need to check the device tree bindings for the correct name. The current reference for the gpios is here:BBAI64 Documents

• The pps-gen-gpios property defines the pin you want to use as a PPS output:

    pps-gen {
    	status = "okay";
    	pinctrl-names = "default";
    	pinctrl-0 = <&pps_gen_pins>;
    	compatible = "pps-gen-gpio";
    	pps-gen-gpio = <&gpio1 19 GPIO_ACTIVE_HIGH>;
    	default-state = "off";
  

Please note that in order to use the module with any other board using the device tree infrastructure, the following matching definitions are required in the device tree:

	pps-gen               node defined for the PPS GPIO
	pps-gen-gpio          value of ".compatible" property in pps-gen node
	pps-gen-gpio          property in pps-gen node that defines which GPIO pin is used

After modifying the device tree, add the files into drivers/pps/generators and configure the driver to be built as a module. You need to enable PPS support in the kernel.