AN4908

Application note
STM32 USART automatic baud rate detection

Introduction
Correct USART communication requires the transmission and reception baud rates to be
matched reasonably closely, otherwise communication errors may occur.
Automatic baud rate detection is useful when establishing a communication link between
two devices, where the slave device is able to detect the baud rate of the master controller
and self-adjust accordingly. This requires an automatic mechanism to determine the baud
rate.
The USART peripheral embedded in some STM32 devices offers many features, including
automatic baud rate detection hardware.
The purpose of this application note is to present the automatic baud rate detection feature
of STM32 microcontrollers and to give an alternative software approach for STM32 devices
that do not implement this feature in hardware.
This application note applies to the products listed in Table 1.

Table 1. Applicable products

Type Product series

STM32F0 Series, STM32F1 Series, STM32F3 Series, STM32F2 Series,

Microcontrollers STM32F4 Series, STM32F7 Series, STM32L0 Series, STM32L1 Series,
STM32L4 Series.

November 2016 DocID029725 Rev 1 1/22

www.st.com 1
Contents AN4908

Contents

1 Hardware automatic baud rate detection . . . . . . . . . . . . . . . . . . . . . . . . 5

1.1 Feature overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.2 Automatic baud rate detection modes . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
1.3 Error calculation for ABR . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

2 Software automatic baud rate detection . . . . . . . . . . . . . . . . . . . . . . . . . 9

3 Setups for software and hardware approaches . . . . . . . . . . . . . . . . . . 10

3.1 USART1 configuration example . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
3.2 Hardware auto baud rate detection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11
3.3 Software automatic baud rate detection . . . . . . . . . . . . . . . . . . . . . . . . . 13
3.4 Analysis of results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
3.4.1 Error calculation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
3.4.2 Comparison of software and hardware approaches . . . . . . . . . . . . . . . 17

4 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

5 Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

2/22 DocID029725 Rev 1

AN4908 List of figures

List of figures

Figure 1. Software automatic baud rate detection overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

Figure 2. Error calculation for ABR at fCK = 72 MHz, 115200 bits/s desired baud rate . . . . . . . . . . 16
Figure 3. ABR error comparison (fCK = HSI clock ,Mode 2 for HW detection) . . . . . . . . . . . . . . . . . 17
Figure 4. ABR error comparison (fCK = 72MHz, Mode 2 for HW detection) . . . . . . . . . . . . . . . . . . . 18
Figure 5. Baud rate comparison (fCK = 72MHz,
desired baud rate = 9 Mbits/s, Mode 2 for hardware detection. . . . . . . . . . . . . . . . . . . . . . 19

DocID029725 Rev 1 3/22

3
List of tables AN4908

List of tables

Table 1. Applicable products . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

Table 2. USART hardware automatic baud rate detection on STM32 Series. . . . . . . . . . . . . . . . . . . 5
Table 3. Hardware automatic baud rate detection on STM32 USART interfaces. . . . . . . . . . . . . . . . 6
Table 4. Automatic baud rate detection modes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Table 5. Software automatic baud rate detection details . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Table 6. Document revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

4/22 DocID029725 Rev 1

AN4908 Hardware automatic baud rate detection

1 Hardware automatic baud rate detection

1.1 Feature overview

Automatic baud rate detection (ABR) allows the receiving device to accept data from a
variety of transmitting devices operating at different speeds without needing to establish
data rates in advance.
In some STM32 products, the USART is able to automatically determine the baud rate using
dedicated hardware.
Table 2 gives an overview of the STM32 series devices that support automatic baud rate
detection.

Table 2. USART hardware automatic baud rate detection on STM32 Series

Product ABR support

Mainstream

STM32F0 Yes
STM32F1 No
STM32F3 Yes

High-performance

STM32F2 No
STM32F4 No
STM32F7 Yes

Ultra-low power

STM32L0 Yes
STM32L1 No
STM32L4 Yes

For STM32 Series devices that embed ABR, not all of the instantiated USART interfaces
support the automatic baud rate feature. This constraint is detailed in Table 3.

DocID029725 Rev 1 5/22

21
 Table 3. Hardware automatic baud rate detection on STM32 USART interfaces(1)(2)(3)
6/22

Hardware automatic baud rate detection

STM32 F0 STM32 F3 STM32 F7 STM32L0 STM32L4

STM32F302xB/C

STM32F303xB/C

STM32F303xD/E
STM32F302xD/E

STM32F302x6/8

STM32F303x6/8

STM32F301x6/8
STM32F030x4,

STM32F358xC
STM32F070xB

STM32F030xC

STM32F398xE
STM32F030x6

STM32F030x8

STM32F070x6

STM32F328x8

STM32F334xx

STM32F318x8

STM32F745xx
STM32F746xx

STM32F756xx

STM32L4x1 /
STM32L4x2 /
STM32L4x3 /
STM32L4x5 /
STM32F37xx
Port

STM32F03x

STM32F05x

STM32F04x

STM32F04x
STM32F07x

STM32F09x

STM32L0x1

STM32L0x2

STM32L0x3

STM32L4x6
USART 1 X X X X X X X X X X X X X X X X X X X X X X X

USART 2 0 - - X X 0 - - X X X X - X - - X X X X X X X

USART 3 0 0 0 - - 0 0 0 - X X X - X - - X X X 0 0 0 X
USART 4 0 0 0 - - 0 0 0 - - - - 0 - 0 0 0 X X - - - X
DocID029725 Rev 1

USART 5 0 0 0 0 0 0 0 0 0 - - - 0 - 0 0 0 X X - - - X
USART 6 0 0 0 0 0 0 0 0 0 - - 0 0 0 0 0 0 X X 0 0 0 0
USART 7 0 0 0 0 0 0 0 0 0 - - 0 0 0 0 0 0 X X 0 0 0 0
USART 8 0 0 0 0 0 0 0 0 0 - - 0 0 0 0 0 0 X X 0 0 0 0
1. X: supported
2. -: not supported
3. 0: USART instance not available

AN4908
AN4908 Hardware automatic baud rate detection

1.2 Automatic baud rate detection modes

ABR refers to the process by which a receiving device determines the speed of incoming
data by examining the first character, usually a preselected sign-on character.
The automatic baud rate feature on STM32 products embed various modes based on
different character patterns:
• any character starting with a bit at ‘1’: Mode 0
• any character starting with a 10xx pattern: Mode 1
• 0x7F: Mode 2
• 0x55: Mode 3.

Table 4. Automatic baud rate detection modes

ABR
Description Waveform
mode


The received character can be a character starting
with a bit at 1. In this case, the USART measures
0
the duration of the start bit (falling edge to rising 6WDUWELW 6WRSELW
edge). 7UDQVLWLRQIURPWR
06Y


Any character starting with a 10xx pattern. In this
case, the USART measures the duration of the start
1 and of the 1st data bit. The duration is measured
from falling edge to falling edge, ensuring better 6WDUWELW  6WRSELW
accuracy in the case of slow signal slopes.
06Y

      

0x7F character frame.

2 In this case, the baud rate is updated first at the end
of the start bit then at the end of the bit 6. 
6WDUWELW 6WRSELW
06Y

   
A 0x55 character frame.
In this case, the baud rate is updated first at the end
3 of the start bit, then at the end of bit and finally at the
end of bit 6. In parallel, another check is performed     6WRSELW
6WDUWELW
for each intermediate transition of RX line.
06Y

Before activating the automatic baud rate detection, one of the ABR modes must be
selected through the ABRMOD[1:0] field in the USARTx_CR2 register. In all ABR modes,
the baud rate is measured several times during the synchronization data reception, and is
compared each time to the previous measurement.
Note: In 7-bit data length mode, 0x7F and 0x55 frames detection ABR modes are not supported.

DocID029725 Rev 1 7/22

21
Hardware automatic baud rate detection AN4908

1.3 Error calculation for ABR

The communication speed range (specifically the maximum communication speed) is
determined by the USART clock source (fCK). The receiver implements different user-
configurable oversampling techniques for data recovery by discriminating between valid
incoming data and noise. This allows a trade-off between the maximum communication
speed and immunity to noise / clock inaccuracy.
The oversampling method is selected by programming the OVER8 bit in the USARTx_CR1
register, and can be either 16 or 8 times the baud-rate clock.
The USART clock source frequency must be compatible with the expected communication
speed:
• When oversampling by 16, the baud rate is between fCK/65535 and fCK/16
• When oversampling by 8, the baud rate is between fCK/65535 and fCK/8.
The baud-rate error is dependent on the USART clock source, the oversampling method
and the ABR mode.

Error(%) = desired baud rate – actual baud rate

------------------------------------------------------------------------------------------------ ⋅ 100
desired baud rate

Where:
• desired baud rate is fixed by the transmitter device
• actual baud rate is the baud rate determined by the USART receiver using the
automatic baud rate detection operation.

8/22 DocID029725 Rev 1

AN4908 Software automatic baud rate detection

2 Software automatic baud rate detection

When hardware auto baud rate detection is not supported, the software approach described
in this section can be adopted.
The idea of the software approach is to send a 0x7F data frame to the USARTx_RX pin.
This is connected to the EXTI line, which is configured to generate an interrupt on each
rising edge.
The duration of the interval between the two rising edges is measured using the Systick
timer. This duration corresponds to the duration of 8 bits, so
• bit time = calculated duration / 8
• baud rate = 1/bit time
The USARTx_BRR register is then programmed, based on the calculated baud rate value.

Figure 1. Software automatic baud rate detection overview

ELWGXUDWLRQWRPHDVXUH

       6WRSELW

6WDUWELW

)LUVWULVLQJHGJH 6HFRQGULVLQJHGJH

06Y

DocID029725 Rev 1 9/22

21
Setups for software and hardware approaches AN4908

3 Setups for software and hardware approaches

The STM32F303xD/E embedding the hardware automatic baud rate feature, is used for this
set-up example.
The HyperTerminal PC application is used to send and receive data frames to and from the
STM32F303. Consequently, standard baud rates in the range 600 bits/s to 115200 bits/s are
tested.The highest baud-rate value that can be reached (9 Mbits/s) is tested using another
STM32F3 device as a transmitter.

3.1 USART1 configuration example

In both examples, the STM32 USART1 is configured as follows:

/*##-1- Configure the UART peripheral

######################################*/
/* Put the USART peripheral in the Asynchronous mode (UART Mode) */
/* UART configured as follows:
- Word Length = 8 Bits
- Stop Bit = One Stop bit
- Parity = NONE parity
- BaudRate = 115200 baud It can be any other value as the USARTx_BRR
register will be reprogrammed
- Hardware flow control disabled (RTS and CTS signals)
- The oversampling mode is 8 or 16 (Both are tested)
*/
UartHandle.Instance = USARTx;
UartHandle.Init.BaudRate = 115200;
UartHandle.Init.WordLength = UART_WORDLENGTH_8B;
UartHandle.Init.StopBits = UART_STOPBITS_1;
UartHandle.Init.Parity = UART_PARITY_NONE;
UartHandle.Init.HwFlowCtl = UART_HWCONTROL_NONE;
UartHandle.Init.Mode = UART_MODE_TX_RX;
UartHandle.Init.OverSampling = UART_OVERSAMPLING_16;
Note: The USART1 clock source is a system clock at 72 MHz, using the HSE PLL clock source.
(Some tests are made using the HSI clock as USART1 clock source. This is to check the
impact of the HSI inaccuracy on the results.)

10/22 DocID029725 Rev 1

AN4908 Setups for software and hardware approaches

3.2 Hardware auto baud rate detection

The USART1 is configured to detect baud rate automatically. The user has to select the
ABR mode in the USART1 initialization function as follows:

/*##-2- Configure the AutoBaudRate method */

UartHandle.AdvancedInit.AdvFeatureInit =UART_ADVFEATURE_AUTOBAUDRATE_INIT;
UartHandle.AdvancedInit.AutoBaudRateEnable =
UART_ADVFEATURE_AUTOBAUDRATE_ENABLE;

/*Uncomment your appropriate mode */

//UartHandle.AdvancedInit.AutoBaudRateMode =
UART_ADVFEATURE_AUTOBAUDRATE_ONSTARTBIT;
//UartHandle.AdvancedInit.AutoBaudRateMode =
UART_ADVFEATURE_AUTOBAUDRATE_ONFALLINGEDGE;
//UartHandle.AdvancedInit.AutoBaudRateMode =
UART_ADVFEATURE_AUTOBAUDRATE_ON0X7FFRAME;
//UartHandle.AdvancedInit.AutoBaudRateMode =
UART_ADVFEATURE_AUTOBAUDRATE_ON0X55FRAME;

if (HAL_UART_Init(&UartHandle) != HAL_OK)
{
/* Initialization Error */
Error_Handler();
}

/* Wait until Receive enable acknowledge flag is set */

while(__HAL_UART_GET_FLAG(&UartHandle,UART_FLAG_REACK) == RESET)
{}

/* Wait until Transmit enable acknowledge flag is set */

while(__HAL_UART_GET_FLAG(&UartHandle,UART_FLAG_TEACK) == RESET)
{}

/* Loop until the end of Autobaudrate phase */

while(__HAL_UART_GET_FLAG(&UartHandle,UART_FLAG_ABRF) == RESET)
{}

DocID029725 Rev 1 11/22

21
Setups for software and hardware approaches AN4908

Once the whole initialization is complete, the USART waits until data is received from the
HyperTerminal before launching the automatic baud rate detection phase. The end of this
phase is monitored by the ABRF flag.
• If the auto baud rate operation is unsuccessful, the ABRE flag is set
• If the auto baud rate operation is completed successfully, an acknowledgment data is
transmitted to the HyperTerminal.

/* If AutoBaudBate error occurred */

if (__HAL_UART_GET_FLAG(&UartHandle, UART_FLAG_ABRE)!= RESET)

{
Error_Handler();
}
else
{
/* Wait until RXNE flag is set */
while(__HAL_UART_GET_FLAG(&UartHandle,UART_FLAG_RXNE) == RESET)
{}
/* Send acknowledgement message*/
if (HAL_UART_Transmit_DMA(&UartHandle, (uint8_t *)aTxBuffer, TXBUFFERSIZE)
!= HAL_OK)
{
/* Transfer error in transmission process */
Error_Handler();
}
while (HAL_UART_GetState(&UartHandle) != HAL_UART_STATE_READY)
{
}
}

12/22 DocID029725 Rev 1

AN4908 Setups for software and hardware approaches

3.3 Software automatic baud rate detection

Table 5 details the software approach.

Table 5. Software automatic baud rate detection details

Action Code

HAL library initialization. HAL_Init();

Suspend Tick increment to HAL_SuspendTick();
prevent wakeup by Systick
interrupt.
Configure the system clock to System Clock source = PLL (HSE)
72 MHz. PLLMUL = RCC_PLL_MUL9 (9)
The SystemCoreClockUpdate Flash Latency(WS) = 2
function can eventually be
executed in the main to verify
the CPU operating frequency.

Configure the UART

See Section 3.1: USART1 configuration example.
peripheral.

static void EXTILine1_Config(void)

{
GPIO_InitTypeDef GPIO_InitStructure;
/* Enable GPIOE clock */
__GPIOE_CLK_ENABLE();
/* Configure PE1 pin as input floating */
Configure the USARTx RX pin GPIO_InitStructure.Mode = GPIO_MODE_IT_RISING;
to generate an interrupt on GPIO_InitStructure.Pull = GPIO_NOPULL;
each rising edge. GPIO_InitStructure.Pin = GPIO_PIN_1;
HAL_GPIO_Init(GPIOE, &GPIO_InitStructure);
/* Enable and set EXTI Line0 Interrupt to the lowest
priority */
HAL_NVIC_SetPriority(EXTI1_IRQn, 2, 2);
HAL_NVIC_EnableIRQ(EXTI1_IRQn);
}

DocID029725 Rev 1 13/22

21
Setups for software and hardware approaches AN4908

Table 5. Software automatic baud rate detection details (continued)

Action Code

/*Wait until the end of interrupt */

while (end_interrupt_flag != 1) {
BSP_LED_On(LED2);
}
/* Autobaudrate sequence : Update BRR register */
Autobaudrate();
/* Send acknowledgement */
0x7F received on Rx pin, wait if (HAL_UART_Transmit_DMA(&UartHandle, (uint8_t
until the end of the interrupt. *)aTxBuffer, TXBUFFERSIZE) != HAL_OK)
Launch the automatic baud
rate sequence described in {
Section 2: Software automatic /* Transfer error in transmission process */
baud rate detection. Error_Handler();
}
while (HAL_UART_GetState(&UartHandle) !=
HAL_UART_STATE_READY)
{}
/* Infinite loop */
while (1)
{}
static void Autobaudrate(void)
{
float tmp=0, elapsed;
uint32_t USART1_clk=0;
uint32_t start_time_val=0;
uint32_t BRR=0;
tmp += 0xFFFFFF - stop_time_val;
tmp -= start_time_val;
elapsed =(tmp/(SystemCoreClock/1000000))/8;
USART1_clk=SystemCoreClock;
if( (USART1->CR1 & 0x8000)== 0x8000)
Automatic baud rate function {
/*In case of oversampling by 8*/
BRR =(elapsed*((2*USART1_clk)/1000000))+1;
USART1->BRR= BRR;
}
else
{
/*In case of oversampling by 16*/
BRR =(elapsed* ((USART1_clk)/1000000))+1;
USART1->BRR=BRR;
}
}

14/22 DocID029725 Rev 1

AN4908 Setups for software and hardware approaches

Table 5. Software automatic baud rate detection details (continued)

Action Code

void EXTI1_IRQHandler()
{
HAL_GPIO_EXTI_IRQHandler(GPIO_PIN_1);
if(temp==0)
{
HAL_SYSTICK_Config(0xFFFFFF);
temp++;
External line 1 interrupt }
request: else
First rising: temp=0 start the
{
systick timer
SysTick->CTRL &= SysTick_Counter_Disable;
Second rising:
– disable the systick counter /* Stop the Timer and get the encoding time */
– get the encoding time GETMYTIME(&stop_time_val);
– clear the SysTick counter /* Clear the SysTick Counter */
SysTick->VAL = SysTick_Counter_Clear;
/* Clear the temp flag*/
temp=0;
/*end of interrupt*/
interrupt_flag=1;
}
}
#define SysTick_Counter_Disable ((uint32_t)0xFFFFFFFE)
#define SysTick_Counter_Enable ((uint32_t)0x00000001)
Required project defines
#define SysTick_Counter_Clear ((uint32_t)0x00000000)
#define GETMYTIME(_t) (*_t=SysTick->VAL)

DocID029725 Rev 1 15/22

21
Setups for software and hardware approaches AN4908

3.4 Analysis of results

3.4.1 Error calculation

Figure 2 shows that ABR modes 2 and 3 are more precise than modes 0 and 1; they provide
a lower baud-rate error figure.
Nevertheless, all modes exhibit good results, as the error between the desired and actual
baud rates is less than 1%.

Figure 2. Error calculation for ABR at fCK = 72 MHz,

115200 bits/s desired baud rate

(UURU














0RGH 0RGH 0RGH 0RGH
$XWREDXGUDWHPRGHV

06Y

16/22 DocID029725 Rev 1

AN4908 Setups for software and hardware approaches

3.4.2 Comparison of software and hardware approaches

Figure 3 shows that in general, when the USART is clocked by the system clock at 72 MHz
(with the HSE as the PLL clock source), the results are better than when the USART clock
source is used as the HSI clock. This is due to the relative inaccuracy of the HSI.

Figure 3. ABR error comparison (fCK = HSI clock ,Mode 2 for HW detection)

(UURU


































%DXGUDWH

6:HUURU
+:HUURU
06Y

DocID029725 Rev 1 17/22

21
Setups for software and hardware approaches AN4908

Figure 4 shows that in most cases the hardware approach provides better results than the
software approach. The software approach nevertheless provides good results that are in
some cases comparable to those obtained using the hardware approach.

Figure 4. ABR error comparison (fCK = 72MHz, Mode 2 for HW detection)

(UURU




































%DXGUDWH

6:HUURU
+:HUURU
06Y

18/22 DocID029725 Rev 1

AN4908 Setups for software and hardware approaches

Figure 5 shows that:

• With the hardware approach we reach the maximum baud rate of 9 Mbits/s with 0%
error.
• With the software approach the error at maximum baud rate is about 30%, which is
explained by the CPU cycles spent executing the interrupt handler.

Figure 5. Baud rate comparison (fCK = 72MHz,

desired baud rate = 9 Mbits/s, Mode 2 for hardware detection

ĂƵĚƌĂƚĞ
ϭϬ
ϵ
ϴ
ϳ
ϲ
ϱ
ϰ
ϯ
Ϯ
ϭ
Ϭ
,ĂƌĚǁĂƌĞ
^ŽĨƚǁĂƌĞ
;ZDŽĚĞϮͿ
ĂƵĚͲƌĂƚĞĚĞƚĞĐƚŝŽŶŵĞƚŚŽĚƐ
06Y

DocID029725 Rev 1 19/22

21
Conclusion AN4908

4 Conclusion

This application note describes the hardware automatic baud rate detection feature
embedded in some STM32 devices. It also provides a technique for implementing this
feature in software, as a solution for STM32 devices not implementing this feature in
hardware.
Although the automatic baud rate detection is applied at the beginning of the examples, it
could be extended and used whenever the transmitter and receiver devices detect
communication errors. This allows a robust application where the host varies its baud rate
between communications.

20/22 DocID029725 Rev 1

AN4908 Revision history

5 Revision history

Table 6. Document revision history

Date Revision Changes

15-Nov-2016 1 Initial release

DocID029725 Rev 1 21/22

21
 AN4908

IMPORTANT NOTICE – PLEASE READ CAREFULLY

STMicroelectronics NV and its subsidiaries (“ST”) reserve the right to make changes, corrections, enhancements, modifications, and
improvements to ST products and/or to this document at any time without notice. Purchasers should obtain the latest relevant information on
ST products before placing orders. ST products are sold pursuant to ST’s terms and conditions of sale in place at the time of order
acknowledgement.

Purchasers are solely responsible for the choice, selection, and use of ST products and ST assumes no liability for application assistance or
the design of Purchasers’ products.

No license, express or implied, to any intellectual property right is granted by ST herein.

Resale of ST products with provisions different from the information set forth herein shall void any warranty granted by ST for such product.

ST and the ST logo are trademarks of ST. All other product or service names are the property of their respective owners.

Information in this document supersedes and replaces information previously supplied in any prior versions of this document.

© 2016 STMicroelectronics – All rights reserved

22/22 DocID029725 Rev 1