F411 ADC1 VBAT Reading Too High

[HDaji.1]

Hi,

I have a PCB with F411 as MCU.

MCU VBAT pin is directly connected to battery positive pin.

I try to use ADC1 to read the VBAT value. 

Here is the ADC1 init code:

  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 = ENABLE;
  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 = ENABLE;
  hadc1.Init.EOCSelection = ADC_EOC_SINGLE_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_VBAT;
  sConfig.Rank = 1;
  sConfig.SamplingTime = ADC_SAMPLETIME_15CYCLES;
  if (HAL_ADC_ConfigChannel(&hadc1, &sConfig) != HAL_OK)
  {
    Error_Handler();
  }

Here is the ADC reading code:

	adc_read_complete = 0;
    HAL_ADC_Start_DMA(&hadc1, (uint32_t *)adc_reading, adc_ch_cnt);
	while(adc_read_complete == 0);

	float v1 = (float)adc_reading[0] / 4096.0 * 4 * 3.3;

My issue is that the reading value is higher than the measured one; and the difference is quite constant as showed below:

ADC reading right after boot-up * Similar to Measured Value	Reading 1	Reading 2
4.119	4.863	4.844
3.609	4.122	4.141

The first reading after boot-up is almost same as measured value; however, subsequent reading values are constantly higher.

For the measured value: we remove battery from PCB and use multi-meter to measure.

Can anyone have any idea on why this could happen, what I have done wrongly?

[HDaji.1]

Forgot the add the DMA for ADC1 code:

  /* DMA controller clock enable */
  __HAL_RCC_DMA2_CLK_ENABLE();

  /* DMA interrupt init */
  /* DMA2_Stream0_IRQn interrupt configuration */
  HAL_NVIC_SetPriority(DMA2_Stream0_IRQn, 0, 0);
  HAL_NVIC_EnableIRQ(DMA2_Stream0_IRQn);

DMA handler

void DMA2_Stream0_IRQHandler(void)
{
  /* USER CODE BEGIN DMA2_Stream0_IRQn 0 */

  /* USER CODE END DMA2_Stream0_IRQn 0 */
  HAL_DMA_IRQHandler(&hdma_adc1);
  /* USER CODE BEGIN DMA2_Stream0_IRQn 1 */
  adc_read_complete = 1;
  /* USER CODE END DMA2_Stream0_IRQn 1 */
}

[AScha.3 · Accepted solution]

Hi,

did you set sampling time , long enough :

You didnt show the adc clock/setting, but i assume 15 cycles time is too short. (set 387 cycles , for test).

[HDaji.1]

yes, you're right. @AScha.3 

Once sampling time is changed to maximum: for F411, it's 480 cycles.

The reading becomes accurately close to measured value.