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)."
Possibly a better way to handle the script: