ADC Clock and Conversion Time question

[giwonKIM]

Hi, I'm giwon KIM in South Korea.

I'm using NUCLEO-401RE board.

In STM32F401RE datasheet and reference manual, It is said to be capable of operating at up to 30 MHz.

So its conversion time was set by calculating (12+3) / 30 MHz = 500 nsec.

I generated a sine wave of 250Hz and advertised it, but as calculated, I had to get 8000 samples per cycle, but I only got about 1300.

Is there a reason for this?

Below is actually a waveform drawn with Excel by acquiring a cycle sample.

And this is my ADC1 init function.

/**

* @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_ChannelConfTypeDef sConfig = {0};

/* USER CODE BEGIN ADC1_Init 1 */

/* USER CODE END ADC1_Init 1 */

/** Configure the global features of the ADC (Clock, Resolution, Data Alignment and number of conversion)

*/

hadc1.Instance = ADC1;

hadc1.Init.ClockPrescaler = ADC_CLOCK_SYNC_PCLK_DIV2;

hadc1.Init.Resolution = ADC_RESOLUTION_12B;

hadc1.Init.ScanConvMode = DISABLE;

hadc1.Init.ContinuousConvMode = ENABLE;

hadc1.Init.DiscontinuousConvMode = DISABLE;

hadc1.Init.ExternalTrigConvEdge = ADC_EXTERNALTRIGCONVEDGE_NONE;

hadc1.Init.ExternalTrigConv = ADC_SOFTWARE_START;

hadc1.Init.DataAlign = ADC_DATAALIGN_RIGHT;

hadc1.Init.NbrOfConversion = 1;

hadc1.Init.DMAContinuousRequests = DISABLE;

hadc1.Init.EOCSelection = ADC_EOC_SEQ_CONV;

if (HAL_ADC_Init(&hadc1) != HAL_OK)

{

Error_Handler();

}

/** Configure for the selected ADC regular channel its corresponding rank in the sequencer and its sample time.

*/

sConfig.Channel = ADC_CHANNEL_0;

sConfig.Rank = 1;

sConfig.SamplingTime = ADC_SAMPLETIME_15CYCLES;

if (HAL_ADC_ConfigChannel(&hadc1, &sConfig) != HAL_OK)

{

Error_Handler();

}

/* USER CODE BEGIN ADC1_Init 2 */

/* USER CODE END ADC1_Init 2 */

}

If you have any question, please ask me.

Thank you.

[TDK]

> sConfig.SamplingTime = ADC_SAMPLETIME_15CYCLES;

Conversion time will be 15 + 12 ticks in this case. 15 for sampling and 12 for conversion. With a 30 MHz clock, this is 900 ns

A 250 Hz sine wave period is 4 ms.

4 ms / 900 ns = 4444.

So why is yours different? Perhaps show your code.

• What is your ADC clock? Show your clock setup.
• Are you using continuous ADC conversion to a buffer using DMA? It's not going to work well with polling.

[MasterT]

ADC_SAMPLETIME_15CYCLES

12 + 15 / 30 MHz = ?

[giwonKIM]

HI,

I modified my code and 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_ChannelConfTypeDef sConfig = {0};

/* USER CODE BEGIN ADC1_Init 1 */

/* USER CODE END ADC1_Init 1 */

/** Configure the global features of the ADC (Clock, Resolution, Data Alignment and number of conversion)

*/

hadc1.Instance = ADC1;

hadc1.Init.ClockPrescaler = ADC_CLOCK_SYNC_PCLK_DIV4;

hadc1.Init.Resolution = ADC_RESOLUTION_12B;

hadc1.Init.ScanConvMode = DISABLE;

hadc1.Init.ContinuousConvMode = ENABLE;

hadc1.Init.DiscontinuousConvMode = DISABLE;

hadc1.Init.ExternalTrigConvEdge = ADC_EXTERNALTRIGCONVEDGE_NONE;

hadc1.Init.ExternalTrigConv = ADC_SOFTWARE_START;

hadc1.Init.DataAlign = ADC_DATAALIGN_RIGHT;

hadc1.Init.NbrOfConversion = 1;

hadc1.Init.DMAContinuousRequests = DISABLE;

hadc1.Init.EOCSelection = ADC_EOC_SEQ_CONV;

if (HAL_ADC_Init(&hadc1) != HAL_OK)

{

Error_Handler();

}

/** Configure for the selected ADC regular channel its corresponding rank in the sequencer and its sample time.

*/

sConfig.Channel = ADC_CHANNEL_0;

sConfig.Rank = 1;

sConfig.SamplingTime = ADC_SAMPLETIME_3CYCLES;

if (HAL_ADC_ConfigChannel(&hadc1, &sConfig) != HAL_OK)

{

Error_Handler();

}

/* USER CODE BEGIN ADC1_Init 2 */

/* USER CODE END ADC1_Init 2 */

}

The output sine waveform is 250Hz as it is, and how many samples should be produced at this time?

My calculation is (12+3) / 84MHz = 22400, But if you sample one cycle, there are about 13,000.

DMA is not in use and operates in polling mode.

Thank you.

[giwonKIM]

ADCCLK is shown in the Datasheet like this.

If I set APB2 to 60Mhz and clock divide to 2, is it correct that ADC operates at 30MHz?

 

[TDK]

> DMA is not in use and operates in polling mode.

So that's your problem. The chip takes time to run code, don't expect to get 5.6 Msps in polling mode.

[giwonKIM]

I understand what you're saying. I'll have to approach it in a different way.

Thank you.