PWM
=====

The phyCORE-AM65x SOM brings out a selection of GPIOs that can be muxed to output a PWM signal. On the development kit you can access one of these signals through the PHYTEC Expansion board (PCM-957) at pin 35D. This guide walks through the basic steps of exporting this channel, configuring it, and enabling it to control the brightness of an LED.

.. list-table:: Connecting an LED
   :header-rows: 1

   * - Signal Name
     - Expansion Board Pin #
     - Description
   * - X_ECAP0_IN_APWM_OUT
     - 35D
     - 3.3V PWM Channel


.. warning:: The signal X_ECAP0_IN_APWM_OUT is used for OLDI backlight display by default. Disconnect the display when using this PWM for other purposes.

Set up Hardware Configuration
----------------------------------

.. image:: ../../images/phycore-am65x/pcm-067-gpio-expansion_pins.webp
  :width: 500px

.. image:: ../../images/phycore-am65x/pcm-067-pwm-expansion-pin.webp
  :width: 500px

.. image:: ../../images/phycore-am65x/pcm-067-pwm-circuit.webp
  :width: 400px


Set up Software Configuration
-------------------------------

In order to be accessible in Linux, the PWM channels need to be configured in the Linux device tree. PHYTEC has provided a sample device tree overlay in the boot directory of the root file system which configures expansion connector interfaces used in this Peripheral Guide. To enable this device tree overlay:

* The device tree overlay is set in U-Boot.

* To enter U-Boot, hit any key within 3 seconds of powering on the hardware to halt autoboot.

* Clear the "default_overlays" variable and modify the "extra_overlays" variable to point to the sample dtbo: "k3-am65xx-phytec-expansion-sample.dtbo". The "default_overlays" variable needs to be cleared because it is set to k3-am65xx-phytec-lcd-018.dtbo which conflicts with the sample.

  .. code-block:: none
    :caption: Target (U-Boot)

    setenv default_overlays
    setenv extra_overlays k3-am65xx-phytec-expansion-sample.dtbo
    saveenv

* To exit U-Boot and continue booting into Linux, type boot

  .. code-block:: none
    :caption: Target (U-Boot)

    boot

.. warning:: The Expansion Sample overlay k3-am65xx-phytec-expansion-sample.dtbo conflicts with the LCD-018 overlay due to the PWM being used for backlight in the LCD-018. Make sure to disconnect the display when using this overlay before powering on the development kit.

Access PWM in Linux
-----------------------

* To request a PWM channel from the kernel, write a 0 to the export attribute:

  .. code-block:: none
    :caption: Target (Linux)

    echo 0 > /sys/class/pwm/pwmchip0/export

* The duty cycle and period are set to 0 by default. You will need to configure these before enabling the PWM channel:

  .. code-block:: none
    :caption: Target (Linux)

    echo 1000000 > /sys/class/pwm/pwmchip0/pwm0/period
    echo 100000 > /sys/class/pwm/pwmchip0/pwm0/duty_cycle

.. note:: The duty cycle and period attributes are expressed in nanoseconds (10-9 seconds). For this example we are using a period of 1 ms with a duty cycle of .1 ms which is the same as saying our signal is at 1 kHz with a 10 percent duty cycle.

* Enable the channel:

  .. code-block:: none
    :caption: Target (Linux)

    echo 1 > /sys/class/pwm/pwmchip0/pwm0/enable

* You should now see that the LED is very dim. To increase the brightness, you must increase the duty cycle. The following command changes the duty cycle to 50%.

  .. code-block:: none
    :caption: Target (Linux)

    echo 500000 > /sys/class/pwm/pwmchip0/pwm0/duty_cycle

* The following command increases the duty cycle to 90%.

  .. code-block:: none
    :caption: Target (Linux)

    echo 900000 > /sys/class/pwm/pwmchip0/pwm0/duty_cycle

Disabling the Channel
-------------------------

  .. code-block:: none
    :caption: Target (Linux)

    echo 0 > /sys/class/pwm/pwmchip0/pwm0/enable

Un-Exporting the Channel
--------------------------

  .. code-block:: none
    :caption: Target (Linux)

    echo 0 > /sys/class/pwm/pwmchip0/unexport

Revert to the Default Software Configuration
-----------------------------------------------

This guide requires disabling the LCD-018 and enabling a sample expansion overlay. In order to revert back to the default overlay settings, the following needs to be run in U-Boot:

* To enter U-Boot, hit any key within 3 seconds of powering on the hardware to halt autoboot.

  .. code-block:: none
    :caption: Target (U-Boot)

    env default -f -a
    saveenv



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