docs: Add hardware RPi PWM documentation (#4418)

Signed-off-by: Martin van Es <martin@mrvanes.com>
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Martin 2021-07-15 19:24:02 +02:00 committed by GitHub
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@ -154,3 +154,39 @@ gpiochip1 - 8 lines:
line 6: unnamed unused input active-high line 6: unnamed unused input active-high
line 7: unnamed unused input active-high line 7: unnamed unused input active-high
``` ```
Optional: Hardware PWM
=======================================
Raspberry Pi's have two PWM channels (PWM0 and PWM1) which are exposed on the header or if not, can be routed to existing gpio pins.
The Linux mcu daemon uses the pwmchip sysfs interface to control hardware pwm devices on Linux hosts.
The pwm sysfs interface is not exposed by default on a Raspberry and can be activated by adding a line to ```/boot/config.txt```:
```
# Enable pwmchip sysfs interface
dtoverlay=pwm,pin=12,func=4
```
This example enables only PWM0 and routes it to gpio12. If both PWM channels need to be enabled you can use ```pwm-2chan```.
The overlay does not expose the pwm line on sysfs on boot and needs to be exported by echo'ing the number of the pwm channel to ```/sys/class/pwm/pwmchip0/export```:
```
echo 0 > /sys/class/pwm/pwmchip0/export
```
This will create device ```/sys/class/pwm/pwmchip0/pwm0``` in the filesystem.
The easiest way to do this is by adding this to ```/etc/rc.local``` before the ```exit 0``` line.
With the sysfs in place, you can now use either the pwm channel(s) by adding the following piece of configuration to your ```printer.cfg```:
```
[output_pin caselight]
pin: host:pwmchip0/pwm0
pwm: True
hardware_pwm: True
cycle_time: 0.000001
```
This will add hardware pwm control to gpio12 on the Pi (because the overlay was configured to route pwm0 to pin=12).
PWM0 can be routed to gpio12 and gpio18, PWM1 can be routed to gpio13 and gpio19:
|PWM|gpio PIN|Func|
|---|--------|----|
| 0| 12| 4|
| 0| 18| 2|
| 1| 13| 4|
| 1| 19| 2|