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UART (Serial Communication)

Introduction​

The serial port is a widely used communication interface. Many industrial control products and wireless transparent transmission modules use serial ports to send/receive commands and transfer data, allowing users to flexibly use various serial module functions without worrying about the underlying implementation. You can also communicate with other development boards via serial ports to exchange data, such as STM32, ESP32, Arduino, etc.

Experiment Objective​

Program serial port data transmission and reception.

Experiment Explanation​

The Walnut Pi GPIO header provides multiple serial ports. This tutorial uses UART2, i.e., PB0--TX2, PB1--RX2. uart1

Enable UART2​

Enter the following command in the terminal:

sudo set-device enable uart2

Restart the development board:

sudo reboot

Check the status after startup:

gpio pins

The following output indicates successful activation: uart1

For more GPIO configuration tutorials, see: GPIO Device Configuration

Serial Object​

Walnut Pi serial communication can use the Serial standard library built into the Linux system. Details are as follows:

Constructor​

serial.Serial("dev",baudrate)

Build a UART object

  • "dev" : Device number. For Walnut Pi, uart2 is "/dev/ttyS2";
  • baudrate : Serial baud rate, can be set to commonly used values like 9600, 115200, etc.

Usage​

Serial.inWaiting()

Returns the number of characters received and stored in the buffer, int type. Can be used to check if data has been received.


Serial.read(num)

Read data, returns byte string.

  • num : Number of characters to read.

Serial.write(b'str')

Send data, requires byte string format.

  • b'str' : Content to send.

For more Python usage of Serial, see the official documentation: https://pyserial.readthedocs.io/en/latest/pyserial_api.html#module-serial

After understanding the UART object usage, we can use a USB-to-TTL tool along with a computer serial assistant (terminal software) to communicate with the Walnut Pi via serial port. These tools are generally similar. Note: if the tool has 3.3V and 5V level switching, the jumper cap should be set to 3.3V, because the Walnut Pi GPIO level is 3.3V.

uart2

This experiment uses UART2, i.e., TX2(PI5) and RX2(PI6). The wiring diagram is as follows: (3.3V can be left unconnected)

uart3

In this experiment, we first initialize the serial port, then send a message so that the PC serial assistant displays it in the receive area. Then enter a loop: when the Walnut Pi detects receivable data, it reads and prints the data via the terminal. The code flow chart is as follows:

Reference Code​

'''
Experiment Name: UART (Serial Communication)
Experiment Platform: Walnut Pi
'''

# Import related modules
import serial,time

# Configure serial port
com = serial.Serial("/dev/ttyS2", 115200)

# Send prompt characters
com.write(b'Hello WalnutPi!')

while True:

    # Get the number of characters in the receive buffer, int
    count = com.inWaiting()
    
    if count != 0: # Data received
        
        # Read content and print
        recv = com.read(count)
        print(recv)
        
        # Send data back
        com.write(recv)
        
        # Clear receive buffer
        com.flushInput()
        
    # Delay 100ms, receive interval
    time.sleep(0.1)

Experiment Results​

Enter the following command in the terminal to confirm UART2 activation:

gpio pins

The following output indicates successful activation:

uart1

If not enabled, follow the steps above to enable: Enable UART2

Use a USB-to-TTL tool to connect the Walnut Pi and the computer.

uart5

On the computer, open the serial assistant, select the COM port corresponding to the USB-to-TTL, baud rate 115200. Click open and wait to receive data:

uart6

Here we use Thonny to remotely run the above Python code on the Walnut Pi. For instructions on running Python code on the Walnut Pi, please refer to: Running Python Code

uart4

After running, you can see the computer serial assistant receiving the message:

uart7

Enter a message in the serial assistant's send field and click send. You can see the received data printed in the Thonny terminal below (data received by the Walnut Pi development board):

uart8

Serial data communication is widely used. Besides communicating with a computer as in this example, it can also communicate with other microcontroller development boards or serial module devices.