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 and Multi MCU sample config.

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