pub fn rtc_calib_cal_code(_unit: u8, atten: Attenuation) -> Option<u16> {
        let cal_code: u16 = Self::read_field_le(match atten {
            Attenuation::_0dB => ADC1_HI_DOUT_ATTEN0,
            Attenuation::_2p5dB => ADC1_HI_DOUT_ATTEN1,
            Attenuation::_6dB => ADC1_HI_DOUT_ATTEN2,
            Attenuation::_11dB => ADC1_HI_DOUT_ATTEN3,
        });
        let cal_code: u16 = if atten == Attenuation::_6dB {
            2970 + cal_code
        } else {
            2900 + cal_code
        };
        Some(cal_code)
    }
Returns the call code

See: <https://github.com/espressif/esp-idf/blob/17a2461297076481858b7f76482676a521cc727a/components/efuse/esp32h2/esp_efuse_rtc_calib.c#L91>
esp-hal/src/soc/esp32h2/efuse/mod.rs