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112 lines
3.6 KiB
ReStructuredText
112 lines
3.6 KiB
ReStructuredText
Analog to Digital Converter
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===========================
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Overview
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--------
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ESP32 integrates two 12-bit SAR ("Successive Approximation Register") ADCs (Analog to Digital Converters) and supports measurements on 18 channels (analog enabled pins). Some of these pins can be used to build a programmable gain amplifier which is used for the measurement of small
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analog signals.
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The ADC driver API currently only supports ADC1 (9 channels, attached to GPIOs 32-39).
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Taking an ADC reading involves configuring the ADC with the desired precision and attentuation settings, and then calling adc1_get_raw() to read the channel.
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It is also possible to read the internal hall effect sensor via ADC1.
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Application Example
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-------------------
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A full example using the ADC driver and the esp_adc_cal is available in esp-idf: :example:`peripherals/adc`
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Reading voltage on ADC1 channel 0 (GPIO 36)::
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#include <driver/adc.h>
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...
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adc1_config_width(ADC_WIDTH_12Bit);
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adc1_config_channel_atten(ADC1_CHANNEL_0,ADC_ATTEN_0db);
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int val = adc1_get_raw(ADC1_CHANNEL_0);
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Reading the internal hall effect sensor::
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#include <driver/adc.h>
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...
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adc1_config_width(ADC_WIDTH_12Bit);
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int val = hall_sensor_read();
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The value read in both these examples is 12 bits wide (range 0-4095).
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API Reference
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-------------
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.. include:: /_build/inc/adc.inc
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GPIO Lookup Macros
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^^^^^^^^^^^^^^^^^^
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Some useful macros can be used to specified the GPIO number of a ADC channel, or vice versa.
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e.g.
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1. ``ADC1_CHANNEL_0_GPIO_NUM`` is the GPIO number of ADC1 channel 0 (36);
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2. ``ADC1_GPIO32_CHANNEL`` is the ADC1 channel number of GPIO 32 (ADC1 channel 4).
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.. include:: /_build/inc/adc_channel.inc
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ADC Calibration
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===============
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Overview
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--------
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The esp_adc_cal API provides functions to correct for differences in measured voltages caused by non-ideal ADC reference voltages in ESP32s. The ideal ADC reference voltage is 1100mV however the reference voltage of different ESP32s can range from 1000mV to 1200mV.
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Correcting the measured voltage using the esp_adc_cal API involves referencing a lookup table of voltages. The voltage obtained from the lookup table is the scaled and shifted by a gain and offset factor that is based on the ADC's reference voltage.
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The reference voltage of the ADCs can be routed to certain GPIOs and measured manually using the ADC driver’s adc2_vref_to_gpio() function.
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Application Example
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-------------------
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Reading the ADC and obtaining a result in mV::
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#include <driver/adc.h>
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#include <esp_adc_cal.h>
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...
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#define V_REF 1100 //ADC reference voltage
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//Config ADC and characteristics
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adc1_config_width(ADC_WIDTH_12Bit);
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adc1_config_channel_atten(ADC1_CHANNEL_6, ADC_ATTEN_11db);
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//Calculate ADC characteristics i.e. gain and offset factors
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esp_adc_cal_characteristics_t characteristics;
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esp_adc_cal_get_characteristics(V_REF, ADC_ATTEN_11db, ADC_WIDTH_12Bit, &characteristics);
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//Read ADC and obtain result in mV
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uint32_t voltage = adc1_to_voltage(ADC1_CHANNEL_6, &characteristics);
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printf("%d mV\n",voltage);
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Routing ADC reference voltage to GPIO::
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#include <driver/adc.h>
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#include <driver/gpio.h>
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#include <esp_err.h>
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...
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esp_err_t status = adc2_vref_to_gpio(GPIO_NUM_25);
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if (status == ESP_OK){
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printf("v_ref routed to GPIO\n");
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}else{
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printf("failed to route v_ref\n");
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}
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API Reference
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-------------
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.. include:: /_build/inc/esp_adc_cal.inc
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