RPi4 Serial port not working on either Raspberry OS or Ubuntu

Question

I've been struggling to get my RPi4 serial working over pins 8,10.

I have both a Raspberry OS image and an Ubuntu 18.04 image for the RPi4. In either case, when I use RASPI-CONFIG to disable console and enable uart, I'm still unable to use minicom to see characters or to receive data over serial.

dmesg | grep tty
console [tty1] enabled
serial: ttyAMA0 at MMIO is a PL011 rev2 
serial: ttySo at MMIO is a 16550

ls -l /dev/serial*
/dev/serial0 -> ttyS0
/dev/serial1 -> ttyAMA0

setserial -g /dev/tty*
/dev/ttyAMA0, UART undefined, Port: 0x0000, IRQ: 19
/dev/ttyS0, UART: 16550, Port 0x0000, IRQ: 21

sudo minicom -o -D /dev/serial0 or 1
no characters reflected back with loopback in place

enable_uart=1 is in config.txt

I'm getting the exact same results on both Ubuntu 18.04 and Raspberry OS. Neither enables the serial port to work. What else could I try or am I missing?

• MatsK: If you loop TX with RX on the same device you can see the char that you send is also received on the same device. This is a common trouble shooting pratice for serial devices.
• CoderMike: What are you actually trying to connect to via serial?
• BenignFun: Using a loopback wire for the test. Ultimately connecting to Roboclaw motor driver via serial. It does not indicate it needs a level shifter but I'm now wondering if my past experimentation with the connection to the Roboclaw put 5v on Rx somwhow.

Answer 1

Here's some things I needed to do specifically to connect to a device like the Roboclaw. I got this to work on Ubuntu20.04, Ubuntu18.04 and raspberry pi OS (raspbian)

• add enable_uart=1 to /boot/config.txt
• remove console=serial0,115200 from /boot/firmware/cmdline.txt on Ubuntu and /boot/cmdline.txt on Raspberry Pi OS
• disable the serial console: sudo systemctl stop [email protected] && sudo systemctl disable [email protected]
• make sure you have pyserial installed if you're using the python serial library, not python-serial from apt.
• create the following udev file:

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

and reload your udev rules: sudo udevadm control --reload-rules && sudo udevadm trigger

• change the group of the new serial devices

sudo chgrp -h tty /dev/serial0
sudo chgrp -h tty /dev/serial1

• The devices are now under the tty group. Need to add the user to the tty group and dialout group:

sudo adduser $USER tty
sudo adduser $USER dialout

• update the permissions for group read on the devices

sudo chmod g+r /dev/ttyS0
sudo chmod g+r /dev/ttyAMA0

• reboot

• Serhii Korol: Thanks! You saved my day. I had issues with the serial port probably conflicting with BT on RPi Zero 2W and RPi 3 A+ with Ubuntu Server 20.04. Now it works as expected!

Answer 2

I found how to fix UARTs of raspberry by using another UARTs of raspberry pi 4 because rpi4 has 6 UARTs port .

Example :

• UART0 , UART1 , UART2 , UART3 , UART4 , UART5

My Devices :

• RPI : Raspberry Pi 4 Model B (MASTER)
• OS : Ubuntu Mate 20.04
• SLAVE : Teensy 4.1 (Communicate with raspberry pi 4)

References

• Raspberry Pi Pinout
• Raspberry Pi UARTs Configuration

You can enable UARTs on RPi4 by editing files /boot/firmware/config.txt and add

dtoverlay=uartx

x is the number of your UARTs

or

Use can use my python script to enable UARTs and testing serial communication

• https://github.com/NonStopBle/PiAttachSerial

Question (What is path of UARTs):

• /dev/ttyACM0 - /dev/ttyACM5