UART not working at High speed

[Rohit007]

Hello, 

I’m trying to transmit data at 2.5 Mbps (2500000 baud), but I’m not receiving any output. The same code works correctly at 2 Mbps, so the issue only appears when using 2.5 Mbps.

I tested both 64 MHz and 40 MHz clock configurations for the UART. At 64 MHz, CubeMX shows a warning:

“Real Baudrate set at BRR register is 2.461538 Mbits/sec. Error: -1.54%.”

Because of this error, I switched to a 40 MHz configuration, which gives the exact 2.5 Mbps baud rate—but I still don’t receive any data.

Is there any additional configuration required to make the MCU reliably transmit at 2.5 Mbps?

[Rohit007]

These are the settings i have applied to uart at 2.5Mbps

static void MX_USART2_UART_Init(void)
{

 /* USER CODE BEGIN USART2_Init 0 */

 /* USER CODE END USART2_Init 0 */

 /* USER CODE BEGIN USART2_Init 1 */

 /* USER CODE END USART2_Init 1 */
 huart2.Instance = USART2;
 huart2.Init.BaudRate = 2500000;
 huart2.Init.WordLength = UART_WORDLENGTH_8B;
 huart2.Init.StopBits = UART_STOPBITS_1;
 huart2.Init.Parity = UART_PARITY_NONE;
 huart2.Init.Mode = UART_MODE_TX_RX;
 huart2.Init.HwFlowCtl = UART_HWCONTROL_NONE;
 huart2.Init.OverSampling = UART_OVERSAMPLING_16;
 huart2.Init.OneBitSampling = UART_ONE_BIT_SAMPLE_DISABLE;
 huart2.Init.ClockPrescaler = UART_PRESCALER_DIV1;
 huart2.AdvancedInit.AdvFeatureInit = UART_ADVFEATURE_NO_INIT;
 if (HAL_UART_Init(&huart2) != HAL_OK)
 {
 Error_Handler();
 }
 if (HAL_UARTEx_SetTxFifoThreshold(&huart2, UART_TXFIFO_THRESHOLD_1_8) != HAL_OK)
 {
 Error_Handler();
 }
 if (HAL_UARTEx_SetRxFifoThreshold(&huart2, UART_RXFIFO_THRESHOLD_1_8) != HAL_OK)
 {
 Error_Handler();
 }
 if (HAL_UARTEx_DisableFifoMode(&huart2) != HAL_OK)
 {
 Error_Handler();
 }
 /* USER CODE BEGIN USART2_Init 2 */

 /* USER CODE END USART2_Init 2 */

}

[Andrew Neil]

You  haven't given any details about your hardware.

You also haven't shown your clock configuration.

Please see: 

How to write your question to maximize your chances to find a solution

 

---

@Rohit007 wrote:

I’m not receiving any output. 

---

How have you verified that?

 

---

@Rohit007 wrote:

I still don’t receive any data.

---

How do you know that the problem is in the transmitter - not the receiver?

Or both!

 

[Rohit007]

apologies @Andrew Neil,
I am using stm32g0c1ret6 MCU board with 40 Mhz for now, to get accurate 2.5Mbps baud rate. but my actual application is on 64Mhz.  i have attached the system clock settings and clock config snapshot for reference.

void SystemClock_Config(void)
{
 RCC_OscInitTypeDef RCC_OscInitStruct = {0};
 RCC_ClkInitTypeDef RCC_ClkInitStruct = {0};

 /** Configure the main internal regulator output voltage
 */
 HAL_PWREx_ControlVoltageScaling(PWR_REGULATOR_VOLTAGE_SCALE1);

 /** Initializes the RCC Oscillators according to the specified parameters
 * in the RCC_OscInitTypeDef structure.
 */
 RCC_OscInitStruct.OscillatorType = RCC_OSCILLATORTYPE_HSI;
 RCC_OscInitStruct.HSIState = RCC_HSI_ON;
 RCC_OscInitStruct.HSIDiv = RCC_HSI_DIV1;
 RCC_OscInitStruct.HSICalibrationValue = RCC_HSICALIBRATION_DEFAULT;
 RCC_OscInitStruct.PLL.PLLState = RCC_PLL_ON;
 RCC_OscInitStruct.PLL.PLLSource = RCC_PLLSOURCE_HSI;
 RCC_OscInitStruct.PLL.PLLM = RCC_PLLM_DIV1;
 RCC_OscInitStruct.PLL.PLLN = 10;
 RCC_OscInitStruct.PLL.PLLP = RCC_PLLP_DIV2;
 RCC_OscInitStruct.PLL.PLLQ = RCC_PLLQ_DIV2;
 RCC_OscInitStruct.PLL.PLLR = RCC_PLLR_DIV4;
 if (HAL_RCC_OscConfig(&RCC_OscInitStruct) != HAL_OK)
 {
 Error_Handler();
 }

 /** Initializes the CPU, AHB and APB buses clocks
 */
 RCC_ClkInitStruct.ClockType = RCC_CLOCKTYPE_HCLK|RCC_CLOCKTYPE_SYSCLK
 |RCC_CLOCKTYPE_PCLK1;
 RCC_ClkInitStruct.SYSCLKSource = RCC_SYSCLKSOURCE_PLLCLK;
 RCC_ClkInitStruct.AHBCLKDivider = RCC_SYSCLK_DIV1;
 RCC_ClkInitStruct.APB1CLKDivider = RCC_HCLK_DIV1;

 if (HAL_RCC_ClockConfig(&RCC_ClkInitStruct, FLASH_LATENCY_1) != HAL_OK)
 {
 Error_Handler();
 }
}

[Andrew Neil]

You still haven't given any details of your hardware!

 

---

@Rohit007 wrote:

I am using stm32g0c1ret6 MCU board

---

What board, exactly?

 

Your clock setup is using HSI.

Internal RC oscillators are rarely a good choice for UART comms - especially at higher speeds...

 

You didn't answer how you are testing this.

 

[Rohit007]

@Andrew Neil I have a EVK of G0 series it is using USART2 connected to serial terminal minicom in PC using Virtual com port and I am trying to print the normal logs on serial terminal.

[Gyessine]

hello @Rohit007 
can you specify if it's a ST board or custom board and what serial monitor are you using?
because I tried replicating your case and honestly, I didn't encounter any issue
it worked fine. So, can you provide more details?
I'm going to attach some screenshots if you want to compare with your own project or maybe if you want to replicate it.

 

int main(void)
{

 /* USER CODE BEGIN 1 */

 /* USER CODE END 1 */

 /* MCU Configuration--------------------------------------------------------*/

 /* Reset of all peripherals, Initializes the Flash interface and the Systick. */
 HAL_Init();

 /* USER CODE BEGIN Init */

 /* USER CODE END Init */

 /* Configure the system clock */
 SystemClock_Config();

 /* USER CODE BEGIN SysInit */

 /* USER CODE END SysInit */

 /* Initialize all configured peripherals */
 MX_GPIO_Init();
 MX_USART2_UART_Init();
 /* USER CODE BEGIN 2 */

 /* USER CODE END 2 */

 /* Infinite loop */
 /* USER CODE BEGIN WHILE */
 while (1)
 {
 /* USER CODE END WHILE */

 /* USER CODE BEGIN 3 */
	
	 HAL_UART_Transmit(&huart2, (uint8_t*)msg, sizeof(msg) - 1, HAL_MAX_DELAY);
	 
 }
 /* USER CODE END 3 */
}

 

void SystemClock_Config(void)
{
 RCC_OscInitTypeDef RCC_OscInitStruct = {0};
 RCC_ClkInitTypeDef RCC_ClkInitStruct = {0};

 /** Configure the main internal regulator output voltage
 */
 HAL_PWREx_ControlVoltageScaling(PWR_REGULATOR_VOLTAGE_SCALE1);

 /** Initializes the RCC Oscillators according to the specified parameters
 * in the RCC_OscInitTypeDef structure.
 */
 RCC_OscInitStruct.OscillatorType = RCC_OSCILLATORTYPE_HSI;
 RCC_OscInitStruct.HSIState = RCC_HSI_ON;
 RCC_OscInitStruct.HSIDiv = RCC_HSI_DIV1;
 RCC_OscInitStruct.HSICalibrationValue = RCC_HSICALIBRATION_DEFAULT;
 RCC_OscInitStruct.PLL.PLLState = RCC_PLL_ON;
 RCC_OscInitStruct.PLL.PLLSource = RCC_PLLSOURCE_HSI;
 RCC_OscInitStruct.PLL.PLLM = RCC_PLLM_DIV1;
 RCC_OscInitStruct.PLL.PLLN = 8;
 RCC_OscInitStruct.PLL.PLLP = RCC_PLLP_DIV2;
 RCC_OscInitStruct.PLL.PLLQ = RCC_PLLQ_DIV2;
 RCC_OscInitStruct.PLL.PLLR = RCC_PLLR_DIV2;
 if (HAL_RCC_OscConfig(&RCC_OscInitStruct) != HAL_OK)
 {
 Error_Handler();
 }

 /** Initializes the CPU, AHB and APB buses clocks
 */
 RCC_ClkInitStruct.ClockType = RCC_CLOCKTYPE_HCLK|RCC_CLOCKTYPE_SYSCLK
 |RCC_CLOCKTYPE_PCLK1;
 RCC_ClkInitStruct.SYSCLKSource = RCC_SYSCLKSOURCE_PLLCLK;
 RCC_ClkInitStruct.AHBCLKDivider = RCC_SYSCLK_DIV1;
 RCC_ClkInitStruct.APB1CLKDivider = RCC_HCLK_DIV1;

 if (HAL_RCC_ClockConfig(&RCC_ClkInitStruct, FLASH_LATENCY_2) != HAL_OK)
 {
 Error_Handler();
 }
}

/**
 * @brief USART2 Initialization Function
 * @param None
 * @retval None
 */
static void MX_USART2_UART_Init(void)
{

 /* USER CODE BEGIN USART2_Init 0 */

 /* USER CODE END USART2_Init 0 */

 /* USER CODE BEGIN USART2_Init 1 */

 /* USER CODE END USART2_Init 1 */
 huart2.Instance = USART2;
 huart2.Init.BaudRate = 250000;
 huart2.Init.WordLength = UART_WORDLENGTH_8B;
 huart2.Init.StopBits = UART_STOPBITS_1;
 huart2.Init.Parity = UART_PARITY_NONE;
 huart2.Init.Mode = UART_MODE_TX_RX;
 huart2.Init.HwFlowCtl = UART_HWCONTROL_NONE;
 huart2.Init.OverSampling = UART_OVERSAMPLING_16;
 huart2.Init.OneBitSampling = UART_ONE_BIT_SAMPLE_DISABLE;
 huart2.Init.ClockPrescaler = UART_PRESCALER_DIV1;
 huart2.AdvancedInit.AdvFeatureInit = UART_ADVFEATURE_NO_INIT;
 if (HAL_UART_Init(&huart2) != HAL_OK)
 {
 Error_Handler();
 }
 /* USER CODE BEGIN USART2_Init 2 */

 /* USER CODE END USART2_Init 2 */

}

 

Hope that helps!
Gyessine

[Rohit007]

@Gyessine   Thank you for reply.

I am using the stm32G0C1RET6 board it is a ST EVK board and the supports the UART comm upto 8Mbps. 
i have connected the UART-to-USB TTL to USART2 of this MCU and tried communicating using the Minicom as a serial terminal with Baud rate - 2500000 (you have given one less zero).

[RobK1]

---

@Rohit007 wrote:

[...] the UART-to-USB TTL [...]

Which converter, specifically? Are you certain it can actually support 2,500,000 bps? I know for certain the one I use can't, eventhough it can go up to 3Mbps.

You also still haven't answered @Andrew Neil's questions about what you have done to verify your USART is actually producing the output you are expecting. 

[gbm]

Usually USB-UART modules support baud rates up to 921600. Are you sure your module is able to handle 2.5 Mb/s?

Check the lower speed setting operation first, then try to increase speed.

[Gyessine]

Hello, @Rohit007 

Apologies for the inconvenience regarding the baud rate.

As a first point, I want to clarify that if you are using a 40 MHz clock, the maximum theoretical throughput achievable is 5 Mbps with an oversampling of 8 bits. The 8 Mbps throughput is achievable only if you use a 64 MHz clock and 8-bit oversampling, as stated in the referance manual page 1003.

However, this baud rate can be limited by the maximum frequency of the GPIO pins being used, which I suspect is one of the root causes to your issue. You need to verify whether the pins you are using can handle the desired frequency that you want to achieve.

For the warning sign, Due to the limited resolution of the USARTDIV register, the actual baud rate generated by the USART may not exactly match the desired baud rate calculated from the clock and prescalers. Table 189 of the referance manual details how USARTDIV is encoded, and the maximum allowable baud rate error tolerance. If the difference between the desired and actual baud rate exceeds these limits, a warning or error is triggered to indicate potential communication issues.

Based on testing conducted on the G0 product, the transmission was capped at 2.25 Mbps with correct data transmission. At 2.5 Mbps, scrambled data was observed, indicating unequal baud rates. This reinforces the conclusion that the issue is caused by exceeding the tolerated error rate.

To conclude, first please verify that your pins can reach your desired baud rate frequency. Second, verify that you are respecting the tolerated error in baudrate(especially since you are using an HSI clock which will augment this error as @Andrew Neil, clock may have an effect, please consider using an HSE) 
you can calculate it using the BRR register (referance manual page 1051) that you can access using SFR in debug session
Hope that helps

Gyessine