Since PWM is a little fraught with gotchas, this is mostly a message to future me-
(Note to self, rtfm - https://datasheets.raspberrypi.com/rp1/rp1-peripherals.pdf)
| pin | a0 | a3 |
|---|---|---|
| GPIO19 | PWM0_CHAN3 | |
| GPIO18 | PWM0_CHAN2 | |
| GPIO15 | PWM0_CHAN3 | |
| GPIO14 | PWM0_CHAN2 | |
| GPIO13 | PWM0_CHAN1 | |
| GPIO12 | PWM0_CHAN0 |
TODO: Figure out how to tell if pwm0 is on /sys/class/pwm/pwmchip1 or /sys/class/pwm/pwmchip2. pwm1 on the Pi 5 might have
device/consumer:platform:cooling_fan/
Life is short, this single dtoverlay configures GPIO12, GPIO13, GPIO18 and GPIO19 to their respective alt modes on boot and enables pwm0:
/dts-v1/;
/plugin/;
/{
compatible = "brcm,bcm2712";
fragment@0 {
target = <&rp1_gpio>;
__overlay__ {
pwm_pins: pwm_pins {
pins = "gpio12", "gpio13", "gpio18", "gpio19";
function = "pwm0", "pwm0", "pwm0", "pwm0";
};
};
};
fragment@1 {
target = <&rp1_pwm0>;
frag1: __overlay__ {
pinctrl-names = "default";
pinctrl-0 = <&pwm_pins>;
status = "okay";
};
};
};
Save as "pwm-pi5-overlay.dts" and compile with:
dtc -I dts -O dtb -o pwm-pi5.dtbo pwm-pi5-overlay.dts
Install:
sudo cp pwm-pi5.dtbo /boot/firmware/overlays/
Don't forget to add dtoverlay=pwm-pi5 to /boot/firmware/config.txt...
Then use this janky script to stick some safety rails on poking PWM:
#!/bin/bash
NODE=/sys/class/pwm/pwmchip1
CHANNEL="$1"
PERIOD="$2"
DUTY_CYCLE="$3"
function usage {
printf "Usage: $0 <channel> <period> <duty_cycle>\n"
printf " channel - number from 0-3\n"
printf " period - PWM period in nanoseconds\n"
printf " duty_cycle - Duty Cycle (on period) in nanoseconds\n"
exit 1
}
if [[ ! $CHANNEL =~ ^[0-3]+$ ]]; then
usage
fi
if [ -d "$NODE/device/consumer:platform:cooling_fan/" ]; then
echo "Hold your horses, looks like this is pwm1?"
exit 1
fi
if [ ! -d "$NODE/pwm$CHANNEL" ]; then
echo "0" | sudo tee -a "$NODE/export"
fi
echo "0" | sudo tee -a "$NODE/pwm$CHANNEL/enable" > /dev/null
echo "$PERIOD" | sudo tee -a "$NODE/pwm$CHANNEL/period" > /dev/null
if [ $? -ne 0 ]; then
echo "^ don't worry, handling it!"
echo "$DUTY_CYCLE" | sudo tee -a "$NODE/pwm$CHANNEL/duty_cycle" > /dev/null
echo "$PERIOD" | sudo tee -a "$NODE/pwm$CHANNEL/period" > /dev/null
else
echo "$DUTY_CYCLE" | sudo tee -a "$NODE/pwm$CHANNEL/duty_cycle" > /dev/null
fi
echo "1" | sudo tee -a "$NODE/pwm$CHANNEL/enable" > /dev/null
case $CHANNEL in
"0")
PIN="12"
FUNC="a0"
;;
"1")
PIN="13"
FUNC="a0"
;;
"2")
PIN="18"
FUNC="a3"
;;
"3")
PIN="19"
FUNC="a3"
esac
# Sure, the pin is set to the correct alt mode by the dtoverlay at startup...
# But we'll do this to protect the user (me, the user is me) from themselves:
pinctrl set $PIN $FUNC
echo "PWM$CHANNEL set to $PERIOD ns, $DUTY_CYCLE, on pin $PIN (func $FUNC)."
Kind soul, thank you for this, I tried many different solutions, including using the raspian devicetree overlays and directly setting memory-mapped registers to no avail. Your solution was the only one that worked, and it worked out of the box for me.
It's crazy to think it is this difficult to unlock PWM, and that there isn't a better tutorial for it.