# RPi microcontroller

This document describes the process of running Klipper on a RPi
and use the same RPi as secondary mcu.

## Why use RPi as a secondary MCU?

Often the MCUs dedicated to controlling 3D printers have a limited and
pre-configured number of exposed pins to manage the main printing
functions (thermal resistors, extruders, steppers ...).
Using the RPi where Klipper is installed as a secondary MCU gives the
possibility to directly use the GPIOs and the buses (i2c, spi) of the RPi
inside klipper without using Octoprint plugins (if used) or external
programs giving the ability to control everything within the print GCODE.

**Warning**: If your platform is a _Beaglebone_ and you have correctly followed the installation steps, the linux mcu is already installed and configured for your system.

## Install the rc script

If you want to use the host as a secondary MCU the klipper_mcu process must run before the klippy process.

After installing Klipper, install the script. run:
```
cd ~/klipper/
sudo cp "./scripts/klipper-mcu-start.sh" /etc/init.d/klipper_mcu
sudo update-rc.d klipper_mcu defaults
```

## Enabling SPI

Make sure the Linux SPI driver is enabled by running sudo raspi-config and enabling SPI under the "Interfacing options" menu.

## Building the micro-controller code

To compile the Klipper micro-controller code, start by configuring it
for the "Linux process":
```
cd ~/klipper/
make menuconfig
```
In the menu, set "Microcontroller Architecture" to "Linux process," then save and exit.

To build and install the new micro-controller code, run:
```
sudo service klipper stop
make flash
sudo service klipper start
```

If klippy.log reports a "Permission denied" error when attempting to connect
to `/tmp/klipper_host_mcu` then you need to add your user to the tty group.
The following command will add the "pi" user to the tty group:
```
sudo usermod -a -G tty pi
```

## Remaining configuration

Complete the installation by configuring Klipper secondary MCU
following the instructions in
[RaspberryPi sample config](../config/sample-raspberry-pi.cfg) and
[Multi MCU sample config](../config/sample-multi-mcu.cfg).

## Optional: Identify the correct gpiochip

On Rasperry and on many clones the pins exposed on the GPIO belong to the first gpiochip. They can therefore be used on klipper simply by referring them with the name `gpio0..n`.
However, there are cases in which the exposed pins belong to gpiochips other than the first. For example in the case of some OrangePi models or if a Port Expander is used. In these cases it is useful to use the commands to access the _Linux GPIO character device_ to verify the configuration.

To install the _Linux GPIO character device - binary_ on a debian based distro like octopi run:
```
sudo apt-get install gpiod
```

To check available gpiochip run:
```
gpiodetect
```

To check the pin number and the pin availability tun:
```
gpioinfo
```

The chosen pin can thus be used within the configuration as `gpiochip<n>/gpio<o>` where **n** is the chip number as seen by the `gpiodetect` command and **o** is the line number seen by the` gpioinfo` command.

***Warning:*** only gpio marked as `unused` can be used. It is not possible for a _line_ to be used by multiple processes simultaneously.

For example on a RPi 3B+ where klipper use the GPIO20 for a switch:
```
$ gpiodetect
gpiochip0 [pinctrl-bcm2835] (54 lines)
gpiochip1 [raspberrypi-exp-gpio] (8 lines)

$ gpioinfo
gpiochip0 - 54 lines:
        line   0:      unnamed       unused   input  active-high
        line   1:      unnamed       unused   input  active-high
        line   2:      unnamed       unused   input  active-high
        line   3:      unnamed       unused   input  active-high
        line   4:      unnamed       unused   input  active-high
        line   5:      unnamed       unused   input  active-high
        line   6:      unnamed       unused   input  active-high
        line   7:      unnamed       unused   input  active-high
        line   8:      unnamed       unused   input  active-high
        line   9:      unnamed       unused   input  active-high
        line  10:      unnamed       unused   input  active-high
        line  11:      unnamed       unused   input  active-high
        line  12:      unnamed       unused   input  active-high
        line  13:      unnamed       unused   input  active-high
        line  14:      unnamed       unused   input  active-high
        line  15:      unnamed       unused   input  active-high
        line  16:      unnamed       unused   input  active-high
        line  17:      unnamed       unused   input  active-high
        line  18:      unnamed       unused   input  active-high
        line  19:      unnamed       unused   input  active-high
        line  20:      unnamed    "klipper"  output  active-high [used]
        line  21:      unnamed       unused   input  active-high
        line  22:      unnamed       unused   input  active-high
        line  23:      unnamed       unused   input  active-high
        line  24:      unnamed       unused   input  active-high
        line  25:      unnamed       unused   input  active-high
        line  26:      unnamed       unused   input  active-high
        line  27:      unnamed       unused   input  active-high
        line  28:      unnamed       unused   input  active-high
        line  29:      unnamed       "led0"  output  active-high [used]
        line  30:      unnamed       unused   input  active-high
        line  31:      unnamed       unused   input  active-high
        line  32:      unnamed       unused   input  active-high
        line  33:      unnamed       unused   input  active-high
        line  34:      unnamed       unused   input  active-high
        line  35:      unnamed       unused   input  active-high
        line  36:      unnamed       unused   input  active-high
        line  37:      unnamed       unused   input  active-high
        line  38:      unnamed       unused   input  active-high
        line  39:      unnamed       unused   input  active-high
        line  40:      unnamed       unused   input  active-high
        line  41:      unnamed       unused   input  active-high
        line  42:      unnamed       unused   input  active-high
        line  43:      unnamed       unused   input  active-high
        line  44:      unnamed       unused   input  active-high
        line  45:      unnamed       unused   input  active-high
        line  46:      unnamed       unused   input  active-high
        line  47:      unnamed       unused   input  active-high
        line  48:      unnamed       unused   input  active-high
        line  49:      unnamed       unused   input  active-high
        line  50:      unnamed       unused   input  active-high
        line  51:      unnamed       unused   input  active-high
        line  52:      unnamed       unused   input  active-high
        line  53:      unnamed       unused   input  active-high
gpiochip1 - 8 lines:
        line   0:      unnamed       unused   input  active-high
        line   1:      unnamed       unused   input  active-high
        line   2:      unnamed       "led1"  output   active-low [used]
        line   3:      unnamed       unused   input  active-high
        line   4:      unnamed       unused   input  active-high
        line   5:      unnamed       unused   input  active-high
        line   6:      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|
