pub fn rtc_calib_cal_mv(_unit: u8, atten: Attenuation) -> u16 {
        const INPUT_VOUT_MV: [[u16; 4]; 1] = [
            [750, 1000, 1500, 2800], // Calibration V1 coefficients
        ];

        let version = Self::rtc_calib_version();

        // https://github.com/espressif/esp-idf/blob/master/components/efuse/esp32h2/include/esp_efuse_rtc_calib.h#L15C9-L17
        // ESP_EFUSE_ADC_CALIB_VER1     1
        // ESP_EFUSE_ADC_CALIB_VER_MIN  ESP_EFUSE_ADC_CALIB_VER1
        // ESP_EFUSE_ADC_CALIB_VER_MAX  ESP_EFUSE_ADC_CALIB_VER1
        if version != 1 {
            // The required efuse bits for this chip are not burnt.
            // 1100 is the middle of the reference voltage range.
            // See: <https://github.com/espressif/esp-idf/blob/465b159cd8771ffab6be70c7675ecf6705b62649/docs/en/api-reference/peripherals/adc_calibration.rst?plain=1#L9>
            return 1100;
        }

        INPUT_VOUT_MV[version as usize - 1][atten as usize]
    }
Get ADC reference point voltage for specified attenuation in millivolts

See: <https://github.com/espressif/esp-idf/blob/be06a6f/components/efuse/esp32h2/esp_efuse_rtc_calib.c#L91>
esp-hal/src/soc/esp32h2/efuse/mod.rs