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Basic UART usage - Apalis Evaluation Board - Apalis TK1

 

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Getting Started with UART

Overview

In this lesson, you will:

  • Learn the basics of using UART on Linux
  • Read and write from UART using the command line
  • Read and write from UART using an application written in C

The information provided in this guide is based in Toradex's knowledge base article UART (Linux).

Materials Required

The Apalis Evaluation Board offers 4 UART interfaces:

Serial-to-USB converter RS232 (DB9 connector) Pin header Notes
UART1 X? X28 (Bottom) X11/X12
UART2 X29 X28 (Top) X11/X12
UART3 - - X11/X12 X30 - IrDA
UART4 - - X38 - Mezzanine

Attention: Never connect a RS-232 output pin to a TTL input. This might lead to permanent damage.

For this tutorial, we will use the UART2 interface. You will need:

  • Jumper wires
  • USB Type-B to Type-A cable

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 numbers (UART1, UART2, etc.).

Step 1

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:

  • RX, the pin that will receive data from another device
  • TX, the pin that will transmit data to another device
  • RTS, request-to-send
  • CTS, clear-to-send

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.

Step 2

The following table contains the available UART interfaces along with their designators on the module, the board and Linux device.

Module Base Board Linux device Enabled
UART1 UART1 /dev/ttyS0
UART2 UART2 /dev/ttyTHS1
UART3 UART3 /dev/ttyTHS2
UART4 UART4 /dev/ttyTHS3

Note: Please refer to the FAQ for more information on how to use UARTs that are disabled in the default BSP configuration.

Step 3

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.

UART signals and pins

Step 4

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 UART2 TX pin (X28.U3) to its RX pin (X28.U2).


  • Apalis UART wiring

    Apalis UART wiring

Step 5

From a serial terminal connected to the module, configure the UART 2 baud rate using the stty command. We will use a baud rate of 9600 baud:

stty -F /dev/ttyTHS1 9600 -echo

Use the cat command to listen for incoming data on the serial port:

cat < /dev/ttyTHS1 &

Write to the serial port. The characters sent will be printed back to you in the next line:

echo "Testing UART" > /dev/ttyTHS1
Testing UART

Step 6

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:

License
Loopback example - Initializing

Step 7

To use the serial port, open the serial device using the standard system call open(), passing the device path and the flags as arguments.

Loopback example - Opening device

Step 8

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.

Loopback example - Configuring device

Step 9

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.

Loopback example - Reading and writing

Step 10

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.

Loopback example - Closing device

Step 11

Test the complete code. Create and configure an Eclipse project as described in Module 2.

Build and run the following code:

Loopback test code

Step 12

We can improve upon the code by adding error checks:

Improved loopback test

Step 13

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 UART2 "first_uart" and UART3 "second_uart".

Connect UART2_RXD (X20.10) to UART3_TXD (X3.B38) and UART2_TXD (X20.8) to UART3_RXD (X3.B39).

Build and run the following code:

Example communicating between UARTs on the same board

FAQ

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.

How can use another UART interface instead of UART2
Can UART1 be used for general purpose
How can I disable DMA for RX