Enable Uart Communication on Pi4 Ubuntu 20.04

I had a bunch of trouble talking to a roboclaw motor on an rpi4 (model B) under Ubuntu 20.04 (64 bit). I was able to resolve it, though I had to disable Bluetooth to get there. Unfortunately, I'm not sure at this point how to get there without disabling Bluetooth. Reader beware.

Confirmed this works with 5 roboclaw motor controllers on the same serial bus.

That said, here's how I did it (note that this is cliffsnotes of this procedure):

1. Disable

   sudo systemctl stop 
   sudo systemctl disable 
   sudo systemctl mask 
   

2. Setup udev rules
   • put below content in new file /etc/udev/rules.d/10-local.rules

     KERNEL=="ttyS0", SYMLINK+="serial0" GROUP="tty" MODE="0660"
     KERNEL=="ttyAMA0", SYMLINK+="serial1" GROUP="tty" MODE="0660"
     

   • Reload udev rules: sudo udevadm control --reload-rules && sudo udevadm trigger
3. Add user to tty group: sudo adduser ubuntu tty
4. Delete substring console=serial0,115200 from /boot/firmware/cmdline.txt
5. Add newline dtoverlay=disable-bt to /boot/firmware/config.txt (I put it right under the cmdline=cmdline.txt line)
6. Restart
7. (Optional) Feel free to test with this script:

Note that after this set up, you need to use ttyAMA0 (or alias serial1), not ttyS0. The first uses the full hardware UART, the second uses a "mini-UART" implementation that seems to not work so well under Ubuntu 20.04 (at least with the roboclaw, in my hardware setup. YMMV.). Some more discussion here on GitHub.

I have a setup using both Bluetooth/hci0 and the miniUART/ttyS0 at the same time with Ubuntu 20.04 (Ubuntu 5.4.0-1034.37-raspi 5.4.101) 64bit on Raspberry Pi 4 Model B Rev 1.1 (4G memory). Here is what I did:

1. Back up the original config.txt and cmdline.txt files

   sudo cp -pr /boot/firmware/cmdline.txt /boot/firmware/cmdline.txt-orig
   sudo cp -pr /boot/firmware/config.txt /boot/firmware/config.txt-orig
   

2. Edit /boot/firmware/config.txt to comment out the enable_uart=1 like below,

   #enable_uart=1
   
   cmdline=cmdline.txt
   

3. Remove the console setting console=serial0,115200 from /boot/firmware/cmdline.txt

4. Disable the Serial Service which used the miniUART

   sudo systemctl stop 
   sudo systemctl disable 
   sudo systemctl mask 
   

5. Add the user which will use the miniUART to tty and dialout group

   sudo adduser ${USER} tty
   sudo adduser ${USER} dialout
   

6. Finally, reboot Ubuntu 20.04, then both hci0 and /dev/ttyS0 can work at the same time for me.

Detailed documentation about the UART can be found at

exactly the same problem I also have. A workaround for the wrong path of the raspi-config script may be a softlink like

ln -s /boot/firmware/cmdline.txt /boot/cmdline.txt

But this doesn't solve the origin problem. I'm sorry.

A simple solution is to use a USB to TTL Adapter to connect the roboclaw to a USB on the pi rather than UART GPIO pins. This will work out of the box without the need for config changes on the pi other than specifying the USB port in the roboclaw driver code. In my experience it is best to use a SiLabs CP2012 based USB to TTL adaptor (as for example sold by adafruit or available on amazon ) as that chipset is widely supported on linux and other OS'es and certainly on Ubuntu does not need custom drivers.