STM32H743 - ADC wrong sampling rate

[NW.2]

Hi STM32-Community,

I'm using an STM32H743 with ADC3 in circular DMA mode. ADC has a clockfrequency of 48 MHz. I'm using 8 channels within this ADC3 and also configured the DMA with said channels each with a sampling time of 64.5 cyles. I'm using an async prescaler of 6.

Calculating the sampling rate for a single ADC Channel I get:

48 MHz
-----------------                                    =  15,504 Hz ~ 15.5 kHz
(8 Channels* 6 (Prescaler) * 64.5)

I've put a variable within my DMA routine (which does nothing else) and everytime the DMA toggles this variable, I send out the time needed between each DMA interrupt. I consistenly receive 1169 ms with a DMA Size of 65528.

65528
---------------------------------   ~ 7.005 kHz.
(1.169)*8 (ADC-Channels)

Where is my error?

Here are the ADC Config:

/* ADC3 init function */
void MX_ADC3_Init(void)
{

  /* USER CODE BEGIN ADC3_Init 0 */

  /* USER CODE END ADC3_Init 0 */

  ADC_ChannelConfTypeDef sConfig = {0};

  /* USER CODE BEGIN ADC3_Init 1 */

  /* USER CODE END ADC3_Init 1 */

  /** Common config
  */
  hadc3.Instance = ADC3;
  hadc3.Init.ClockPrescaler = ADC_CLOCK_ASYNC_DIV6;
  hadc3.Init.Resolution = ADC_RESOLUTION_16B;
  hadc3.Init.ScanConvMode = ADC_SCAN_ENABLE;
  hadc3.Init.EOCSelection = ADC_EOC_SINGLE_CONV;
  hadc3.Init.LowPowerAutoWait = DISABLE;
  hadc3.Init.ContinuousConvMode = ENABLE;
  hadc3.Init.NbrOfConversion = 8;
  hadc3.Init.DiscontinuousConvMode = DISABLE;
  hadc3.Init.ExternalTrigConv = ADC_SOFTWARE_START;
  hadc3.Init.ExternalTrigConvEdge = ADC_EXTERNALTRIGCONVEDGE_NONE;
  hadc3.Init.ConversionDataManagement = ADC_CONVERSIONDATA_DMA_CIRCULAR;
  hadc3.Init.Overrun = ADC_OVR_DATA_PRESERVED;
  hadc3.Init.LeftBitShift = ADC_LEFTBITSHIFT_NONE;
  hadc3.Init.OversamplingMode = DISABLE;
  if (HAL_ADC_Init(&hadc3) != HAL_OK)
  {
    Error_Handler();
  }

  /** Configure Regular Channel
  */
  sConfig.Channel = ADC_CHANNEL_5;
  sConfig.Rank = ADC_REGULAR_RANK_1;
  sConfig.SamplingTime = ADC_SAMPLETIME_64CYCLES_5;
  sConfig.SingleDiff = ADC_SINGLE_ENDED;
  sConfig.OffsetNumber = ADC_OFFSET_NONE;
  sConfig.Offset = 0;
  sConfig.OffsetSignedSaturation = DISABLE;
  if (HAL_ADC_ConfigChannel(&hadc3, &sConfig) != HAL_OK)
  {
    Error_Handler();
  }

  /** Configure Regular Channel
  */
  sConfig.Channel = ADC_CHANNEL_9;
  sConfig.Rank = ADC_REGULAR_RANK_2;
  if (HAL_ADC_ConfigChannel(&hadc3, &sConfig) != HAL_OK)
  {
    Error_Handler();
  }

  /** Configure Regular Channel
  */
  sConfig.Channel = ADC_CHANNEL_4;
  sConfig.Rank = ADC_REGULAR_RANK_3;
  if (HAL_ADC_ConfigChannel(&hadc3, &sConfig) != HAL_OK)
  {
    Error_Handler();
  }

  /** Configure Regular Channel
  */
  sConfig.Channel = ADC_CHANNEL_8;
  sConfig.Rank = ADC_REGULAR_RANK_4;
  if (HAL_ADC_ConfigChannel(&hadc3, &sConfig) != HAL_OK)
  {
    Error_Handler();
  }

  /** Configure Regular Channel
  */
  sConfig.Channel = ADC_CHANNEL_3;
  sConfig.Rank = ADC_REGULAR_RANK_5;
  if (HAL_ADC_ConfigChannel(&hadc3, &sConfig) != HAL_OK)
  {
    Error_Handler();
  }

  /** Configure Regular Channel
  */
  sConfig.Channel = ADC_CHANNEL_7;
  sConfig.Rank = ADC_REGULAR_RANK_6;
  if (HAL_ADC_ConfigChannel(&hadc3, &sConfig) != HAL_OK)
  {
    Error_Handler();
  }

  /** Configure Regular Channel
  */
  sConfig.Channel = ADC_CHANNEL_2;
  sConfig.Rank = ADC_REGULAR_RANK_7;
  if (HAL_ADC_ConfigChannel(&hadc3, &sConfig) != HAL_OK)
  {
    Error_Handler();
  }

  /** Configure Regular Channel
  */
  sConfig.Channel = ADC_CHANNEL_6;
  sConfig.Rank = ADC_REGULAR_RANK_8;
  if (HAL_ADC_ConfigChannel(&hadc3, &sConfig) != HAL_OK)
  {
    Error_Handler();
  }
  /* USER CODE BEGIN ADC3_Init 2 */

  /* USER CODE END ADC3_Init 2 */

}

CubeMX ADC-Frequency:

I know I'm not 100% correct with my method measuring the sampling frequency, but being that far off left me wondering what I'm doing wrong.

Kind regards

[TDK · Accepted solution]

You're missing:

An x2 prescaler on the ADC clock.

Extra samples due to the conversion (would be 8.5 for 16-bit conversion).

[NW.2]

Thank you so much!

Have a nice week end! 

[Ab1]

Query: Using STM32H743XI EVAL BOARD,

ERROR IN READING 2 ADC1 CHANNEL (PA0_C, PA1_C) IN DMA CIRCULAR MODE -

1ST CHANNEL GIVING ACCURATE COUNTS,

2ND CHANNEL SET AT 2V - ALWAYS SHOWING COUNTS SAME AS OF 1ST CHANNEL, WHERE POT IS CONNECTED?

ADC CLOCK, CONFIGS AND DMA SETTINGS AS FOLLOWS:

Tried example code also from STM32H7cubmx package (1.11.0), same result? anything missing from above configuration?

[TDK]

Please don't duplicate post, especially in unrelated threads.

https://community.st.com/t5/stm32-mcus-products/stm32-gpio-sink-source-currents/m-p/628313#M232513