STM32H755 dual ADC DMA sample rate is half

[mosamasa]

Hi,

I'm using STM32H755 nucleo board and evaluating ADC. I'm using ADC1 and ADC2. I like to get 1.92MSPS per ADC (12bit). According to my understanding I need minimum 8 cycles for conversion and therefor I was using 15.36MHz clock rate. With these settings, I'm getting 960kSPS rate, which is half what I expected. Then I doubled clock rate to 30.72MHz, which gives correct 1.92MSPS per ADC. According to AN5354, maximum ADC frequency is 17MHz (LQFP144), which is much lower than my current setting. Can I use 30.72MHz clock for ADC and why it has to be double?

[CMYL]

Hello @mosamasa 

Regarding the sampling rate, the ADC needs a minimum of 2.5 ADC clock cycles for the sample period and 12.5 ADC clock cycles for conversion (12-bit resolution), resulting in a total of 15 ADC clock cycles. This aligns with your observation that doubling the clock rate to 30.72 MHz achieves the desired 1.92 MSPS per ADC.

If you can share more details on the ADCs settings or a code snippet as you are using DMA which probably triggered by Timer?

Best regards

[MasterT]

Get RM0399, I think there is a divider /2 in the ADC clock interface, same as on H743 /753 adc  version V.

[mosamasa]

My understanding is that I need 6.5 ADC_CLKs + 1.5 => 8 ADC_CLKs according to STM32H7 - ADC presentation.

Here are my ADC settings:

/**
  * @brief ADC1 Initialization Function
  * @PAram None
  * @retval None
  */
static void MX_ADC1_Init(void)
{

  /* USER CODE BEGIN ADC1_Init 0 */

  /* USER CODE END ADC1_Init 0 */

  ADC_MultiModeTypeDef multimode = {0};
  ADC_ChannelConfTypeDef sConfig = {0};

  /* USER CODE BEGIN ADC1_Init 1 */

  /* USER CODE END ADC1_Init 1 */

  /** Common config
  */
  hadc1.Instance = ADC1;
  hadc1.Init.ClockPrescaler = ADC_CLOCK_ASYNC_DIV1;
  hadc1.Init.Resolution = ADC_RESOLUTION_12B;
  hadc1.Init.ScanConvMode = ADC_SCAN_DISABLE;
  hadc1.Init.EOCSelection = ADC_EOC_SEQ_CONV;
  hadc1.Init.LowPowerAutoWait = DISABLE;
  hadc1.Init.ContinuousConvMode = ENABLE;
  hadc1.Init.NbrOfConversion = 1;
  hadc1.Init.DiscontinuousConvMode = DISABLE;
  hadc1.Init.NbrOfDiscConversion = 1;
  hadc1.Init.ExternalTrigConv = ADC_SOFTWARE_START;
  hadc1.Init.ExternalTrigConvEdge = ADC_EXTERNALTRIGCONVEDGE_NONE;
  hadc1.Init.ConversionDataManagement = ADC_CONVERSIONDATA_DMA_ONESHOT;
  hadc1.Init.Overrun = ADC_OVR_DATA_PRESERVED;
  hadc1.Init.LeftBitShift = ADC_LEFTBITSHIFT_NONE;
  hadc1.Init.OversamplingMode = DISABLE;
  hadc1.Init.Oversampling.Ratio = 1;
  if (HAL_ADC_Init(&hadc1) != HAL_OK)
  {
    Error_Handler();
  }

  /** Configure the ADC multi-mode
  */
  multimode.Mode = ADC_MODE_INDEPENDENT;
  if (HAL_ADCEx_MultiModeConfigChannel(&hadc1, &multimode) != HAL_OK)
  {
    Error_Handler();
  }

  /** Configure Regular Channel
  */
  sConfig.Channel = ADC_CHANNEL_2;
  sConfig.Rank = ADC_REGULAR_RANK_1;
  sConfig.SamplingTime = ADC_SAMPLETIME_1CYCLE_5;
  sConfig.SingleDiff = ADC_SINGLE_ENDED;
  sConfig.OffsetNumber = ADC_OFFSET_NONE;
  sConfig.Offset = 0;
  if (HAL_ADC_ConfigChannel(&hadc1, &sConfig) != HAL_OK)
  {
    Error_Handler();
  }
  /* USER CODE BEGIN ADC1_Init 2 */

  /* USER CODE END ADC1_Init 2 */

}

/**
  * @brief ADC2 Initialization Function
  * @PAram None
  * @retval None
  */
static void MX_ADC2_Init(void)
{

  /* USER CODE BEGIN ADC2_Init 0 */

  /* USER CODE END ADC2_Init 0 */

  ADC_ChannelConfTypeDef sConfig = {0};

  /* USER CODE BEGIN ADC2_Init 1 */

  /* USER CODE END ADC2_Init 1 */

  /** Common config
  */
  hadc2.Instance = ADC2;
  hadc2.Init.ClockPrescaler = ADC_CLOCK_ASYNC_DIV1;
  hadc2.Init.Resolution = ADC_RESOLUTION_12B;
  hadc2.Init.ScanConvMode = ADC_SCAN_DISABLE;
  hadc2.Init.EOCSelection = ADC_EOC_SEQ_CONV;
  hadc2.Init.LowPowerAutoWait = DISABLE;
  hadc2.Init.ContinuousConvMode = ENABLE;
  hadc2.Init.NbrOfConversion = 1;
  hadc2.Init.DiscontinuousConvMode = DISABLE;
  hadc2.Init.ExternalTrigConv = ADC_SOFTWARE_START;
  hadc2.Init.ExternalTrigConvEdge = ADC_EXTERNALTRIGCONVEDGE_NONE;
  hadc2.Init.ConversionDataManagement = ADC_CONVERSIONDATA_DMA_ONESHOT;
  hadc2.Init.Overrun = ADC_OVR_DATA_PRESERVED;
  hadc2.Init.LeftBitShift = ADC_LEFTBITSHIFT_NONE;
  hadc2.Init.OversamplingMode = DISABLE;
  hadc2.Init.Oversampling.Ratio = 1;
  if (HAL_ADC_Init(&hadc2) != HAL_OK)
  {
    Error_Handler();
  }

  /** Configure Regular Channel
  */
  sConfig.Channel = ADC_CHANNEL_15;
  sConfig.Rank = ADC_REGULAR_RANK_1;
  sConfig.SamplingTime = ADC_SAMPLETIME_1CYCLE_5;
  sConfig.SingleDiff = ADC_SINGLE_ENDED;
  sConfig.OffsetNumber = ADC_OFFSET_NONE;
  sConfig.Offset = 0;
  sConfig.OffsetSignedSaturation = DISABLE;
  if (HAL_ADC_ConfigChannel(&hadc2, &sConfig) != HAL_OK)
  {
    Error_Handler();
  }
  /* USER CODE BEGIN ADC2_Init 2 */

  /* USER CODE END ADC2_Init 2 */

}

And my clock settings:

void PeriphCommonClock_Config(void)
{
  RCC_PeriphCLKInitTypeDef PeriphClkInitStruct = {0};

  /** Initializes the peripherals clock
  */
  PeriphClkInitStruct.PeriphClockSelection = RCC_PERIPHCLK_ADC;
  PeriphClkInitStruct.PLL2.PLL2M = 4;
  PeriphClkInitStruct.PLL2.PLL2N = 384;
  PeriphClkInitStruct.PLL2.PLL2P = 25;
  PeriphClkInitStruct.PLL2.PLL2Q = 2;
  PeriphClkInitStruct.PLL2.PLL2R = 2;
  PeriphClkInitStruct.PLL2.PLL2RGE = RCC_PLL2VCIRANGE_1;
  PeriphClkInitStruct.PLL2.PLL2VCOSEL = RCC_PLL2VCOWIDE;
  PeriphClkInitStruct.PLL2.PLL2FRACN = 0;
  PeriphClkInitStruct.AdcClockSelection = RCC_ADCCLKSOURCE_PLL2;
  if (HAL_RCCEx_PeriphCLKConfig(&PeriphClkInitStruct) != HAL_OK)
  {
    Error_Handler();
  }
}

I'm currently using software to trigger ADC. I was trying to use TIM2, but for some reason I was not able to find settings, which are working.