Analog to Digital Converter

Overview

Configuration and Reading ADC

Each ADC unit supports two work modes, ADC-RTC or ADC-DMA mode. ADC-RTC is controlled by the RTC controller and is suitable for low-frequency sampling operations. ADC-DMA is controlled by a digital controller and is suitable for high-frequency continuous sampling actions.

ADC-RTC mode

The ADC should be configured before reading is taken.

Attenuation configuration is done per channel, see adc1_channel_t and adc2_channel_t, set as a parameter of above functions.

Then it is possible to read ADC conversion result with adc1_get_raw() and adc2_get_raw(). Reading width of ADC2 should be set as a parameter of adc2_get_raw() instead of in the configuration functions.

Note

Since the ADC2 is shared with the WIFI module, which has higher priority, reading operation of adc2_get_raw() will fail between esp_wifi_start() and esp_wifi_stop(). Use the return code to see whether the reading is successful.

There is another specific function adc_vref_to_gpio() used to route internal reference voltage to a GPIO pin. It comes handy to calibrate ADC reading and this is discussed in section Minimizing Noise.

Application Examples

Reading voltage on ADC1 channel 0 (GPIO 0):

#include <driver/adc.h>

...

    adc1_config_width(ADC_WIDTH_BIT_12);
    adc1_config_channel_atten(ADC1_CHANNEL_0,ADC_ATTEN_DB_0);
    int val = adc1_get_raw(ADC1_CHANNEL_0);

The input voltage in the above example is from 0 to 1.1 V (0 dB attenuation). The input range can be extended by setting a higher attenuation, see adc_atten_t. An example of using the ADC driver including calibration (discussed below) is available at esp-idf: peripherals/adc

Reading voltage on ADC2 channel 7 (GPIO 0):

#include <driver/adc.h>

...

    int read_raw;
    adc2_config_channel_atten( ADC2_CHANNEL_7, ADC_ATTEN_0db );

    esp_err_t r = adc2_get_raw( ADC2_CHANNEL_7, ADC_WIDTH_12Bit, &read_raw);
    if ( r == ESP_OK ) {
        printf("%d\n", read_raw );
    } else if ( r == ESP_ERR_TIMEOUT ) {
        printf("ADC2 used by Wi-Fi.\n");
    }

The reading may fail due to collision with Wi-Fi, should check it. An example using the ADC2 driver to read the output of DAC is available in esp-idf: peripherals/adc

Minimizing Noise

The ESP32-C3 ADC can be sensitive to noise leading to large discrepancies in ADC readings. To minimize noise, users may connect a 0.1uF capacitor to the ADC input pad in use. Multisampling may also be used to further mitigate the effects of noise.

ADC noise mitigation

Graph illustrating noise mitigation using capacitor and multisampling of 64 samples.

ADC Calibration

The esp_adc_cal/include/esp_adc_cal.h API provides functions to correct for differences in measured voltages caused by variation of ADC reference voltages (Vref) between chips. Per design the ADC reference voltage is 1100 mV, however the true reference voltage can range from 1000 mV to 1200 mV amongst different ESP32-C3s.

ADC reference voltage comparison

Graph illustrating effect of differing reference voltages on the ADC voltage curve.

Correcting ADC readings using this API involves characterizing one of the ADCs at a given attenuation to obtain a characteristics curve (ADC-Voltage curve) that takes into account the difference in ADC reference voltage. The characteristics curve is in the form of y = coeff_a * x + coeff_b and is used to convert ADC readings to voltages in mV. Calculation of the characteristics curve is based on calibration values which can be stored in eFuse or provided by the user.

Calibration Values

Calibration values are used to generate characteristic curves that account for the variation of ADC reference voltage of a particular ESP32-C3 chip. There are currently three sources of calibration values on ESP32, and one source on ESP32-S2. The availability of these calibration values will depend on the type and production date of the ESP32-C3 chip/module.

Application Example

For a full example see esp-idf: peripherals/adc

Characterizing an ADC at a particular attenuation:

#include "driver/adc.h"
#include "esp_adc_cal.h"

...

    //Characterize ADC at particular atten
    esp_adc_cal_characteristics_t *adc_chars = calloc(1, sizeof(esp_adc_cal_characteristics_t));
    esp_adc_cal_value_t val_type = esp_adc_cal_characterize(unit, atten, ADC_WIDTH_BIT_12, DEFAULT_VREF, adc_chars);
    //Check type of calibration value used to characterize ADC
    if (val_type == ESP_ADC_CAL_VAL_EFUSE_VREF) {
        printf("eFuse Vref");
    } else if (val_type == ESP_ADC_CAL_VAL_EFUSE_TP) {
        printf("Two Point");
    } else {
        printf("Default");
    }

Reading an ADC then converting the reading to a voltage:

#include "driver/adc.h"
#include "esp_adc_cal.h"

...
    uint32_t reading =  adc1_get_raw(ADC1_CHANNEL_5);
    uint32_t voltage = esp_adc_cal_raw_to_voltage(reading, adc_chars);

Routing ADC reference voltage to GPIO, so it can be manually measured (for Default Vref):

#include "driver/adc.h"

...

    esp_err_t status = adc_vref_to_gpio(ADC_UNIT_1, GPIO_NUM_25);
    if (status == ESP_OK) {
        printf("v_ref routed to GPIO\n");
    } else {
        printf("failed to route v_ref\n");
    }

GPIO Lookup Macros

There are macros available to specify the GPIO number of a ADC channel, or vice versa. e.g.

  1. ADC1_CHANNEL_0_GPIO_NUM is the GPIO number of ADC1 channel 0 (36);

  2. ADC1_GPIO32_CHANNEL is the ADC1 channel number of GPIO 32 (ADC1 channel 4).

API Reference

This reference covers three components:

ADC driver

Functions

esp_err_t adc_digi_filter_reset(adc_digi_filter_idx_t idx)

Reset adc digital controller filter.

Return

  • ESP_OK Success

Parameters

  • idx: Filter index.

esp_err_t adc_digi_filter_set_config(adc_digi_filter_idx_t idx, adc_digi_filter_t *config)

Set adc digital controller filter configuration.

Return

  • ESP_OK Success

Parameters

esp_err_t adc_digi_filter_get_config(adc_digi_filter_idx_t idx, adc_digi_filter_t *config)

Get adc digital controller filter configuration.

Return

  • ESP_OK Success

Parameters

esp_err_t adc_digi_filter_enable(adc_digi_filter_idx_t idx, bool enable)

Enable/disable adc digital controller filter. Filtering the ADC data to obtain smooth data at higher sampling rates.

Return

  • ESP_OK Success

Parameters

  • idx: Filter index.

  • enable: Enable/Disable filter.

esp_err_t adc_digi_monitor_set_config(adc_digi_monitor_idx_t idx, adc_digi_monitor_t *config)

Config monitor of adc digital controller.

Return

  • ESP_OK Success

Parameters

esp_err_t adc_digi_monitor_enable(adc_digi_monitor_idx_t idx, bool enable)

Enable/disable monitor of adc digital controller.

Return

  • ESP_OK Success

Parameters

  • idx: Monitor index.

  • enable: True or false enable monitor.

Structures

struct adc_digi_pattern_table_t

ADC digital controller (DMA mode) conversion rules setting.

Public Members

uint8_t atten : 2

ADC sampling voltage attenuation configuration. Modification of attenuation affects the range of measurements. 0: measurement range 0 - 800mV, 1: measurement range 0 - 1100mV, 2: measurement range 0 - 1350mV, 3: measurement range 0 - 2600mV.

uint8_t channel : 3

ADC channel index.

uint8_t unit : 1

ADC unit index.

uint8_t reserved : 2

reserved0

uint8_t val

Raw data value

struct adc_digi_output_data_t

ADC digital controller (DMA mode) output data format. Used to analyze the acquired ADC (DMA) data.

Public Members

uint32_t data : 12

ADC real output data info. Resolution: 12 bit.

uint32_t reserved12 : 1

Reserved12.

uint32_t channel : 3

ADC channel index info. If (channel < ADC_CHANNEL_MAX), The data is valid. If (channel > ADC_CHANNEL_MAX), The data is invalid.

uint32_t unit : 1

ADC unit index info. 0: ADC1; 1: ADC2.

uint32_t reserved17_31 : 15

Reserved17.

struct adc_digi_output_data_t::[anonymous]::[anonymous] type2

When the configured output format is 12bit. ADC_DIGI_FORMAT_11BIT

uint32_t val

Raw data value

struct adc_digi_clk_t

ADC digital controller (DMA mode) clock system setting. Calculation formula: controller_clk = (APLL or APB) / (div_num + div_a / div_b + 1).

Note

: The clocks of the DAC digital controller use the ADC digital controller clock divider.

Public Members

bool use_apll

true: use APLL clock; false: use APB clock.

uint32_t div_num

Division factor. Range: 0 ~ 255. Note: When a higher frequency clock is used (the division factor is less than 9), the ADC reading value will be slightly offset.

uint32_t div_b

Division factor. Range: 1 ~ 63.

uint32_t div_a

Division factor. Range: 0 ~ 63.

struct adc_digi_config_t

CONFIG_IDF_TARGET_ESP32.

ADC digital controller (DMA mode) configuration parameters.

Example setting: When using ADC1 channel0 to measure voltage, the sampling rate is required to be 1 kHz: 

+---------------------+--------+--------+--------+
| sample rate         | 1 kHz  | 1 kHz  | 1 kHz  |
+---------------------+--------+--------+--------+
| conv_mode           | single |  both  |  alter |
| adc1_pattern_len    |    1   |    1   |    1   |
| dig_clk.use_apll    |    0   |    0   |    0   |
| dig_clk.div_num     |   99   |   99   |   99   |
| dig_clk.div_b       |    0   |    0   |    0   |
| dig_clk.div_a       |    0   |    0   |    0   |
| interval            |  400   |  400   |  200   |
+---------------------+--------+--------+--------+
| `trigger_meas_freq` | 1 kHz  | 1 kHz  | 2 kHz  |
+---------------------+--------+--------+--------+

Explanation of the relationship between conv_limit_num, dma_eof_num and the number of DMA outputs: 

+---------------------+--------+--------+--------+
| conv_mode           | single |  both  |  alter |
+---------------------+--------+--------+--------+
| trigger meas times  |    1   |    1   |    1   |
+---------------------+--------+--------+--------+
| conv_limit_num      |   +1   |   +1   |   +1   |
| dma_eof_num         |   +1   |   +2   |   +1   |
| dma output (byte)   |   +2   |   +4   |   +2   |
+---------------------+--------+--------+--------+

Public Members

bool conv_limit_en

Enable the function of limiting ADC conversion times. If the number of ADC conversion trigger count is equal to the limit_num, the conversion is stopped.

uint32_t conv_limit_num

Set the upper limit of the number of ADC conversion triggers. Range: 1 ~ 255.

uint32_t adc_pattern_len

Pattern table length for digital controller. Range: 0 ~ 7 (0: Don’t change the pattern table setting). The pattern table that defines the conversion rules for each SAR ADC. Each table has 7 items, in which channel selection, resolution and attenuation are stored. When the conversion is started, the controller reads conversion rules from the pattern table one by one. For each controller the scan sequence has at most 16 different rules before repeating itself.

adc_digi_pattern_table_t *adc_pattern

Pointer to pattern table for digital controller. The table size defined by adc_pattern_len.

uint32_t sample_freq_hz

The expected ADC sampling frequency in Hz. Range: 611Hz ~ 83333Hz Fs = Fd / interval / 2 Fs: sampling frequency; Fd: digital controller frequency, no larger than 5M for better performance interval: interval between 2 measurement trigger signal, the smallest interval should not be smaller than the ADC measurement period, the largest interval should not be larger than 4095

struct adc_arbiter_t

ADC arbiter work mode and priority setting.

Note

ESP32-S2: Only ADC2 support arbiter.

Public Members

adc_arbiter_mode_t mode

Refer to adc_arbiter_mode_t. Note: only support ADC2.

uint8_t rtc_pri

RTC controller priority. Range: 0 ~ 2.

uint8_t dig_pri

Digital controller priority. Range: 0 ~ 2.

uint8_t pwdet_pri

Wi-Fi controller priority. Range: 0 ~ 2.

struct adc_digi_filter_t

ADC digital controller (DMA mode) filter configuration.

Note

For ESP32-S2, The filter object of the ADC is fixed.

Note

For ESP32-S2, The filter object is always all enabled channels.

Public Members

adc_unit_t adc_unit

Set adc unit number for filter. For ESP32-S2, Filter IDX0/IDX1 can only be used to filter all enabled channels of ADC1/ADC2 unit at the same time.

adc_channel_t channel

Set adc channel number for filter. For ESP32-S2, it’s always ADC_CHANNEL_MAX

adc_digi_filter_mode_t mode

Set adc filter mode for filter. See adc_digi_filter_mode_t.

struct adc_digi_monitor_t

ADC digital controller (DMA mode) monitor configuration.

Note

For ESP32-S2, The monitor object of the ADC is fixed.

Note

For ESP32-S2, The monitor object is always all enabled channels.

Public Members

adc_unit_t adc_unit

Set adc unit number for monitor. For ESP32-S2, monitor IDX0/IDX1 can only be used to monitor all enabled channels of ADC1/ADC2 unit at the same time.

adc_channel_t channel

Set adc channel number for monitor. For ESP32-S2, it’s always ADC_CHANNEL_MAX

adc_digi_monitor_mode_t mode

Set adc monitor mode. See adc_digi_monitor_mode_t.

uint32_t h_threshold

Set monitor threshold of adc digital controller.

uint32_t l_threshold

Set monitor threshold of adc digital controller.

Macros

ADC_ARBITER_CONFIG_DEFAULT()

ADC arbiter default configuration.

Note

ESP32S2: Only ADC2 supports (needs) an arbiter.

Enumerations

enum adc_unit_t

ADC unit enumeration.

Note

For ADC digital controller (DMA mode), ESP32 doesn’t support ADC_UNIT_2, ADC_UNIT_BOTH, ADC_UNIT_ALTER.

Values:

ADC_UNIT_1 = 1

SAR ADC 1.

ADC_UNIT_2 = 2

SAR ADC 2.

ADC_UNIT_BOTH = 3

SAR ADC 1 and 2.

ADC_UNIT_ALTER = 7

SAR ADC 1 and 2 alternative mode.

ADC_UNIT_MAX
enum adc_channel_t

ADC channels handle. See adc1_channel_t, adc2_channel_t.

Note

For ESP32 ADC1, don’t use ADC_CHANNEL_8, ADC_CHANNEL_9. See adc1_channel_t.

Values:

ADC_CHANNEL_0 = 0

ADC channel

ADC_CHANNEL_1

ADC channel

ADC_CHANNEL_2

ADC channel

ADC_CHANNEL_3

ADC channel

ADC_CHANNEL_4

ADC channel

ADC_CHANNEL_5

ADC channel

ADC_CHANNEL_6

ADC channel

ADC_CHANNEL_7

ADC channel

ADC_CHANNEL_8

ADC channel

ADC_CHANNEL_9

ADC channel

ADC_CHANNEL_MAX
enum adc_atten_t

ADC attenuation parameter. Different parameters determine the range of the ADC. See adc1_config_channel_atten.

Values:

ADC_ATTEN_DB_0 = 0

No input attenumation, ADC can measure up to approx. 800 mV.

ADC_ATTEN_DB_2_5 = 1

The input voltage of ADC will be attenuated, extending the range of measurement to up to approx. 1100 mV.

ADC_ATTEN_DB_6 = 2

The input voltage of ADC will be attenuated, extending the range of measurement to up to approx. 1350 mV.

ADC_ATTEN_DB_11 = 3

The input voltage of ADC will be attenuated, extending the range of measurement to up to approx. 2600 mV.

ADC_ATTEN_MAX
enum adc_i2s_source_t

ESP32 ADC DMA source selection.

Values:

ADC_I2S_DATA_SRC_IO_SIG = 0

I2S data from GPIO matrix signal

ADC_I2S_DATA_SRC_ADC = 1

I2S data from ADC

ADC_I2S_DATA_SRC_MAX
enum adc_bits_width_t

ADC resolution setting option.

Values:

ADC_WIDTH_BIT_12 = 3

ADC capture width is 12Bit.

ADC_WIDTH_MAX
enum adc_digi_convert_mode_t

ADC digital controller (DMA mode) work mode.

Note

The conversion mode affects the sampling frequency: SINGLE_UNIT_1: When the measurement is triggered, only ADC1 is sampled once. SINGLE_UNIT_2: When the measurement is triggered, only ADC2 is sampled once. BOTH_UNIT : When the measurement is triggered, ADC1 and ADC2 are sampled at the same time. ALTER_UNIT : When the measurement is triggered, ADC1 or ADC2 samples alternately.

Values:

ADC_CONV_SINGLE_UNIT_1 = 1

SAR ADC 1.

ADC_CONV_SINGLE_UNIT_2 = 2

SAR ADC 2.

ADC_CONV_BOTH_UNIT = 3

SAR ADC 1 and 2.

ADC_CONV_ALTER_UNIT = 7

SAR ADC 1 and 2 alternative mode.

ADC_CONV_UNIT_MAX
enum adc_digi_output_format_t

ADC digital controller (DMA mode) output data format option.

Values:

ADC_DIGI_FORMAT_12BIT

ADC to DMA data format, [15:12]-channel, [11: 0]-12 bits ADC data (adc_digi_output_data_t). Note: For single convert mode.

ADC_DIGI_FORMAT_11BIT

ADC to DMA data format, [15]-adc unit, [14:11]-channel, [10: 0]-11 bits ADC data (adc_digi_output_data_t). Note: For multi or alter convert mode.

ADC_DIGI_FORMAT_MAX
enum adc_arbiter_mode_t

ADC arbiter work mode option.

Note

ESP32-S2: Only ADC2 support arbiter.

Values:

ADC_ARB_MODE_SHIELD

Force shield arbiter, Select the highest priority controller to work.

ADC_ARB_MODE_FIX

Fixed priority switch controller mode.

ADC_ARB_MODE_LOOP

Loop priority switch controller mode. Each controller has the same priority, and the arbiter will switch to the next controller after the measurement is completed.

enum adc_digi_filter_idx_t

ADC digital controller (DMA mode) filter index options.

Note

For ESP32-S2, The filter object of the ADC is fixed.

Values:

ADC_DIGI_FILTER_IDX0 = 0

The filter index 0. For ESP32-S2, It can only be used to filter all enabled channels of ADC1 unit at the same time.

ADC_DIGI_FILTER_IDX1

The filter index 1. For ESP32-S2, It can only be used to filter all enabled channels of ADC2 unit at the same time.

ADC_DIGI_FILTER_IDX_MAX
enum adc_digi_filter_mode_t

ADC digital controller (DMA mode) filter type options. Expression: filter_data = (k-1)/k * last_data + new_data / k.

Values:

ADC_DIGI_FILTER_DIS = -1

Disable filter

ADC_DIGI_FILTER_IIR_2 = 0

The filter mode is first-order IIR filter. The coefficient is 2.

ADC_DIGI_FILTER_IIR_4

The filter mode is first-order IIR filter. The coefficient is 4.

ADC_DIGI_FILTER_IIR_8

The filter mode is first-order IIR filter. The coefficient is 8.

ADC_DIGI_FILTER_IIR_16

The filter mode is first-order IIR filter. The coefficient is 16.

ADC_DIGI_FILTER_IIR_64

The filter mode is first-order IIR filter. The coefficient is 64.

ADC_DIGI_FILTER_IIR_MAX
enum adc_digi_monitor_idx_t

ADC digital controller (DMA mode) monitor index options.

Note

For ESP32-S2, The monitor object of the ADC is fixed.

Values:

ADC_DIGI_MONITOR_IDX0 = 0

The monitor index 0. For ESP32-S2, It can only be used to monitor all enabled channels of ADC1 unit at the same time.

ADC_DIGI_MONITOR_IDX1

The monitor index 1. For ESP32-S2, It can only be used to monitor all enabled channels of ADC2 unit at the same time.

ADC_DIGI_MONITOR_IDX_MAX
enum adc_digi_monitor_mode_t

Set monitor mode of adc digital controller. MONITOR_HIGH:If ADC_OUT > threshold, Generates monitor interrupt. MONITOR_LOW: If ADC_OUT < threshold, Generates monitor interrupt.

Values:

ADC_DIGI_MONITOR_DIS = 0

Disable monitor.

ADC_DIGI_MONITOR_EN

If ADC_OUT < threshold, Generates monitor interrupt. If ADC_OUT > threshold, Generates monitor interrupt.

ADC_DIGI_MONITOR_MAX

Functions

void adc_power_on(void)

Enable ADC power.

void adc_power_off(void)

Power off SAR ADC.

void adc_power_acquire(void)

Increment the usage counter for ADC module. ADC will stay powered on while the counter is greater than 0. Call adc_power_release when done using the ADC.

void adc_power_release(void)

Decrement the usage counter for ADC module. ADC will stay powered on while the counter is greater than 0. Call this function when done using the ADC.

esp_err_t adc1_pad_get_io_num(adc1_channel_t channel, gpio_num_t *gpio_num)

Get the GPIO number of a specific ADC1 channel.

Return

  • ESP_OK if success

  • ESP_ERR_INVALID_ARG if channel not valid

Parameters

  • channel: Channel to get the GPIO number

  • gpio_num: output buffer to hold the GPIO number

esp_err_t adc1_config_channel_atten(adc1_channel_t channel, adc_atten_t atten)

Set the attenuation of a particular channel on ADC1, and configure its associated GPIO pin mux.

The default ADC voltage is for attenuation 0 dB and listed in the table below. By setting higher attenuation it is possible to read higher voltages.

Due to ADC characteristics, most accurate results are obtained within the “suggested range” shown in the following table. 

+----------+-------------+-----------------+
|          | attenuation | suggested range |
|    SoC   |     (dB)    |      (mV)       |
+==========+=============+=================+
|          |       0     |    100 ~  950   |
|          +-------------+-----------------+
|          |       2.5   |    100 ~ 1250   |
|   ESP32  +-------------+-----------------+
|          |       6     |    150 ~ 1750   |
|          +-------------+-----------------+
|          |      11     |    150 ~ 2450   |
+----------+-------------+-----------------+
|          |       0     |      0 ~  750   |
|          +-------------+-----------------+
|          |       2.5   |      0 ~ 1050   |
| ESP32-S2 +-------------+-----------------+
|          |       6     |      0 ~ 1300   |
|          +-------------+-----------------+
|          |      11     |      0 ~ 2500   |
+----------+-------------+-----------------+

For maximum accuracy, use the ADC calibration APIs and measure voltages within these recommended ranges.

Note

For any given channel, this function must be called before the first time adc1_get_raw() is called for that channel.

Note

This function can be called multiple times to configure multiple ADC channels simultaneously. You may call adc1_get_raw() only after configuring a channel.

Return

  • ESP_OK success

  • ESP_ERR_INVALID_ARG Parameter error

Parameters

  • channel: ADC1 channel to configure

  • atten: Attenuation level

esp_err_t adc1_config_width(adc_bits_width_t width_bit)

Configure ADC1 capture width, meanwhile enable output invert for ADC1. The configuration is for all channels of ADC1.

Return

  • ESP_OK success

  • ESP_ERR_INVALID_ARG Parameter error

Parameters

  • width_bit: Bit capture width for ADC1

int adc1_get_raw(adc1_channel_t channel)

Take an ADC1 reading from a single channel.

Note

ESP32: When the power switch of SARADC1, SARADC2, HALL sensor and AMP sensor is turned on, the input of GPIO36 and GPIO39 will be pulled down for about 80ns. When enabling power for any of these peripherals, ignore input from GPIO36 and GPIO39. Please refer to section 3.11 of ‘ECO_and_Workarounds_for_Bugs_in_ESP32’ for the description of this issue. As a workaround, call adc_power_acquire() in the app. This will result in higher power consumption (by ~1mA), but will remove the glitches on GPIO36 and GPIO39.

Note

Call adc1_config_width() before the first time this function is called.

Note

For any given channel, adc1_config_channel_atten(channel) must be called before the first time this function is called. Configuring a new channel does not prevent a previously configured channel from being read.

Return

  • -1: Parameter error

  • Other: ADC1 channel reading.

Parameters

  • channel: ADC1 channel to read

esp_err_t adc2_pad_get_io_num(adc2_channel_t channel, gpio_num_t *gpio_num)

Get the GPIO number of a specific ADC2 channel.

Return

  • ESP_OK if success

  • ESP_ERR_INVALID_ARG if channel not valid

Parameters

  • channel: Channel to get the GPIO number

  • gpio_num: output buffer to hold the GPIO number

esp_err_t adc2_config_channel_atten(adc2_channel_t channel, adc_atten_t atten)

Configure the ADC2 channel, including setting attenuation.

The default ADC voltage is for attenuation 0 dB and listed in the table below. By setting higher attenuation it is possible to read higher voltages.

Due to ADC characteristics, most accurate results are obtained within the “suggested range” shown in the following table. 

+----------+-------------+-----------------+
|          | attenuation | suggested range |
|    SoC   |     (dB)    |      (mV)       |
+==========+=============+=================+
|          |       0     |    100 ~  950   |
|          +-------------+-----------------+
|          |       2.5   |    100 ~ 1250   |
|   ESP32  +-------------+-----------------+
|          |       6     |    150 ~ 1750   |
|          +-------------+-----------------+
|          |      11     |    150 ~ 2450   |
+----------+-------------+-----------------+
|          |       0     |      0 ~  750   |
|          +-------------+-----------------+
|          |       2.5   |      0 ~ 1050   |
| ESP32-S2 +-------------+-----------------+
|          |       6     |      0 ~ 1300   |
|          +-------------+-----------------+
|          |      11     |      0 ~ 2500   |
+----------+-------------+-----------------+

For maximum accuracy, use the ADC calibration APIs and measure voltages within these recommended ranges.

Note

This function also configures the input GPIO pin mux to connect it to the ADC2 channel. It must be called before calling adc2_get_raw() for this channel.

Note

For any given channel, this function must be called before the first time adc2_get_raw() is called for that channel.

Return

  • ESP_OK success

  • ESP_ERR_INVALID_ARG Parameter error

Parameters

  • channel: ADC2 channel to configure

  • atten: Attenuation level

esp_err_t adc2_get_raw(adc2_channel_t channel, adc_bits_width_t width_bit, int *raw_out)

Take an ADC2 reading on a single channel.

Note

ESP32: When the power switch of SARADC1, SARADC2, HALL sensor and AMP sensor is turned on, the input of GPIO36 and GPIO39 will be pulled down for about 80ns. When enabling power for any of these peripherals, ignore input from GPIO36 and GPIO39. Please refer to section 3.11 of ‘ECO_and_Workarounds_for_Bugs_in_ESP32’ for the description of this issue. As a workaround, call adc_power_acquire() in the app. This will result in higher power consumption (by ~1mA), but will remove the glitches on GPIO36 and GPIO39.

Note

ESP32: For a given channel, adc2_config_channel_atten() must be called before the first time this function is called. If Wi-Fi is started via esp_wifi_start(), this function will always fail with ESP_ERR_TIMEOUT.

Note

ESP32-S2: ADC2 support hardware arbiter. The arbiter is to improve the use efficiency of ADC2. After the control right is robbed by the high priority, the low priority controller will read the invalid ADC2 data. Default priority: Wi-Fi > RTC > Digital;

Return

  • ESP_OK if success

  • ESP_ERR_TIMEOUT ADC2 is being used by other controller and the request timed out.

  • ESP_ERR_INVALID_STATE The controller status is invalid. Please try again.

Parameters

  • channel: ADC2 channel to read

  • width_bit: Bit capture width for ADC2

  • raw_out: the variable to hold the output data.

esp_err_t adc_vref_to_gpio(adc_unit_t adc_unit, gpio_num_t gpio)

Output ADC1 or ADC2’s reference voltage to adc2_channe_t’s IO.

This function routes the internal reference voltage of ADCn to one of ADC2’s channels. This reference voltage can then be manually measured for calibration purposes.

Note

ESP32 only supports output of ADC2’s internal reference voltage.

Return

  • ESP_OK: v_ref successfully routed to selected GPIO

  • ESP_ERR_INVALID_ARG: Unsupported GPIO

Parameters

  • [in] adc_unit: ADC unit index

  • [in] gpio: GPIO number (Only ADC2’s channels IO are supported)

esp_err_t adc2_vref_to_gpio(gpio_num_t gpio)

Output ADC2 reference voltage to adc2_channe_t’s IO.

This function routes the internal reference voltage of ADCn to one of ADC2’s channels. This reference voltage can then be manually measured for calibration purposes.

Return

  • ESP_OK: v_ref successfully routed to selected GPIO

  • ESP_ERR_INVALID_ARG: Unsupported GPIO

Parameters

  • [in] gpio: GPIO number (ADC2’s channels are supported)

esp_err_t adc_digi_controller_config(const adc_digi_config_t *config)

Setting the digital controller.

Return

  • ESP_ERR_INVALID_STATE Driver state is invalid.

  • ESP_ERR_INVALID_ARG If the combination of arguments is invalid.

  • ESP_OK On success

Parameters

esp_err_t adc_digi_initialize(const adc_digi_init_config_t *init_config)

Initialize the Digital ADC.

Return

  • ESP_ERR_INVALID_ARG If the combination of arguments is invalid.

  • ESP_ERR_NOT_FOUND No free interrupt found with the specified flags

  • ESP_ERR_NO_MEM If out of memory

  • ESP_OK On success

Parameters

  • init_config: Pointer to Digital ADC initilization config. Refer to adc_digi_init_config_t.

esp_err_t adc_digi_start(void)

Start the Digital ADC and DMA peripherals. After this, the hardware starts working.

Return

  • ESP_ERR_INVALID_STATE Driver state is invalid.

  • ESP_OK On success

esp_err_t adc_digi_stop(void)

Stop the Digital ADC and DMA peripherals. After this, the hardware stops working.

Return

  • ESP_ERR_INVALID_STATE Driver state is invalid.

  • ESP_OK On success

esp_err_t adc_digi_read_bytes(uint8_t *buf, uint32_t length_max, uint32_t *out_length, uint32_t timeout_ms)

Read bytes from Digital ADC through DMA.

Return

  • ESP_ERR_INVALID_STATE Driver state is invalid. Usually it means the ADC sampling rate is faster than the task processing rate.

  • ESP_ERR_TIMEOUT Operation timed out

  • ESP_OK On success

Parameters

  • [out] buf: Buffer to read from ADC.

  • [in] length_max: Expected length of data read from the ADC.

  • [out] out_length: Real length of data read from the ADC via this API.

  • [in] timeout_ms: Time to wait for data via this API, in millisecond.

esp_err_t adc_digi_deinitialize(void)

Deinitialize the Digital ADC.

Return

  • ESP_ERR_INVALID_STATE Driver state is invalid.

  • ESP_OK On success

Structures

struct adc_digi_init_config_s

Digital ADC DMA configuration.

Public Members

uint32_t max_store_buf_size

Max length of the converted data that driver can store before they are processed. When this length is reached, driver will dump out all the old data and start to store them again.

uint32_t conv_num_each_intr

Bytes of data that can be converted in 1 interrupt.

uint32_t adc1_chan_mask

Channel list of ADC1 to be initialized.

uint32_t adc2_chan_mask

Channel list of ADC2 to be initialized.

Macros

ADC_ATTEN_0db

ADC rtc controller attenuation option.

Note

This definitions are only for being back-compatible

ADC_ATTEN_2_5db
ADC_ATTEN_6db
ADC_ATTEN_11db
ADC_WIDTH_BIT_DEFAULT

The default (max) bit width of the ADC of current version. You can also get the maximum bitwidth by SOC_ADC_MAX_BITWIDTH defined in soc_caps.h.

ADC_WIDTH_9Bit
ADC_WIDTH_10Bit
ADC_WIDTH_11Bit
ADC_WIDTH_12Bit
ADC_MAX_DELAY

Digital ADC DMA read max timeout value, it may make the adc_digi_read_bytes block forever if the OS supports.

Type Definitions

typedef struct adc_digi_init_config_s adc_digi_init_config_t

Digital ADC DMA configuration.

Enumerations

enum adc1_channel_t

Values:

ADC1_CHANNEL_0 = 0

ADC1 channel 0 is GPIO0

ADC1_CHANNEL_1

ADC1 channel 1 is GPIO1

ADC1_CHANNEL_2

ADC1 channel 2 is GPIO2

ADC1_CHANNEL_3

ADC1 channel 3 is GPIO3

ADC1_CHANNEL_4

ADC1 channel 4 is GPIO4

ADC1_CHANNEL_MAX
enum adc2_channel_t

Values:

ADC2_CHANNEL_0 = 0

ADC2 channel 0 is GPIO5

ADC2_CHANNEL_MAX
enum adc_i2s_encode_t

ADC digital controller encode option.

Values:

ADC_ENCODE_12BIT

ADC to DMA data format, , [15:12]-channel [11:0]-12 bits ADC data

ADC_ENCODE_11BIT

ADC to DMA data format, [15]-unit, [14:11]-channel [10:0]-11 bits ADC data

ADC_ENCODE_MAX

ADC Calibration

Functions

esp_err_t esp_adc_cal_check_efuse(esp_adc_cal_value_t value_type)

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

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)

Parameters

  • value_type: Type of calibration value (ESP_ADC_CAL_VAL_EFUSE_VREF or ESP_ADC_CAL_VAL_EFUSE_TP)

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)

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.

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

Parameters

  • [in] adc_num: ADC to characterize (ADC_UNIT_1 or ADC_UNIT_2)

  • [in] atten: Attenuation to characterize

  • [in] bit_width: Bit width configuration of ADC

  • [in] default_vref: Default ADC reference voltage in mV (Only in ESP32, used if eFuse values is not available)

  • [out] chars: Pointer to empty structure used to store ADC characteristics

uint32_t esp_adc_cal_raw_to_voltage(uint32_t adc_reading, const esp_adc_cal_characteristics_t *chars)

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())

Return

Voltage in mV

Parameters

  • [in] adc_reading: ADC reading

  • [in] chars: Pointer to initialized structure containing ADC characteristics

esp_err_t esp_adc_cal_get_voltage(adc_channel_t channel, const esp_adc_cal_characteristics_t *chars, uint32_t *voltage)

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())

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

Parameters

  • [in] channel: ADC Channel to read

  • [in] chars: Pointer to initialized ADC characteristics structure

  • [out] voltage: Pointer to store converted voltage

Structures

struct esp_adc_cal_characteristics_t

Structure storing characteristics of an ADC.

Note

Call esp_adc_cal_characterize() to initialize the structure

Public Members

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)

Enumerations

enum esp_adc_cal_value_t

Type of calibration value used in characterization.

Values:

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

GPIO Lookup Macros

Macros

ADC1_GPIO1_CHANNEL
ADC1_CHANNEL_0_GPIO_NUM
ADC1_GPIO2_CHANNEL
ADC1_CHANNEL_1_GPIO_NUM
ADC1_GPIO3_CHANNEL
ADC1_CHANNEL_2_GPIO_NUM
ADC1_GPIO4_CHANNEL
ADC1_CHANNEL_3_GPIO_NUM
ADC1_GPIO5_CHANNEL
ADC1_CHANNEL_4_GPIO_NUM
ADC2_GPIO5_CHANNEL
ADC2_CHANNEL_0_GPIO_NUM

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