Boris Lovosevic edited this page Sep 6, 2018 · 19 revisions

machine Module

Class ADC


ESP32 integrates two 12-bit SAR (Successive Approximation Register) ADCs (Analog to Digital Converters) and supports measurements on 18 channels (analog enabled pins).
The ADC driver API supports:

  • ADC1 (8 channels, attached to GPIOs 32 - 39)
  • ADC2 (10 channels, attached to GPIOs 0, 2, 4, 12 - 15 and 25 - 27).

However, there’re some restrictions for the application to use ADC2:

  • The application can use ADC2 only when Wi-Fi driver is not started, since the ADC is also used by the Wi-Fi driver, which has higher priority.
  • Some of the ADC2 pins are used as strapping pins (GPIO 0, 2, 15), so they cannot be used freely.
    For examples, for official Develop Kits:
    • ESP32 Core Board V2 / ESP32 DevKitC: GPIO 0 cannot be used due to external auto program circuits.
    • ESP-WROVER-KIT V3: GPIO 0, 2, 4 and 15 cannot be used due to external connections for different purposes.


This class includes full support for using ESP32 ADC peripheral.
Functions are added to set the attenuation and to calibrate the ADC.

ESP32 ADC input voltage range depends on attenuation setting:

Attenuation Voltage range
0 dB 1.1 V
2.5 dB 1.5 V
6 dB 2.2 V
11 dB 3.9 V

Due to ADC characteristics, most accurate results are obtained within the following approximate voltage ranges:

  • 0 dB attenuaton (ATTN_0DB) between 100 and 950mV
  • 2.5 dB attenuation (ATTN_2_5DB) between 100 and 1250mV
  • 6 dB attenuation (ATTN_6DB) between 150 to 1750mV
  • 11 dB attenuation (ATTN_11DB) between 150 to 2450mV

Note: Using ADC2 works, but as support for ADC2 in esp-idf is not fully functional, some error logs may br printed while using ADC2.


Create the adc instance object


adc = machine.ADC(pin [,unit=1])

pin argument defines the gpio which will will be used as adc input.
It can be given as integer pin number, or machine.Pin(n) object.
For ADC1 only GPIOs 32-39 can be used as ADC inputs.
For ADC2 gpios 4, 0, 2, 15, 13, 12, 14, 27, 25, 26 can be used as ADC inputs.

Be very carefull not to use the pins used by ESP32, especially the bootstrapping pins.

Use machine.ADC.HALL constant to select ESP32 Hall sensor as input.
If Hall sensor is used, gpio#36 and gpio#39 cannot be used as adc inputs at the same time.

Optional unit argument select ESP32 ADC unit for this instance. Values 1 (ADC1, default) or 2 (ADC2) can be selected.

Initially, the attenuation is set to 0 dB, and resolution to 12 bits.

>>> import machine
>>> adc=machine.ADC(34)
>>> adc
ADC(Pin(34): unit=ADC1, chan=6, width=12 bits, atten=0dB (1.1V), Vref=1100 mV)
>>> adc2=machine.ADC(25, unit=2)
>>> adc2
ADC(Pin(25): unit=ADC2, chan=8, width=12 bits, atten=0dB (1.1V), Vref=1100 mV)
>>> 

adc.deinit()

Deinitialize the adc, free the pin used.


adc.atten(value)

Set the attenuation value.

The following attenuation constants can be used for value:
ATTN_0DB - attenuation 0 dB (range: 0 - 1.1 V)
ATTN_2_5DB - attenuation 2.5 dB (range 0 - 1.5 V)
ATTN_6DB - attenuation 6 dB (range: 0 - 2.5 V)
ATTN_11DB - attenuation 11 dB (range: 0 - 3.9 V'

>>> adc.atten(adc.ATTN_11DB)
>>> adc
ADC(Pin(34): unit=ADC1, chan=6, width=12 bits, atten=11dB (3.9V), Vref=1100 mV)
>>> 

adc.width(value)

Configure ADC capture width..

The following constants can be used for value:
WIDTH_9BIT - capture width is 9Bit
WIDTH_10BIT - capture width is 10Bit
WIDTH_11BIT - capture width is 11Bit
WIDTH_12BIT - capture width is 12Bit

>>> adc.width(adc.WIDTH_10BIT)
>>> adc
ADC(Pin(34): unit=ADC1, chan=6, width=10 bits, atten=11dB (3.9V), Vref=1100 mV)
>>> 

machine.ADC.vref([vref=mv] [,] [vref_topin=pin])

Get or set ADC refference voltage and/or reference output pin.

The gain and offset factors of an ESP32 module's ADC are calculated using the reference voltage and the Gain and Offset curves provided in the lookup tables.
Nominal voltage refernce is 1100 mV, and can be adjusted to compensate for ESP32 chip differences.
The reference voltage can be set to any value in range 1000 ~ 1200 mV.
The internal ESP32 reference can be routed to gpio to be measured and the value used to set the reference voltage for the specific ESP32 chip.

To route the reference voltage to the gpio set the argument vref_topin to the gpio pin number to be used as output.
Valid gpios are only 25, 26 and 27.

Returns tuple containing the current refference voltage in mV and selected output pin.

# Get the reference voltage
>>> machine.ADC.vref()
(1100, 0)
# Route the internal reference voltage to GPIO#25
>>> machine.ADC.vref(vref_topin=25)
# You can now measure the voltage on GPIO#25 with the voltmeter

# Set the reference voltage to measured value in mV
# This value will be used to get more precise readings
>>> machine.ADC.vref(vref=1096)
(1096, 0)

adc.read()

Read the ADC value as voltage (in mV)

Calibrated read is used.
For hall sensor readings, the raw value is returned.


Data collection


adc.collect(freq [,len=0] [,readmv=False] [,data=None] [,callback=None] [,wait=False])

Collect ADC data in background using the ESP32 timer interrupt.

Arg Description
freq the frequency at which the data is collected
valid range: 0.001 - 18000 Hz
readmv optional, default False; collect raw ADC value (False) or calibrated values in mV (True)
len optional, default: 0; number of samples to collect
If not collecting to an array, must be > 0
If collecting to an array, can be omited, if set, only the len samples will be collected
data optional, default: None; Collect data into a given array object
If the array is of type 'H' or 'h', 16-bit values will be collected (the resolution depends on ADC width)
If the array is of type 'B', 8-bit values will be collected (the ADC value will be converted to 8-bit value)
callback optional, default: None; callback function to be executed after the collection is finished
Function prototype: adc_cb_func(adc_obj)
wait optional, default: False; wait until the collection is finished

During the data collection some statistical data are collected: minimum, maximum, average and rms.
If the data array is not given, only the statictical data are available after the collection.


adc.read_timed(data, freq [,nsamples=-1] [,byte=True] [,data=None] [,wait=False] [,callback=None])

Read the ADC data in background using the ESP32 I2S peripheral.
Mostly used to capture the audio data to an array of file.
The ADC is configured to 12 bit and 11 dB attenuation by I2S, you may need an amplifier to get the required input range.

Arg Description
data array object or filename>br>If an array object is given, read data into array object
If the array is of type 'H' or 'h', 16-bit values will be saved (the resolution depends on ADC width)
If the array is of type 'B', 8-bit values will be saved (the ADC value will be converted to 8-bit value)
If the filename is given, the ADC data will be saved to file
freq the frequency at which the data is collected
valid range: 5000 - 500000 Hz
nsamples number of ADC samples to collect
mandatory if saving to file
optional if saving to an array, if set, only the nsamples samples will be saved
byte optional, default: True; only used if saving to file.
If True, save ADC values as 8-bit values, if False save as 16-bit values
wait optional, default: False; wait until the collection is finished
callback optional, default: None; callback function to be executed after the collection is finished
Function prototype: adc_cb_func(adc_obj)

Warning: Saving data to file on internal file system can be too slow and not all the data will be saved.
Saving data to the file on SD Card usually works without issues.


adc.stopcollect()

Stop previously started collect or read_timed operation running in background.


adc.progress()

Get the status of the previously started collect or read_timed operation.
Returns the 4-item tuple:
(active, collected_len, total_len, elapsed_time)
active True if operation is not yet finished, False if finished
collected_len number of collected values, less than total_len if not yet finished
total_len total number of values that will be collected
elapsed_time elapsed collection time in micro seconds


adc.collected()

Returns the 4-item tuple of statistical data collected by adc.collect() function:
(minimum, maximum, average, rms)

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