In this lesson, you will:
The information provided in this guide is based in Toradex' knowledge base article UART (Linux).
The Colibri Evaluation Board offers 3 UART interfaces:
|Serial-to-USB converter||RS232 (DB9 connector)||Pin header|
Note: Changing the position of jumpers JP17, JP19, JP20 and JP21 routes the UART_A signals from connector X27 to connector X25 (bottom).
For this tutorial, we will use the UART_B interface. You will need:
Note: Depending on the module used, the UART interfaces are indexed with numbers (UART1, UART2, etc.) or letters (UARTA, UARTB, etc.). For this tutorial, we'll stick with the nomenclature used by the carrier board, which indexes the UART peripherals using letters (UART_A, UART_B, etc.).
UART stands for Universal Asynchronous Receiver-Transmitter. These kind of interfaces are used to achieve asynchronous serial communication between devices. The transmission speed and the data format are configurable. There are 4 main pins that may be used on a UART interface:
Note: The RTS and CTS signals are used to achieve hardware flow control. We will not delve into this topic in this tutorial. Therefore, we will only use the RX and TX pins.
The following block diagram shows some ways in which UART can be accessed:
The following table contains the UART interfaces available by default along with their designators on the module, the board and Linux device.
|Module||Base Board||Linux device|
Note: Please refer to the FAQ for more information on how to use UARTs that are disabled in the default BSP configuration.
The correspondence between UART interface signals and SoM/Carrier Board pins is provided in the following table:
Note: The notation CONNECTOR.PIN will be employed in this lesson, e.g. X12.5 means pin 5 on the X12 connector.
A loopback test will be run from command-line. This will send and receive data from the same serial port to verify if it's working.
Connect your board's UART_B TX pin (X12.17) to its RX pin (X12.16).
From a serial terminal connected to the module, configure the UART_B baud rate using the stty command. We will use a baud rate of 9600 baud:
stty -F /dev/ttyLP1 9600 -echo
Use the cat command to listen for incoming data on the serial port:
cat < /dev/ttyLP1 &
Write to the serial port. The characters sent will be printed back to you in the next line:
echo "Testing UART" > /dev/ttyLP1Testing UART
An UART loopback test can also be written in C.
Termios, the Unix API for terminal I/O will be employed.
An example will be introduced step-by-step. First of all, include some libraries and initialize variables:
Warning: The source codes provided in this guide are distributed under the 3-clause BSD license terms. See below:
To use the serial port, open the serial device using the standard system call open(), passing the device path and the flags as arguments.
Using some functions of the termios library, get the serial port attributes and configure the communication parameters and other properties. This example uses the fairly common 9600/8N1 configuration, standing for a baud rate of 9600 baud, 8 data bits, no parity bit and 1 stop bit.
Now you can use the standard system calls write() and read() to write to and read from the serial port.
First, flush the receiving buffer to discard old data. Then, store a string into the buffer variable and write it to the serial port.
After writing to the serial port, you can read from it. Since it's a loopback, you will read what you just wrote and save it to the buffer variable. Also, don't forget to use the standard system call close() to close the device when you're done.
Test the complete code. Create and configure an Eclipse project as described in Module 2.
Build and run the following code:
We can improve upon the code by adding error checks:
The following example demonstrates communication between two UART ports on your board. It shows how to use more than one serial port at a time.
For this example, we will call UART_B "first_uart" and UART_C "second_uart".
Connect UART_B_RXD (X12.16) to UART_C_TXD (X12.3) and UART_B_TXD (X12.17) to UART_C_RXD (X12.2).
Build and run the following code:
This lesson covers the basics of UART usage on Linux. There are other important topics that were not discussed. This FAQ section provides further information and sources.