How to Use the VREFINT in stm32f103 bluepill? - adc

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 VREFINT 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. VREFINT has a fixed voltage of 1.20 ± 0.04 V.
If an ADC input pin is connected to VDDA, you get a reading of 4095. If it's connected to VSSA, you get 0. If there is any other voltage V1 between these limits, you get 4095 * V1 / VDDA. This applies to the VREFINT channel as well.
When you measure VREFINT, ADC1->DR = 4095 * VREFINT / VDDA. Because you know that VREFINT = 1.20V, you can calculate VDDA=4095 * 1.20 / ADC1->DR Volts.