How to Configure UART & Transmit Data

Learn how to configure STM32 UART in CubeMX, set 115200 baud, 8N1 frame, and use HAL_UART_Transmit to send strings & numbers to a serial console. The project is available for download at the end of the post.

This tutorial is the start of a new series on the UART peripheral of STM32 Microcontrollers. In this series we will cover different ways of transmitting and receiving data over the UART protocol. We will also see different UART modes available in the STM32 microcontrollers and how to use them.

In this tutorial, we will understand the connection and configuration of different parameters of UART available in the CubeMX. We will also transmit the data over UART and receive it on a serial console in the computer.

Table Of Contents

Introducing STM32 UART
UART vs USART
CONNECTION
CUBEMX CONFIGURATION
THE CODE
RESULT
VIDEO TUTORIAL
PROJECT DOWNLOAD

Introducing STM32 UART

The UART (Universal Asynchronous Receiver/Transmitter) peripheral in STM32 microcontrollers provides a simple and efficient way to perform serial communication. It allows data exchange between the microcontroller and other devices such as PCs, GPS modules, Bluetooth modules, or other microcontrollers using standard asynchronous protocols.

UART communication is asynchronous, meaning it doesn’t require a separate clock line—only TX (Transmit) and RX (Receive) lines are used. STM32 MCUs often provide multiple UART/USART interfaces, and they can be configured using STM32CubeMX and HAL libraries to suit different baud rates, word lengths, parity, and stop bits.

STM32 supports both UART and USART peripherals, where USART adds synchronous mode (with a clock line) in addition to asynchronous UART mode.

Features of STM32 UART:

Configurable Baud Rate
Supports a wide range of baud rates (e.g., 9600 to 1+ Mbps), allowing flexible communication speed based on the application.
Full-Duplex Communication
Simultaneously sends and receives data through separate TX and RX lines.
Interrupt and DMA Support
UART can be operated in polling, interrupt-driven, or DMA mode for efficient, low-latency data handling.
Flexible Frame Format
Supports custom configurations like 8/9 data bits, 1/2 stop bits, and optional parity for error checking.

UART vs USART

UART stands for Universal Asynchronous Receiver Transmitter whereas the USART stands for Universal Synchronous Asynchronous Receiver Transmitter. The term Synchronous enables the USART to send an additional clock signal to the receiving device. The data is then sampled at a predefined edge (Rising or Falling) of the clock. The USART mode uses 3 pins (clock, Tx and Rx) compared to the 2 pins (Tx and Rx) used in the UART.

The most widely used synchronous communications are I2C, SPI, etc. The USART, in a way, can act as the SPI master to send the data to the slave. The synchronous communication is also used when communicating with a smartcard.

Here is a comparison table between UART and USART based on the information you provided:

Feature	UART	USART
Full Form	Universal Asynchronous Receiver Transmitter	Universal Synchronous/Asynchronous Receiver Transmitter
Communication Type	Asynchronous only	Supports both Synchronous and Asynchronous
Clock Signal	Not used	Uses an additional clock line in synchronous mode
Number of Pins Used	2 (TX and RX)	3 in synchronous mode (TX, RX, and CLK)
Data Sampling	Based on start and stop bits (internal clock)	Data is sampled on the rising or falling edge of external clock
Application Example	Basic serial communication with PCs, Bluetooth modules	Smartcard communication, SPI-like data transfers
Acts Like SPI Master?	❌ No	✅ Yes, in synchronous mode
Use of Start/Stop Bits	Required for data framing	Optional in synchronous mode

We will use the UART for the major part of this series as it has wider applications compared to the USART.

CONNECTION

We will use the STM32 MCU to send the data to the computer. Some of the Nucleo and Discovery dev boards from ST supports the virtual com port. This feature enables the USB connected for the ST-link to be also used for the data transmission between the MCU and the computer.

The Virtual Com Port is supported by many Nucleo and Discovery boards but not all. You need to check the schematic of the board to confirm whether the respective board supports it.

Below are the images from the schematic of the Nucleo F446RE and Discovery F412.

Discovery F412 Schematic — Discovery F412

As you can see in the images above, both Nucleo F446RE and Discovery F412 supports the USB Virtual Com Port. So if you are using either of these boards, you do not need to use an additional module to communicate to the computer. The USB used for the ST link can also be used for the communication.

Not all the boards support this Virtual Com port feature. Below is the image from the schematic of the very famous STM32F4 Discovery board.

STM32F4 Discovery Schematic — STM32F4 Discovery

As you can see in the image above, there is no virtual com port in the F4 Discovery board. In such cases we can use some module to convert the UART signals to the USB, which is connected to the computer.

Below is the image showing the connection between the MCU and the FT232 USB to UART converter.

The UART is always connected in the cross connection, connecting the TX pin of the MCU to the RX of the device and the RX to the TX of the device. The module then connects to the computer using the USB.

CUBEMX CONFIGURATION

Below is the image showing the UART configuration in CubeMX.

We will use the Asynchronous Mode for the communication. Only 2 pins are used in the Asynchronous mode, TX and RX. The baud rate is set to 115200. We need to use the same baud rate for the receiver device also.

Word length consists of the data bits and the parity bit. STM32 supports different word lengths of 7 bits, 8 bits and 9 bits. A typical UART frame is shown in the image below.

The frame consists of a start bit, data bits and stop bits. we are using 8 data bits with no parity bit and 1 stop bit. We need to use the same configuration in the receiver device also. The data direction is set to Receive and Transmit.

The oversampling is used to increase the tolerance of the receiver to the clock deviation. But it also reduces the maximum baud rate the device can achieve. The image below explains the oversampling and max baud rate.

This is all the configuration we need to do in the cubeMX. Let’s see the code now.

THE CODE

We can only send the ascii characters via the UART. Below we will see how to send a simple string and a number, by converting it to the character format.

Send string

We can directly send a string via the UART. The code below sends the string every 1 second.

uint8_t data[] = "Hello world\n"; 
int main()
{
  ....
   while (1)
   {
     HAL_UART_Transmit(&huart2, data, 12, 1000); HAL_Delay(1000);
   }
}

uint8_t data[] = "Hello world\n"; 
int main()
{
  ....
   while (1)
   {
     HAL_UART_Transmit(&huart2, data, 12, 1000); HAL_Delay(1000);
   }
}

Here we define an array (data) which contains the string to be sent to the UART. Then call the function HAL_UART_Transmit in the while loop every 1 second.

The parameters of the function are as follows:

@UART Instance, huart2
@pointer to the data array, data
@size of the data in bytes, 12
@timout in case of error, 1000ms

Send number

As I mentioned we can not send the number directly to the UART. It only transmits the data in the ascii format. To send the number, we first need to convert each digit of the number to the character format and then send the data to the UART.

#include <stdio.h>
uint8_t number = 123;
uint8_t numarray[4];
int main()
{
 ....
 while (1)
 {
   sprintf(numarray, "%d\n", number);
   HAL_UART_Transmit(&huart1, numarray, 4, 1000);
   HAL_Delay(1000);
 }
}

#include <stdio.h>
uint8_t number = 123;
uint8_t numarray[4];
int main()
{
 ....
 while (1)
 {
   sprintf(numarray, "%d\n", number);
   HAL_UART_Transmit(&huart1, numarray, 4, 1000);
   HAL_Delay(1000);
 }
}

Here we will send the number 123 to the UART. We first need to define an array of size at least the number of digits of the number.

The sprintf function is used to convert the number to the character format. The format specifier, %d, is used to convert the integral value (123).

After conversion the characters are stored in the array, numarray. We send this array to the UART every 1 second.

RESULT

The Serial console on the computer should have the same configuration as we did for the STM32 in the cubeMX. The image below shows the configuration with the baud rate of 115200, 8 data bits with 1 stop bit and no parity.

The data sent by the STM32 is received in the serial console. The image below shows the data, “Hello world”, printed several times. It was sent every 1 second by the MCU.