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PWM on a Raspberry Pi 5

There have been reports that PWM on RPi5 is difficult: 1, 2 and others listed in this search. The issues seem to have to do with the various programming languages and libraries that worked with earlier RPi models (prior to RPi 5) - but are now failing on RPi 5. Outlined below is an approach that avoids these failures. This recipe is a follow-up to a couple of Q&A on SE (1, 2).

Before getting into the details, there are a couple of things I feel that I should mention. First, I've not provided much documentation with this recipe. There are two reasons for this; first, I developed this recipe by "following my nose" and reading bits here and there, and second... there really is no documentation! I don't know what's going on in The RPi Organization, but it seems to me there's been a noticeable decrease in the documentation published on the RPi 5. For example: There is no PDF datasheet for the BCM2712 - the RPi 5 processor. I'd encourage anyone who reads this to consider making a comment through the RPi Forums, or on their GitHub site.

All of that said, here are some instructions for setting up and controlling the PWM on RPi 5 using config.txt, sysfs and a tiny bit of bash:

Table of Contents

Single PWM channel

1. Load the single channel PWM overlay; i.e. edit config.txt & reboot:

Note: config.txt overlay documentation is available here.

   $ sudo nano /boot/firmware/config.txt 
   # add 1 line to the file: 
   dtoverlay=pwm
   # use defaults for 'pin' & 'func' parameters: 18,2
   # save the file, close it, and then re-boot 
   $ sudo reboot

2. Review the kernel driver documentation on PWM (Warning: x-arcane)

You can find it here.

3. "Explore" the /sys/class/pwm folder:

   $ ls /sys/class/pwm
   pwmchip0  pwmchip2  pwmchip6 
   
   # on RPi 5 the PWM controller is pwmchip2 
   
   $ cd /sys/class/pwm/pwmchip2 && ls -l
   lrwxrwxrwx 1 root gpio     0 Aug 15 00:25 device -> ../../../1f00098000.pwm
   --w--w---- 1 root gpio 16384 Aug 15 00:25 export
   -r--r--r-- 1 root gpio 16384 Aug 15 00:25 npwm
   drwxrwxr-x 2 root gpio     0 Aug 15 00:25 power
   lrwxrwxrwx 1 root gpio     0 Aug 15 00:25 subsystem -> ../../../../../../../class/pwm
   -rw-rw-r-- 1 root gpio 16384 Aug 15 00:25 uevent
   --w--w---- 1 root gpio 16384 Aug 15 00:25 unexport

4. "Export" channel 2 & Verify that worked properly:

   $ echo 2 > export 
   $ pwd
   /sys/class/pwm/pwmchip2 
   $ ls -l
   lrwxrwxrwx 1 root gpio     0 Aug 15 00:25 device -> ../../../1f00098000.pwm
   --w--w---- 1 root gpio 16384 Aug 15 00:34 export
   -r--r--r-- 1 root gpio 16384 Aug 15 00:25 npwm
   drwxrwxr-x 2 root gpio     0 Aug 15 00:25 power
   drwxrwxr-x 3 root gpio     0 Aug 15 00:34 pwm2   ### <== NOTE NEW FOLDER !!!
   lrwxrwxrwx 1 root gpio     0 Aug 15 00:25 subsystem -> ../../../../../../../class/pwm
   -rw-rw-r-- 1 root gpio 16384 Aug 15 00:25 uevent
   --w--w---- 1 root gpio 16384 Aug 15 00:25 unexport
   
   # Note the addition of the new folder 'pwm2' 
   # "Explore" new folder 'pwm2': 
   
   $ ls -l pwm2
   total 0
   -r--r--r-- 1 root gpio 16384 Aug 15 00:34 capture
   -rw-rw-r-- 1 root gpio 16384 Aug 15 00:36 duty_cycle
   -rw-rw-r-- 1 root gpio 16384 Aug 15 00:36 enable
   -rw-rw-r-- 1 root gpio 16384 Aug 15 00:35 period
   -rw-rw-r-- 1 root gpio 16384 Aug 15 00:34 polarity
   drwxrwxr-x 2 root gpio     0 Aug 15 00:34 power
   -rw-rw-r-- 1 root gpio 16384 Aug 15 00:34 uevent 
   
   # Which "look appropriate" for a PWM control channel

5. "Set" the PWM parameters:

   $ cd /sys/class/pwm/pwmchip2/pwm2
   $ echo 2000000 > period 
   $ echo 1000000 > duty_cycle 
   $ echo 1 > enable

6. Verify the PWM output:

If you've got some lab equipment (freq counter, oscilloscope, etc), connect that to GPIO 18 (header pin 12), and observe the output. If you don't, you can connect a DC voltmeter - or even an LED+Resistor - across GPIO 18 & GND & observe. A DC voltmeter should read about 1.65V with the period & duty-cycle settings from above.

The following is a screen-shot made using my old Agilent MSO9104A. The PWM waveform being measured was extremely stable - as one would expect from a hardware-based PWM output.

ss1-pwm-1ch

Dual PWM channels

As you would expect, the dual-channel version of this recipe closely follows the single-channel version above. We'll try to note all the differences below:

1. Load the 2-channel PWM overlay; i.e. edit config.txt & reboot:

$ sudo nano /boot/firmware/config.txt 
# add 1 line to the file: 
dtoverlay=pwm-2chan 

# REMOVE 'dtoverlay=pwm' if it's in there 
# save & close config.txt, and then reboot: 

$ sudo reboot

4."Export" channel 2 and channel 3; then verify that worked properly:

   $ cd /sys/class/pwm/pwmchip2
   $ echo 2 > export && echo 3 > export
   
   # NOTE the creation of folders 'pwm2' & 'pwm3'

5."Set" the PWM parameters for each channel:

   # Create two different PWM waveforms for 'pwm2' & 'pwm3'
   
   $ cd /sys/class/pwm/pwmchip2/pwm2
   $ echo 2000000 > period && echo 1000000 > duty_cycle && echo 1 > enable
   $ cd /sys/class/pwm/pwmchip2/pwm3
   $ echo 1000000 > period && echo 500000 > duty_cycle && echo 1 > enable

6. Verify :

The following screen-shot was made on an Agilent MSO9104A. As expected, the pwm3 trace is running at twice the frequency of pwm2 (b/c the 'period' is halved), both are extremely stable, and pwm3 appears on GPIO 19 (the default from config.txt).

ss2-pwm-2ch

A trivial bash script for basic setup:

We spent a lot of words describing this setup, but for routine usage this can be handled by a script. Assuming the /boot/firmware/config.txt file has been setup (dynamic device tree loading does not seem to work with PWM), the bash coding boils down to a few very simple echo statements:

#!/usr/bin/env bash

cd /sys/class/pwm/pwmchip2
echo 2 > export
sleep 1

cd ./pwm2
echo 2000000 > period && echo 1000000 > duty_cycle && echo 1 > enable

I'll leave parameterizing the period and duty_cycle up to the reader - it's basic bash coding.