text: // Copyright 2015-2016 Espressif Systems (Shanghai) PTE LTD
  //
  // Licensed under the Apache License, Version 2.0 (the "License");
  // you may not use this file except in compliance with the License.
  // You may obtain a copy of the License at
  
  //     http://www.apache.org/licenses/LICENSE-2.0
  //
  // Unless required by applicable law or agreed to in writing, software
  // distributed under the License is distributed on an "AS IS" BASIS,
  // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
  // See the License for the specific language governing permissions and
  // limitations under the License.
  
  #ifndef __ESP_ADC_CAL_H__
  #define __ESP_ADC_CAL_H__
  
  #ifdef __cplusplus
  extern "C" {
  #endif
  
  #include <stdint.h>
  #include "esp_err.h"
  #include "driver/adc.h"
  
  /**
   * @brief Type of calibration value used in characterization
   */
  typedef enum {
      ESP_ADC_CAL_VAL_EFUSE_VREF = 0,         /**< Characterization based on reference voltage stored in eFuse*/
      ESP_ADC_CAL_VAL_EFUSE_TP = 1,           /**< Characterization based on Two Point values stored in eFuse*/
      ESP_ADC_CAL_VAL_DEFAULT_VREF = 2,       /**< Characterization based on default reference voltage*/
      ESP_ADC_CAL_VAL_MAX,
      ESP_ADC_CAL_VAL_NOT_SUPPORTED = ESP_ADC_CAL_VAL_MAX,
  } esp_adc_cal_value_t;
  
  /**
   * @brief Structure storing characteristics of an ADC
   *
   * @note Call esp_adc_cal_characterize() to initialize the structure
   */
  typedef struct {
      adc_unit_t adc_num;                     /**< ADC number*/
      adc_atten_t atten;                      /**< ADC attenuation*/
      adc_bits_width_t bit_width;             /**< ADC bit width */
      uint32_t coeff_a;                       /**< Gradient of ADC-Voltage curve*/
      uint32_t coeff_b;                       /**< Offset of ADC-Voltage curve*/
      uint32_t vref;                          /**< Vref used by lookup table*/
      const uint32_t *low_curve;              /**< Pointer to low Vref curve of lookup table (NULL if unused)*/
      const uint32_t *high_curve;             /**< Pointer to high Vref curve of lookup table (NULL if unused)*/
  } esp_adc_cal_characteristics_t;
  
  /**
   * @brief Checks if ADC calibration values are burned into eFuse
   *
   * This function checks if ADC reference voltage or Two Point values have been
   * burned to the eFuse of the current ESP32
   *
   * @param   value_type  Type of calibration value (ESP_ADC_CAL_VAL_EFUSE_VREF or ESP_ADC_CAL_VAL_EFUSE_TP)
   * @note in ESP32S2, only ESP_ADC_CAL_VAL_EFUSE_TP is supported. Some old ESP32S2s do not support this, either.
   * In which case you have to calibrate it manually, possibly by performing your own two-point calibration on the chip.
   *
   * @return
   *      - ESP_OK: The calibration mode is supported in eFuse
   *      - ESP_ERR_NOT_SUPPORTED: Error, eFuse values are not burned
   *      - ESP_ERR_INVALID_ARG: Error, invalid argument (ESP_ADC_CAL_VAL_DEFAULT_VREF)
   */
  esp_err_t esp_adc_cal_check_efuse(esp_adc_cal_value_t value_type);
  
  /**
   * @brief Characterize an ADC at a particular attenuation
   *
   * This function will characterize the ADC at a particular attenuation and generate
   * the ADC-Voltage curve in the form of [y = coeff_a * x + coeff_b].
   * Characterization can be based on Two Point values, eFuse Vref, or default Vref
   * and the calibration values will be prioritized in that order.
   *
   * @note
   * For ESP32, Two Point values and eFuse Vref calibration can be enabled/disabled using menuconfig.
   * For ESP32s2, only Two Point values calibration and only ADC_WIDTH_BIT_13 is supported. The parameter default_vref is unused.
   *
   *
   * @param[in]   adc_num         ADC to characterize (ADC_UNIT_1 or ADC_UNIT_2)
   * @param[in]   atten           Attenuation to characterize
   * @param[in]   bit_width       Bit width configuration of ADC
   * @param[in]   default_vref    Default ADC reference voltage in mV (Only in ESP32, used if eFuse values is not available)
   * @param[out]  chars           Pointer to empty structure used to store ADC characteristics
   *
   * @return
   *      - ESP_ADC_CAL_VAL_EFUSE_VREF: eFuse Vref used for characterization
   *      - ESP_ADC_CAL_VAL_EFUSE_TP: Two Point value used for characterization (only in Linear Mode)
   *      - ESP_ADC_CAL_VAL_DEFAULT_VREF: Default Vref used for characterization
   */
  esp_adc_cal_value_t esp_adc_cal_characterize(adc_unit_t adc_num,
                                               adc_atten_t atten,
                                               adc_bits_width_t bit_width,
                                               uint32_t default_vref,
                                               esp_adc_cal_characteristics_t *chars);
  
  /**
   * @brief   Convert an ADC reading to voltage in mV
   *
   * This function converts an ADC reading to a voltage in mV based on the ADC's
   * characteristics.
   *
   * @note    Characteristics structure must be initialized before this function
   *          is called (call esp_adc_cal_characterize())
   *
   * @param[in]   adc_reading     ADC reading
   * @param[in]   chars           Pointer to initialized structure containing ADC characteristics
   *
   * @return      Voltage in mV
   */
  uint32_t esp_adc_cal_raw_to_voltage(uint32_t adc_reading, const esp_adc_cal_characteristics_t *chars);
  
  /**
   * @brief   Reads an ADC and converts the reading to a voltage in mV
   *
   * This function reads an ADC then converts the raw reading to a voltage in mV
   * based on the characteristics provided. The ADC that is read is also
   * determined by the characteristics.
   *
   * @note    The Characteristics structure must be initialized before this
   *          function is called (call esp_adc_cal_characterize())
   *
   * @param[in]   channel     ADC Channel to read
   * @param[in]   chars       Pointer to initialized ADC characteristics structure
   * @param[out]  voltage     Pointer to store converted voltage
   *
   * @return
   *      - ESP_OK: ADC read and converted to mV
   *      - ESP_ERR_TIMEOUT: Error, timed out attempting to read ADC
   *      - ESP_ERR_INVALID_ARG: Error due to invalid arguments
   */
  esp_err_t esp_adc_cal_get_voltage(adc_channel_t channel, const esp_adc_cal_characteristics_t *chars, uint32_t *voltage);
  
  #ifdef __cplusplus
  }
  #endif
  
  #endif /* __ESP_ADC_CAL_H__ */
  