Automatic Baud Rate Detection for STM32 Serial Ports

Regarding the issue of automatically identifying the UART serial port baud rate, those with project experience or who have studied serial ports should know a little about the methods for automatic identification.

Most likely, the common knowledge is to achieve this through baud rate matching, which is the most common and effective method.

The method mentioned above is the well-known software-based approach to detecting baud rate. In fact, there is another method that involves hardware itself completing the baud rate detection.

For STM32, ST has mentioned this in application notes and reference manuals. Below, I will briefly explain the content of hardware automatic baud rate detection based on the documentation.

ABR: Auto Baud Rate allows the receiving device to accept data from various sending devices operating at different rates without establishing the data rate in advance.

1. Applications of ABR

• When the communication speed of the system is unknown in advance.

• When the system is using a relatively low-accuracy clock source, and this mechanism allows obtaining the correct baud rate without measuring clock deviations.

2. Supported ABR Series

In STM32, only certain series support hardware automatic baud rate detection; earlier series do not support it (such as F1 F4). The series released later all support this feature, including the latest STM32H7 and G0 series.

Of course, for STM32 series devices with built-in ABR, not all instantiated USART interfaces support automatic baud rate detection.

3. Automatic Baud Rate Detection Modes

ABR refers to the process where the receiving device determines the incoming data rate by checking the first character (usually a pre-selected flag character).

The automatic baud rate detection function on STM32 products has various built-in modes based on different character patterns:

Mode 0: Any character starting with “1” bit;

Mode 1: Any character starting with 10xx pattern;

Mode 2: 0x7F;

Mode 3: 0x55;

Note:

A. In all ABR modes, the baud rate is checked multiple times during synchronous data reception, comparing each detection value with the previous one.

B. In 7-bit data length mode, 0x7F and 0x55 frame detection ABR modes are not supported.

4. Code Configuration

The relevant code is provided by the official reference examples based on (standard peripheral library, HAL library), such as F0 standard peripheral library reference code:

static void AutoBauRate_StartBitMethod(void){   /* USART enable */  USART_Cmd(EVAL_COM1, ENABLE);
  /* Configure the AutoBaudRate method */  USART_AutoBaudRateConfig(EVAL_COM1, USART_AutoBaudRate_StartBit);
  /* Enable AutoBaudRate feature */  USART_AutoBaudRateCmd(EVAL_COM1, ENABLE);
  /* Wait until Receive enable acknowledge flag is set */  while(USART_GetFlagStatus(EVAL_COM1, USART_FLAG_REACK) == RESET)  {}  
  /* Wait until Transmit enable acknowledge flag is set */    while(USART_GetFlagStatus(EVAL_COM1, USART_FLAG_TEACK) == RESET)  {}  
  /* Loop until the end of Autobaudrate phase */  while(USART_GetFlagStatus(EVAL_COM1, USART_FLAG_ABRF) == RESET)  {}  
  /* If AutoBaudBate error occurred */  if (USART_GetFlagStatus(EVAL_COM1, USART_FLAG_ABRE) != RESET)  {    /* Turn on LED3 */    STM_EVAL_LEDOn(LED3);  }  else  {    /* Turn on LED2 */    STM_EVAL_LEDOn(LED2);
    /* Wait until RXNE flag is set */    while(USART_GetFlagStatus(EVAL_COM1, USART_FLAG_RXNE) == RESET)    {}
    /* Wait until TXE flag is set */        while(USART_GetFlagStatus(EVAL_COM1, USART_FLAG_TXE) == RESET)    {}
    /* Send received character */    USART_SendData(EVAL_COM1, USART_ReceiveData(EVAL_COM1)); 
    /* clear the TE bit (if a transmission is on going or a data is in the TDR, it will be sent before    efectivelly disabling the transmission) */    USART_DirectionModeCmd(EVAL_COM1, USART_Mode_Tx, DISABLE);
    /* Check the Transfer Complete Flag */    while (USART_GetFlagStatus(EVAL_COM1, USART_FLAG_TC) == RESET)    {}  }
  /* USART Disable */  USART_Cmd(EVAL_COM1, DISABLE);}

5. ABR Error Calculation

The communication rate range (especially the maximum communication rate) is determined by the USART clock source (fCK). The receiver uses different user-configurable oversampling techniques to distinguish valid input data from noise, thereby recovering the data. This can achieve a balance between maximum communication rate and noise/clock inaccuracies.

The oversampling method can be selected by programming the OVER8 bit in the USARTx_CR1 register, which can be either 16 times or 8 times the baud rate clock.

The frequency of the USART clock source must be compatible with the expected communication rate:

• With 16 times oversampling, the baud rate is between fCK/65535 and fCK/16.

• With 8 times oversampling, the baud rate is between fCK/65535 and fCK/8.

The baud rate error depends on the USART clock source, oversampling method, and ABR mode.

Where:

• The expected baud rate depends on the sending device

• The actual baud rate is the baud rate determined by the USART receiver using automatic baud rate detection operation.

6. Errors

The following diagram is based on official test data, calculating the ABR error when fCK = 72 MHz and the expected baud rate is 115200 bits/s.

From the diagram above, it can be seen that ABR modes 2 and 3 have higher accuracy than modes 0 and 1; their baud rate error values are lower.

However, since the error between the expected baud rate and the actual baud rate is less than 1%, the results of all modes are considered normal.