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esp_adc_cal/Add eFuse functionality

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This commit updates the esp_adc_cal ocmponent to support new calibration methods
which utilize calibratoin values stored in eFuse. This commit includes LUT mode
This commit is contained in:
Darian Leung
2018-01-24 17:22:13 +08:00
parent ca3faa6186
commit 73cdfbfe79
12 changed files with 1018 additions and 339 deletions
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83
docs/api-reference/peripherals/adc.rst
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@@ -85,55 +85,58 @@ The value read in both these examples is 12 bits wide (range 0-4095).
.. _adc-api-adc-calibration:
Minimizing Noise
----------------
The ESP32 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.
.. figure:: ../../_static/adc-noise-graph.jpg
:align: center
:alt: ADC noise mitigation
Graph illustrating noise mitigation using capacitor and multisampling of 64 samples.
ADC Calibration
---------------
The :component_file:`esp_adc_cal/include/esp_adc_cal.h` 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 1100 mV however the reference voltage of different ESP32s can range from 1000 mV to 1200 mV.
The :component_file:`esp_adc_cal/include/esp_adc_cal.h` API provides functions to correct for differences in measured voltages caused by non-ideal ADC reference voltages and non-linear characteristics (only applicable at 11dB attenuation). The ideal ADC reference voltage is 1100mV, however true reference voltages can range from 1000mV to 1200 mV amongst ESP32s.
Correcting the measured voltage using this API involves referencing a lookup table of voltages. The voltage obtained from the lookup table is then scaled and shifted by a gain and offset factor that is based on the ADC's reference voltage. This is done with function :cpp:func:`esp_adc_cal_get_characteristics`.
The reference voltage of the ADCs can be routed to certain GPIOs and measured manually using the ADC driver’s :cpp:func:`adc2_vref_to_gpio` function.
Example of Reading Calibrated Values
------------------------------------
Reading the ADC and obtaining a result in mV::
#include <driver/adc.h>
#include <esp_adc_cal.h>
.. figure:: ../../_static/adc-vref-graph.jpg
:align: center
:alt: ADC reference voltage comparison
...
#define V_REF 1100 // ADC reference voltage
// Configure ADC
adc1_config_width(ADC_WIDTH_12Bit);
adc1_config_channel_atten(ADC1_CHANNEL_6, ADC_ATTEN_11db);
// Calculate ADC characteristics i.e. gain and offset factors
esp_adc_cal_characteristics_t characteristics;
esp_adc_cal_get_characteristics(V_REF, ADC_ATTEN_DB_11, ADC_WIDTH_BIT_12, &characteristics);
// Read ADC and obtain result in mV
uint32_t voltage = adc1_to_voltage(ADC1_CHANNEL_6, &characteristics);
printf("%d mV\n",voltage);
Routing ADC reference voltage to GPIO, so it can be manually measured and entered in function :cpp:func:`esp_adc_cal_get_characteristics`::
Graph illustrating effect of differing reference voltages on the ADC voltage curve.
#include <driver/adc.h>
#include <driver/gpio.h>
#include <esp_err.h>
Correcting ADC readings using this API involves characterizing one of the ADCs at a given attenuation to obtain a characteristics curve (ADC-Voltage curve). The characteristics curve is used to convert ADC readings to voltages in mV. Representation of characteristics curve will differ under **Linear Mode** and **Lookup Table Mode**. Calculation of the characteristics curve is based on calibration values which can be stored in eFuse or provided by the user.
...
.. _linear-mode:
esp_err_t status = adc2_vref_to_gpio(GPIO_NUM_25);
if (status == ESP_OK){
printf("v_ref routed to GPIO\n");
}else{
printf("failed to route v_ref\n");
}
Linear Mode
^^^^^^^^^^^
An example of using the ADC driver and obtaining calibrated measurements is available in esp-idf: :example:`peripherals/adc`
Linear Mode characterization will generate a linear characteristics curve in the form of ``y = coeff_a * x + coeff_b``. The linear curve will map ADC readings to a voltage in mV. The calibration values which the calculation of ``coeff_a`` and ``coeff_b`` can be based on will be prioritized in the following order
1. Two Point values
2. eFuse Vref
3. Default Vref
.. _lut-mode:
Lookup Table Mode
^^^^^^^^^^^^^^^^^
Lookup Table (LUT) Mode characterization utilizes a LUT to represent an ADC’s characteristics curve. Each LUT consists of a High and Low reference curve which are representative of the characteristic curve of ESP32s with a Vref of 1200mV and 1000mV respectively. Converting an ADC reading to a voltage using a LUT involves interpolating between the High and Low curves based on an ESP32’s true Vref. The true Vref can be read from eFuse (eFuse Vref) or provided by the user (Default Vref) if eFuse Vref is unavailable.
Calibration Values
^^^^^^^^^^^^^^^^^^
Calibration values are used during the characterization processes, and there are currently three possible types of calibration values. Note the availability of these calibration values will depend on the type of version of the ESP32 chip/module.
The **Two Point** calibration values represent each of the ADCs’ readings at 150mV and 850mV. The values are burned into eFuse during factory calibration and are used in Linear Mode to generate a linear characteristics curve. Note that the Two Point values are only available on some versions of ESP32 chips/modules
The **eFuse Vref** value represents the true reference voltage of the ADCs and can be used in both Linear and LUT modes. This value is measured and burned into eFuse during factory calibration. Note that eFuse Vref is not available on older variations of ESP32 chips/modules
**Default Vref** is an estimate of the ADC reference voltage provided by the user as a parameter during characterization. If Two Point or eFuse Vref values are unavailable, Default Vref will be used. To obtain an estimate of an ESP32 modules Vref, users can call the function ``adc2_vref_to_gpio()`` to route the ADC refernce voltage to a GPIO and measure it manually.
GPIO Lookup Macros
------------------