Linux kernel PPS generator using GPIO pins
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README.md

pps-gen-gpio

Linux kernel PPS generator using GPIO pins.

In kernel 4.14 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.

We have tested the driver with kernel 4.14.20 on a Beaglebone Black where P9 pin 16 (GPIO1_19) is used as PPS output. The corresponding modified device tree file is here shown.

Usage

If you use a Beaglebone Black and want to change the PPS output pin, you have to modify your device tree file (am335x-boneblack.dts) accordingly:

  • Make sure you have the right pin multiplexing setting:

      pps_gen_pins: pinmux_pps_gen_pins {
      	pinctrl-single,pins = <
      		0x4C (PIN_OUTPUT_PULLDOWN | MUX_MODE7) /* gpmc_a3.gpio1_19 */
    

Check the BBB System Reference Manual and the am335x Technical Reference Manual to find out the correct pinmux table offset of the chosen pin.

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

      pps-gen {
      		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.