Use eFuse Vref for ESP32 Devices

[chinswain]

Hi all, I'm using a LilyGO-T-SIM7000G to measure sensors on a beehive and would like to get a fairly accurate battery measurement as it's a remote location (Solar powered).

The board has 100K voltage dividers on pin 35:

They have an example sketch for calculating the voltage buy using the fuse ref, is this something that could be integrated into the library?

https://github.com/Xinyuan-LilyGO/LilyGO-T-SIM7000G/blob/master/examples/readBattery/readBattery.ino

void setup()
{
    esp_adc_cal_characteristics_t adc_chars;
    esp_adc_cal_value_t val_type = esp_adc_cal_characterize(ADC_UNIT_1, ADC_ATTEN_DB_11, ADC_WIDTH_BIT_12, 1100, &adc_chars);    //Check type of calibration value used to characterize ADC
    if (val_type == ESP_ADC_CAL_VAL_EFUSE_VREF) {
        Serial.printf("eFuse Vref:%u mV", adc_chars.vref);
        vref = adc_chars.vref;
    } else if (val_type == ESP_ADC_CAL_VAL_EFUSE_TP) {
        Serial.printf("Two Point --> coeff_a:%umV coeff_b:%umV\n", adc_chars.coeff_a, adc_chars.coeff_b);
    } else {
        Serial.println("Default Vref: 1100mV");
    }
}

void loop()
{
     uint16_t v = analogRead(ADC_PIN);
     float battery_voltage = ((float)v / 4095.0) * 2.0 * 3.3 * (vref / 1000.0);
     String voltage = "Voltage :" + String(battery_voltage) + "V\n";
     Serial.println(voltage);
}

[rlogiacco]

Isn't the first parameter of the begin function enough? It's clearly adviced into the library documentation to use a calibration method to obtain more accurate results...

                /**
		 * Initializes the library by optionally setting additional parameters.
		 * To obtain the best results use a calibrated reference using the VoltageReference library or equivalent.
		 * 
		 * @param refVoltage is the board reference voltage, expressed in millivolts
		 * @param dividerRatio is the multiplier used to obtain the real battery voltage
		 * @param mapFunction is a pointer to the function used to map the battery voltage to the remaining capacity percentage (defaults to linear mapping)
		 */
		void begin(uint16_t refVoltage, float dividerRatio, mapFn_t = 0);