tquadrat.org

Raspberry Pi as a USB Device

Raspberry Pi Zero as a USB Keyboard

You can find different descriptions on how to configure a Raspberry Pi Zero as a virtual keyboard:

These are the steps that I followed:

  1. Install the latest version of the Raspberry Pi OS; this is written for the Bookworm releasae.
  2. Enable the Drivers and modules
    1. Add the line 
      dtoverlay=dwc2

      to the end of /boot/firmware/config.txt.

    2. Add the lines 
      dwc2
libcomposite

      to the end of /etc/modules.

  3. Configuring the Gadget

    Now, the Raspberry&ndsb;Pi&ndsb;Zero needs to be defined as a USB keyboard. The configuration is done via ConfigFS, a virtual file system located in /sys/.

    1. Creating the config script

      The configuration is volatile, so it must run on each startup. Create a new file called virtual_usb in /usr/bin/ and make it executable:

      sudo touch /usr/bin/virtual_usb
sudo chmod +x /usr/bin/virtual_usb

    2. Run the script on each startup

      That this script is executed automatically at startup will be achieved by adding the line below to /etc/rc.local before the last line (that one reading exit 0):

      /usr/bin/virtual_usb # libcomposite configuration

    3. Creating the gadget

      For this project, the Raspberry&ndsb;Pi&ndsb;Zero is turned into a USB keyboard (but it could be made to work as a Serial adapter, an Ethernet adapter or Mass Storage).

      Open the scirpt file with:

      joe /usr/bin/virtual_usb

      You can leave the default values as they are, but you could even change the serial number, manufacturer and product name to fit your specific needs.

      #!/bin/bash
cd /sys/kernel/config/usb_gadget/
mkdir -p virtual_usb
cd virtual_usb
echo 0x1d6b > idVendor # Linux Foundation
echo 0x0104 > idProduct # Multifunction Composite Gadget
echo 0x0100 > bcdDevice # v1.0.0
echo 0x0200 > bcdUSB # USB2
mkdir -p strings/0x409
echo "fedcba9876543210" > strings/0x409/serialnumber
echo "tquadrat.org" > strings/0x409/manufacturer
echo "Virtual USB Device" > strings/0x409/product
mkdir -p configs/c.1/strings/0x409
echo "Config 1: ECM network" > configs/c.1/strings/0x409/configuration
echo 250 > configs/c.1/MaxPower

# Add functions here
mkdir -p functions/hid.usb0
echo 1 > functions/hid.usb0/protocol
echo 1 > functions/hid.usb0/subclass
echo 8 > functions/hid.usb0/report_length
echo -ne \\x05\\x01\\x09\\x06\\xa1\\x01\\x05\\x07\\x19\\xe0\\x29\\xe7\\x15\\x00\\x25\\x01\\x75\\x01\\x95\\x08\\x81\\x02\\x95\\x01\\x75\\x08\\x81\\x03\\x95\\x05\\x75\\x01\\x05\\x08\\x19\\x01\\x29\\x05\\x91\\x02\\x95\\x01\\x75\\x03\\x91\\x03\\x95\\x06\\x75\\x08\\x15\\x00\\x25\\x65\\x05\\x07\\x19\\x00\\x29\\x65\\x81\\x00\\xc0 > functions/hid.usb0/report_desc
ln -s functions/hid.usb0 configs/c.1/
# End functions

ls /sys/class/udc > UDC
chmod 666 /dev/hidg0

  4. Connect the Raspberry&ndsb;Pi&ndsb;Zero to the target computer

    After the Raspberry&ndsb;Pi&ndsb;Zero is now prepared, shut it down and connect it to the target computer through the micro USB port that is used for data and peripherals. That micro USB will both power the Pi Zero and act as a keyboard to the connected computer.

  5. Test the new virtual keyboard

    Create this Python script RPi_Keyboard_Example.py somewhere:

    #!/usr/bin/env python3
NULL_CHAR = chr( 0 )

def write_report( report ):
    with open( '/dev/hidg0', 'rb+' ) as fd:
        fd.write( report.encode() )

# Press a
write_report( NULL_CHAR * 2 + chr( 4 ) + NULL _ CHAR * 5 )
        
# Release keys
write_report( NULL_CHAR * 8 )

# Press SHIFT + a = A
write_report( chr( 32 ) + NULL_CHAR + chr( 4 ) + NULL_CHAR * 5 )

# Press b
write_report( NULL_CHAR * 2 + chr( 5 ) + NULL_CHAR * 5 )

# Release keys
write_report( NULL_CHAR * 8 )

# Press SHIFT + b = B
write_report( chr( 32 ) + NULL_CHAR + chr( 5 ) + NULL_CHAR * 5 )

# Press SPACE key
write_report( NULL_CHAR * 2 + chr( 44 ) + NULL_CHAR * 5 )

# Press c key
write_report( NULL_CHAR * 2 + chr( 6 ) + NULL_CHAR * 5 )

# Press d key
write_report( NULL_CHAR * 2 + chr( 7 ) + NULL_CHAR * 5 )

# Press RETURN/ENTER key
write_report( NULL_CHAR * 2 + chr( 40 ) + NULL_CHAR * 5 )

# Press e key
write_report( NULL_CHAR * 2 + chr( 8 ) + NULL_CHAR * 5 )

# Press f key
write_report( NULL_CHAR * 2 + chr( 9 ) + NULL_CHAR * 5 )

# Release all keys
write_report( NULL_CHAR * 8 )

    To start that script, type python3 RPi_Keyboard_Example.py. When you have opened a text editor on the target computer and placed the cursor in there, the sequence

    aAbB cd
ef

    will by “typed” into it.

To drive it further

A program that emulates a keyboard would need the codes for the keys. I have collected some links to lists with these data: