Serial Port Communication

1. Serial Port Pins

Serial port pin mapping (taking M5-R1 as an example):

Serial	Pin	Function
TXD	8	Transmit signal line
RXD	10	Receive signal line

Corresponds to UART3 on the 40-pin header of the board.

2. Checking the Serial Port Device

Check whether the serial port device has been created.

    # Run this command to list the serial devices
    ls /dev/ttyS*

As shown, ttyS3 is UART3 and ttyS8 is UART8 (this port is occupied by the Bluetooth module).

3. Serial Communication Test

Using M5-R1 as an example, the serial port is tested by connecting to a PC through a USB-to-serial adapter.

3.1 Connecting the Serial Port

Connect the TX and RX of the USB-to-serial adapter to UART6_RX and UART6_TX on the 40-pin header, respectively.

The USB-to-TTL serial adapter is shown below:

• TXD -- RXD
• RXD -- TXD
• GND -- GND

3.2 Querying Serial Parameters and Changing the Baud Rate

Use the stty tool to query serial parameters.

    # Run the following command on the board terminal
    root@rk3588-buildroot:/#  stty -F /dev/ttyS6
    speed 9600 baud; line = 0;
    -brkint -imaxbel
    root@rk3588-buildroot:/#

Use the stty tool to change serial parameters.

    # Set the communication rate; ispeed is the input rate and ospeed is the output rate
    root@rk3588-buildroot:/# stty -F /dev/ttyS6 ispeed 115200 ospeed 115200
    root@rk3588-buildroot:/# stty -F /dev/ttyS6
    speed 115200 baud; line = 0;
    -brkint -imaxbel
    root@rk3588-buildroot:/#

Warning

The baud rate must be reconfigured after every boot; rebooting resets the baud rate to 9600 by default.

3.3 Communicating with a PC

After configuring the serial debug assistant on the PC, use the following commands on the board to test sending data over the serial port:

    # Run the following commands on the board terminal
    # Use the echo command to write the strings "Hello!" and "Linux6_1!" to the terminal device file
    echo Hello! > /dev/ttyS6
    echo "Linux6_1" > /dev/ttyS6
    # The serial debug assistant on the PC receives the content

To test receiving data, use the microcom tool:

    # Run the following commands on the board terminal
    # Use the microcom command to read from the terminal device file; the -s option sets the baud rate
    microcom -s 115200 /dev/ttyS6
    # microcom now waits
    # Send a string from the serial debug assistant
    # The board terminal prints the received content