How to Use the VREFINT in stm32f103 bluepill?

Question

I'm not able to understand the use of VREFINT in stm32f103 board. Can anyone explain me how to get adc value in stm32f103 using VREFINT?

  if(HAL_ADC_PollForConversion(&hadc1, 100) == HAL_OK)
  {
      adcVrefInt = HAL_ADC_GetValue(&hadc1);
      vdd = 4095.0 * 1.20 / (float)adcVrefInt;
      vdd += 0.61; // .61 is the difference i'm getting in VDD
      sprintf(buffer, "VREFINT: %ld\tVDD: %.2f\t", adcVrefInt, vdd);
      HAL_UART_Transmit(&huart1, (uint8_t *)buffer, strlen(buffer), 100);

      if(HAL_ADC_PollForConversion(&hadc2, 100) == HAL_OK)
      {
          adcValue = HAL_ADC_GetValue(&hadc2);
          adcVoltage = (vdd/4095.0) * adcValue;
          sprintf(buffer, "ADC_PA0: %ld\tVoltage-PA0: %.2f\n", adcValue, adcVoltage);
          HAL_UART_Transmit(&huart1, (uint8_t *)buffer, strlen(buffer), 100);
      }
  }

Answer 1

You can read the V_REFINT channel (17) much like any other channel on ADC1, after setting the TSVREFE bit in ADC1->CR2. It's an internal analog signal, there is no pin associated with it. V_REFINT has a fixed voltage of 1.20 ± 0.04 V.

If an ADC input pin is connected to V_DDA, you get a reading of 4095. If it's connected to V_SSA, you get 0. If there is any other voltage V₁ between these limits, you get 4095 * V₁ / V_DDA. This applies to the V_REFINT channel as well.

When you measure V_REFINT, ADC1->DR = 4095 * V_REFINT / V_DDA. Because you know that V_REFINT = 1.20V, you can calculate V_DDA=4095 * 1.20 / ADC1->DR Volts.