Four channel PWM on a Raspberry Pi 5

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ferzforz: Posts: 2; Joined: Thu Nov 06, 2025 12:59 pm

Four channel PWM on a Raspberry Pi 5

Quote

Thu Nov 06, 2025 1:34 pm

Hi.
I'm just trying out things. My goal is to move from a Motor HAT to a smaller chip with the RPI5 PWM channels driving an H-bridge.

The rp1-peripherals document very clearly states that there are 4 independent PWM channels. I'm just trying to access them.
First with bash, then later with c++.

Trying to use gpioctrl + sysfs just for proof of concept. No custom overlays loaded.

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uname -a
Linux raspberrypi 6.12.47+rpt-rpi-2712 #1 SMP PREEMPT Debian 1:6.12.47-1+rpt1 (2025年09月16日) aarch64 GNU/Linux

And I do the simplest possible thing.

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#unbind some fan driver holding pwm 3. a bit brute-ish but works.
echo "1f0009c000.pwm" | sudo tee /sys/class/pwm/pwmchip0/device/driver/unbind
# give all the channels!
echo 0 | sudo tee /sys/class/pwm/pwmchip0/export
echo 1 | sudo tee /sys/class/pwm/pwmchip0/export
echo 2 | sudo tee /sys/class/pwm/pwmchip0/export
echo 3 | sudo tee /sys/class/pwm/pwmchip0/export
# after this we excpect to see pwm0-3 in /sys/class/pwm/pwmchip0/
ls -l /sys/class/pwm/pwmchip0/
total 0
lrwxrwxrwx 1 root gpio 0 Nov 6 14:10 device -> ../../../1f0009c000.pwm
--w--w---- 1 root gpio 16384 Nov 6 14:15 export
-r--r--r-- 1 root gpio 16384 Nov 6 14:10 npwm
drwxrwxr-x 2 root gpio 0 Nov 6 14:10 power
drwxrwxr-x 3 root gpio 0 Nov 6 14:15 pwm0
drwxrwxr-x 3 root gpio 0 Nov 6 14:14 pwm1
drwxrwxr-x 3 root gpio 0 Nov 6 14:15 pwm2
drwxrwxr-x 3 root gpio 0 Nov 6 14:15 pwm3
lrwxrwxrwx 1 root gpio 0 Nov 6 14:10 subsystem -> ../../../../../../../class/pwm
-rw-rw-r-- 1 root gpio 16384 Nov 6 14:10 uevent
--w--w---- 1 root gpio 16384 Nov 6 14:10 unexport
# now we set the pins!
sudo pinctrl set 12-15 a0
# and confirm
pinctrl get 12-15
12: a0 pd | lo // GPIO12 = PWM0_CHAN0
13: a0 pd | lo // GPIO13 = PWM0_CHAN1
14: a0 pd | lo // GPIO14 = PWM0_CHAN2
15: a0 pd | lo // GPIO15 = PWM0_CHAN3
#and enable the outputs!
for i in 0 1 2 3; do
 echo "=== pwm$i ==="
 echo 100000 | sudo tee /sys/class/pwm/pwmchip0/pwm$i/period
 echo 50000 | sudo tee /sys/class/pwm/pwmchip0/pwm$i/duty_cycle
 echo 1 | sudo tee /sys/class/pwm/pwmchip0/pwm$i/enable
done

And I believe that I should be able to measure a voltage on those pins. However, its all zeros.
And it's controlling the fan on pwm3!
which is weird because

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 pinctrl | grep -i pwm
12: a0 pd | lo // GPIO12 = PWM0_CHAN0
13: a0 pd | lo // GPIO13 = PWM0_CHAN1
14: a0 pd | lo // GPIO14 = PWM0_CHAN2
15: a0 pd | lo // GPIO15 = PWM0_CHAN3
45: a0 pd | hi // FAN_PWM/GPIO45 = PWM1_CHAN3

Any help would be greatly appreciated.

aBUGSworstnightmare: Posts: 13432; Joined: Tue Jun 30, 2015 1:35 pm

Re: Four channel PWM on a Raspberry Pi 5

Quote

Thu Nov 06, 2025 2:26 pm

how did you enable the PWMs?
here is an overlay for you to get you started viewtopic.php?p=2205275#p2205275

ferzforz: Posts: 2; Joined: Thu Nov 06, 2025 12:59 pm

Re: Four channel PWM on a Raspberry Pi 5

Quote

Fri Nov 07, 2025 8:27 am

aBUGSworstnightmare wrote: ↑
Thu Nov 06, 2025 2:26 pm
how did you enable the PWMs?
here is an overlay for you to get you started viewtopic.php?p=2205275#p2205275

Humble pie for me!

adding
dtoverlay=pwm
in /boot/firmware/config.txt made it work after reboot.

Thanks a lot!