STM32H753 UART Baud is 15% low.

[GlennH]

STM32H753 UART Baud is 15% low. I set it to 115200, but it's running at 98400. Is this a misconfigured clock?

[AScha.3]

Obviously you set something wrong ..in clock tree, or uart setting.

see:

[TDK]

If the baud rate is not what you want, it is not configured correctly.

Some background into your hardware and how the clock rate is configured and how you are measuring that clock rate would be appropriate. Without that, one can only guess at the issue.

• Incorrect system clock setting
• Wrong BRR value
• Misinterpretation of results

[Tesla DeLorean]

Sounds like something is very broken on your system, that you don't describe at all.

Measure HSE / PLL via MCO (PA8)

Check HSE_VALUE define (stm32h7xx_hal_conf.h) and its relationship with reality.

[GlennH]

AScha.3,

Thank you for the reply. I'm not using CUBE MX on this project. I'm using a vendor-supplied onboard computer that runs FreeRTOS and many vendor-specific tasks. I'm creating just one task in this framework to transfer data from the payload to the OBC for processing and then send the desired information back to the ground station.

Their UARTS all seem to work properly. Here is my setup:

oid USART6_RS485_Init()
{
	GPIO_InitTypeDef GPIO_InitStruct = { 0 };

	    /* Peripheral clock enable */
		__HAL_RCC_USART6_CLK_ENABLE();
	    __HAL_RCC_GPIOC_CLK_ENABLE();

	    /**USART1 GPIO Configuration
	    PC6    ------> USART6_TX
	    PC7   ------> USART6_RX
	    PG8   ------> USART6_DE (RTS)
	    */
	    huart6.Instance                    = USART6;
	    huart6.Init.BaudRate               = 115200;
	    huart6.Init.WordLength             = UART_WORDLENGTH_8B;
	    huart6.Init.StopBits               = UART_STOPBITS_1;
	    huart6.Init.Parity                 = UART_PARITY_NONE;
	    huart6.Init.Mode                   = UART_MODE_TX_RX;
	    huart6.Init.HwFlowCtl              = UART_HWCONTROL_NONE;
	    huart6.Init.OverSampling           = UART_OVERSAMPLING_16;
	    huart6.Init.OneBitSampling         = UART_ONE_BIT_SAMPLE_DISABLE;
	    huart6.AdvancedInit.AdvFeatureInit = UART_ADVFEATURE_NO_INIT;

	    if (HAL_UART_Init(&huart6) != HAL_OK)
	    {
	        Error_Handler();
	    }

	    HAL_UART_Receive_IT(&huart6, &rxBuffer[rxIndex], 1);

	    // PIN6 TX from the MCU PIN7 RX to the MCU
	    GPIO_InitStruct.Pin       = GPIO_PIN_6 | GPIO_PIN_7;
	    GPIO_InitStruct.Mode      = GPIO_MODE_AF_PP;
	    GPIO_InitStruct.Pull      = GPIO_NOPULL;
	    GPIO_InitStruct.Speed     = GPIO_SPEED_FREQ_VERY_HIGH;
	    GPIO_InitStruct.Alternate = GPIO_AF7_USART6;
	    HAL_GPIO_Init(GPIOC, &GPIO_InitStruct);

	    /*Configure GPIO pin Output Level */
	    HAL_GPIO_WritePin(GPIOG, GPIO_PIN_8, GPIO_PIN_RESET);
	    HAL_GPIO_WritePin(GPIOD, GPIO_PIN_13, GPIO_PIN_RESET);
	    HAL_GPIO_WritePin(GPIOG, GPIO_PIN_13, GPIO_PIN_RESET);

	    // PG8 not PF10

	    GPIO_InitStruct.Pin   = GPIO_PIN_8;
	    GPIO_InitStruct.Mode  = GPIO_MODE_OUTPUT_PP;
	    GPIO_InitStruct.Pull  = GPIO_NOPULL;
	    GPIO_InitStruct.Speed = GPIO_SPEED_FREQ_LOW;
	    HAL_GPIO_Init(GPIOG, &GPIO_InitStruct);

	    /*Configure GPIO pin : RS485_TE_Pin */
	    GPIO_InitStruct.Pin   = GPIO_PIN_13;
	    GPIO_InitStruct.Mode  = GPIO_MODE_OUTPUT_PP;
	    GPIO_InitStruct.Pull  = GPIO_NOPULL;
	    GPIO_InitStruct.Speed = GPIO_SPEED_FREQ_LOW;
	    HAL_GPIO_Init(GPIOD, &GPIO_InitStruct);

	    // PG13 not PF12

	    /*Configure GPIO pin : RS485_nRE_Pin */
	    GPIO_InitStruct.Pin   = GPIO_PIN_13;
	    GPIO_InitStruct.Mode  = GPIO_MODE_OUTPUT_PP;
	    GPIO_InitStruct.Pull  = GPIO_NOPULL;
	    GPIO_InitStruct.Speed = GPIO_SPEED_FREQ_LOW;
	    HAL_GPIO_Init(GPIOG, &GPIO_InitStruct);

	    /* USART1 interrupt Init */
	    HAL_NVIC_SetPriority(USART6_IRQn, 6U, 0);
	    HAL_NVIC_EnableIRQ(USART6_IRQn);

	    // Receive Enable ACTIVE
	    USART6_RS485ReceptionEnable();

	    // Termination Enable ACTIVE
	    // MacCfg_vSetBusTerminationState(eMAC_Interface_Primary, MacCfg_bGetBusTerminationState(eMAC_Interface_Primary, true), true);

	    // Start with a disabled communication driver...
	    USART6_RS485DriverDisable();
}

[GlennH]

TDK,

Thank you for the response. Please see my answer to AScha.3 above.

[Tesla DeLorean]

Speed is going to be the function of the synchronous clocks involved. The math on the rates is dependent on the validity of the data furnished, ie garbage in, garbage out

HSI would be a known quantity, within a few percent, however HSE is unknown and unmeasured by the MCU

[GlennH]

Tesla DeLorean,

Thank you for the reply.

I will check the HSE / PLL at my first opportunity.

The HSE_VALUE is (uint32_t) 25000000

[GlennH]

57: #define HSE_VALUE ((uint32_t)25000000) /*!< Value of the External oscillator in Hz */

65: #define HSI_VALUE ((uint32_t)64000000) /*!< Value of the Internal oscillator in Hz*/

[Tesla DeLorean]

15% might suggest a reality of closer too 21.25 or 28.75 MHz

An APB of a few 100 MHz should have about zero difficulty hitting 115200 baud, as the bus clock drops the ability to resolve diminishes, but still to a few percent worst case.

Sending 0x55 'U' at 8N1 continuously should result in a square wave half the baud frequency.