PWM

The phyCORE-i.MX7 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, PWM3, through the header X3 at pin 38. This guide walks through the basic steps of exporting this channel, configuring it, and enabling it to control the brightness of an LED.

Connecting an LED

Circuit Drawing

Setup

Step-by-step guide

PWM3 is disabled by default for general purpose and is reserved for controlling the backlight brightness on the PEB-AV-02 Parallel Touch Display. In order to free up this PWM Channel, download this modified device tree blob (imx7d-phyboard-zeta-004-PWM.dtb) and move it to your bootable SD Card. This device tree disables support for the the PEB-AV-02 Display and frees up this PWM Channel.

See the end of this guide for more details regarding the modifications made to the device tree source.

• Poweroff your phyCORE-i.MX7 development kit and remove the SD Card. Connect the SD Card to your Ubuntu Host Machine. 

On your Ubuntu Host Machine:

• Download the above device tree blob.

• Using the Terminal, navigate to the download location:

  Host (Ubuntu)

  cd ~/Downloads

  CODE

• Now transfer the device tree blob to the boot partition of the SD Card:

  Host (Ubuntu)

  sudo cp imx7d-phyboard-zeta-004-PWM.dtb /media/user/Boot\ imx7d-/ && sync

  CODE

• Unmount the SD Card from your Host Machine before ejecting the card.

   Unmount the SD Card

  

• Now insert the SD Card back into the phyCORE-i.MX7 kit and press any key during boot to stop in U-Boot.

• Enter the following commands to utilize the new device tree before booting into Linux:

  Target (U-Boot)

  setenv fdt_file imx7d-phyboard-zeta-004-PWM.dtb
  saveenv
  boot

  CODE

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

  Target (Linux)

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

  CODE

• The duty cycle and period are set to 0 by default so we will need to configure these before enabling the PWM channel: 

  Target (Linux)

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

  CODE
  

  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% duty cycle.

• Enable the channel: 

  Target (Linux)

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

  CODE

• 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%.

  Target (Linux)

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

  CODE

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

  Target (Linux)

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

  CODE

Disabling the Channel

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

Un-exporting the Channel

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

Get Fancy!

• Open a text editor to write a script: 

  Target (Linux)

  vi ~/pulseLED.sh

  CODE

• Enter the following and save the file: 

  Vi Text Editor

  #!/bin/bash
  
  dc=0
  max_dc=1000000
  step=25000
  
  #Re-request the PWM Channel incase its already in use
  echo 0 > /sys/class/pwm/pwmchip0/unexport
  echo 0 > /sys/class/pwm/pwmchip0/export
  
  #PWM config
  echo $max_dc > /sys/class/pwm/pwmchip0/pwm0/period
  echo $dc > /sys/class/pwm/pwmchip0/pwm0/duty_cycle
  echo 1 > /sys/class/pwm/pwmchip0/pwm0/enable
  
  
  while true; do
          while (("$dc" < "$max_dc")); do
                  dc=$((dc + step))
                  echo $dc > /sys/class/pwm/pwmchip0/pwm0/duty_cycle
                  sleep 0.025
          done
          while (("$dc" > 0)); do
                  dc=$((dc - step))
                  echo $dc > /sys/class/pwm/pwmchip0/pwm0/duty_cycle
                  sleep 0.025
          done
  done

  CODE
  

  The vi text editor begins in "Command Mode" and you must first hit the 'i' key in order to enter "Insert Mode". Using the arrow keys to navigate, make the necessary changes and then hit ESC to go back to "Command mode". Now enter ":wq" to write the file and quit.

  
  

  Pro Tip: Use the right click on your mouse to paste. This will only work if you are in "Insert Mode" first.

• Change the permissions in order to execute the script: 

  Target (Linux)

  chmod +x ~/pulseLED.sh

  CODE

• Now run the script in the back ground: 

  Target (Linux)

  ~/pulseLED.sh &

  CODE
• Check out that pulsing LED!

• To end the background process, enter the following: 

  Target (Linux)

  killall pulseLED.sh

  CODE

• The LED will stay on at the brightness it was at when the process was killed. To turn this off enter the following to disable and then un-export the LED. 

  Target (Linux)

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

  CODE

Enabling PWM in the Device Tree

This section is for advanced users hoping to learn more about customizing the Kernel. You will likely want to skip this section if you are just working through the guides to validate your hardware.

In order to enable PWM3 on your own, you will first have to build the BSP by following the BSP Development Guide. Once done, we can modify the device tree source and then recompile just the Kernel (this will take far less time then building the entire BSP). 

• After building the BSP, open the imx7d-phyboard-zeta-004.dts using the Vim Text Editor:  

  Host (Ubuntu)

  vim $YOCTO_DIR/build/tmp/work/imx7d_phyboard_zeta_004-poky-linux-gnueabi/linux-phytec-fsl/4.14.78+git_v4.14.78-phy2-r0/git/arch/arm/boot/dts/imx7d-phyboard-zeta-004.dts

  CODE

• Using the arrow keys to navigate, comment out the line enabling the PEB-AV-02 Parallel Display Adapter. Note that doing this will render the PEB-AV-02 Parallel Display incompatible with the phyCORE-i.MX7. The resulting file after the change is shown below (an arrow was included to show the line that needs to be changed, don't include this arrow in the actual file).

  /*
   * Copyright (C) 2019 PHYTEC America, LLC
   *
   * This program is free software; you can redistribute it and/or modify
   * it under the terms of the GNU General Public License version 2 as
   * published by the Free Software Foundation.
   *
   *
   * DTS for PHYTEC kit KPB-01910-004
   */
  /dts-v1/;
  
  #include "imx7d.dtsi"                   /* Dual Processor */
  #include "imx7-phycore-som.dtsi"        /* Superset - includes all SOM population options */
  #include "imx7d-pba-c-09.dtsi"          /* Carrier board */
  
  /*
  #include "imx7-peb-av-02.dtsi"           Parallel LCD adapter                                                      <---------Comment out this line as shown
  */
  
  #include "imx7-peb-d-rpi.dtsi"          /* PEB-D-RPI module */
  #include "imx7d-pcm-061-2110111c.dtsi"  /* SOM variant */
  
  

  CODE
  

  The vi text editor begins in "Command Mode" and you must first hit the 'i' key in order to enter "Insert Mode". Using the arrow keys to navigate, make the necessary changes and then hit ESC to go back to "Command mode". Now enter ":wq" to write the file and quit.

  
  

  Pro Tip: Use the right click on your mouse to paste. This will only work if you are in "Insert Mode" first.

• Once the file is saved and closed, recompile just the Kernel by using the following build-time optimization: 

  bitbake linux-phytec-fsl -f -c compile && bitbake linux-phytec-fsl

  CODE
• The changes will be reflected in the generated binary $YOCTO_DIR/build/tmp/deploy/images/imx7d-phyboard-zeta-004/imx7d-phyboard-zeta-004.dtb which can be moved to the Boot partition of your SD Card.