STM32G051 ADC Calibration issue

[KDham.1]

Hi I am using the STM23G051C8T6 for the internal RTC and an external battery on Vbat. I noticed if I perform the Calibration using the following line

HAL_ADCEx_Calibration_Start(&hadc1);

my initial ADC reading will be lower then actual reading. For example will read 2.7 instead of 2.9. 

If I don't perform the Calibration I read the 2.9 correctly from the 1st measurement however subsequence measurements read higher. My ADC Config below.

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.DataAlign = ADC_DATAALIGN_RIGHT;
  hadc1.Init.ScanConvMode = ADC_SCAN_DISABLE;
  hadc1.Init.EOCSelection = ADC_EOC_SINGLE_CONV;
  hadc1.Init.LowPowerAutoWait = DISABLE;
  hadc1.Init.LowPowerAutoPowerOff = DISABLE;
  hadc1.Init.ContinuousConvMode = DISABLE;
  hadc1.Init.NbrOfConversion = 1;
  hadc1.Init.DiscontinuousConvMode = DISABLE;
  hadc1.Init.ExternalTrigConv = ADC_SOFTWARE_START;
  hadc1.Init.ExternalTrigConvEdge = ADC_EXTERNALTRIGCONVEDGE_NONE;
  hadc1.Init.DMAContinuousRequests = DISABLE;
  hadc1.Init.Overrun = ADC_OVR_DATA_PRESERVED;
  hadc1.Init.SamplingTimeCommon1 = ADC_SAMPLETIME_12CYCLES_5;
  hadc1.Init.SamplingTimeCommon2 = ADC_SAMPLETIME_12CYCLES_5;
  hadc1.Init.OversamplingMode = DISABLE;
  hadc1.Init.TriggerFrequencyMode = ADC_TRIGGER_FREQ_HIGH;
  if (HAL_ADC_Init(&hadc1) != HAL_OK)
  {
    Error_Handler();
  }

  /** Configure Regular Channel
  */
  sConfig.Channel = ADC_CHANNEL_VBAT;
  sConfig.Rank = ADC_REGULAR_RANK_1;
  sConfig.SamplingTime = ADC_SAMPLINGTIME_COMMON_1;
  if (HAL_ADC_ConfigChannel(&hadc1, &sConfig) != HAL_OK)
  {
    Error_Handler();
  }
  /* USER CODE BEGIN ADC1_Init 2 */

  /* USER CODE END ADC1_Init 2 */

}

And my ADC Measurment code:

  ADC->CCR |= ADC_CCR_VBATEN; // Set Divide by 3 Bridge to Measure Voltage on Vbat

         HAL_ADC_Start(&hadc1); // Start ADC conversion
        // HAL_Delay(100);

         if (HAL_ADC_PollForConversion(&hadc1, 100) == HAL_OK) // Wait for conversion to complete
         {
             adcValue = HAL_ADC_GetValue(&hadc1); // Read ADC value

             // Calculate voltage from ADC value
             battery_voltage_mv = ((float)adcValue / 4096.0f) * 3.04f * 3000;
         }

        // HAL_Delay(100);
         ADC->CCR &= ~ADC_CCR_VBATEN; // Clear Divide by 3 Bridge to Vbat to reduce battery drain
     HAL_ADC_Stop(&hadc1);

[TDK]

Per the datasheet, VBAT has a minimum 12us sampling time. 12.5 cycles is almost assuredly not enough if you're using typical ADC clock speeds. Bump up the sampling time to max.

[KDham.1]

Hi Thank you for the reply. When I increase the sampling time I notice we hit the Actual Voltage right away but then in subsequent measurements we start measuring a Voltage Higher then the actual voltage. This is with Calibration. 

For example we will hit 2.9 quick then in future measurements we start measuring > 3.0

If I sample even Slower then initial Voltage is incorrect but future measurements bring it up to the correct voltage.

Any tips?

Thanks,

Kuljot

[TDK]

What it does when not calibrated, and what it does with <12 us sampling time is immaterial. Focus on what it does with proper calibration and with proper sampling duration.

Note the measurement is only as good as your VREF+. Sounds like this may be slow to come up. Check proper VDDA and VREF+ layout and voltage stability at the time of the measurements.