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
```
Building the micro-controller code
==================================
To compile the Klipper micro-controller code, start by configuring it
for the "Linux process":
```
cd ~/klipper/
make menuconfig
```
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
```