micropython/docs/library/pyb.ADC.rst

.. currentmodule:: pyb
.. _pyb.ADC:

class ADC -- analog to digital conversion
=========================================

Usage::

    import pyb

    adc = pyb.ADC(pin)                  # create an analog object from a pin
    val = adc.read()                    # read an analog value

    adc = pyb.ADCAll(resolution)        # create an ADCAll object
    adc = pyb.ADCAll(resolution, mask)  # create an ADCAll object for selected analog channels
    val = adc.read_channel(channel)     # read the given channel
    val = adc.read_core_temp()          # read MCU temperature
    val = adc.read_core_vbat()          # read MCU VBAT
    val = adc.read_core_vref()          # read MCU VREF
    val = adc.read_vref()               # read MCU supply voltage

 
Constructors
------------

.. class:: pyb.ADC(pin)

   Create an ADC object associated with the given pin.
   This allows you to then read analog values on that pin.

Methods
-------

.. method:: ADC.read()

   Read the value on the analog pin and return it.  The returned value
   will be between 0 and 4095.

.. method:: ADC.read_timed(buf, timer)

   Read analog values into ``buf`` at a rate set by the ``timer`` object.

   ``buf`` can be bytearray or array.array for example.  The ADC values have
   12-bit resolution and are stored directly into ``buf`` if its element size is
   16 bits or greater.  If ``buf`` has only 8-bit elements (eg a bytearray) then
   the sample resolution will be reduced to 8 bits.

   ``timer`` should be a Timer object, and a sample is read each time the timer
   triggers.  The timer must already be initialised and running at the desired
   sampling frequency.

   To support previous behaviour of this function, ``timer`` can also be an
   integer which specifies the frequency (in Hz) to sample at.  In this case
   Timer(6) will be automatically configured to run at the given frequency.

   Example using a Timer object (preferred way)::

       adc = pyb.ADC(pyb.Pin.board.X19)    # create an ADC on pin X19
       tim = pyb.Timer(6, freq=10)         # create a timer running at 10Hz
       buf = bytearray(100)                # creat a buffer to store the samples
       adc.read_timed(buf, tim)            # sample 100 values, taking 10s

   Example using an integer for the frequency::

       adc = pyb.ADC(pyb.Pin.board.X19)    # create an ADC on pin X19
       buf = bytearray(100)                # create a buffer of 100 bytes
       adc.read_timed(buf, 10)             # read analog values into buf at 10Hz
                                           #   this will take 10 seconds to finish
       for val in buf:                     # loop over all values
           print(val)                      # print the value out

   This function does not allocate any heap memory. It has blocking behaviour:
   it does not return to the calling program until the buffer is full.

.. method:: ADC.read_timed_multi((adcx, adcy, ...), (bufx, bufy, ...), timer)

   This is a static method. It can be used to extract relative timing or
   phase data from multiple ADC's.

   It reads analog values from multiple ADC's into buffers at a rate set by
   the *timer* object. Each time the timer triggers a sample is rapidly
   read from each ADC in turn.

   ADC and buffer instances are passed in tuples with each ADC having an
   associated buffer. All buffers must be of the same type and length and
   the number of buffers must equal the number of ADC's.

   Buffers can be ``bytearray`` or ``array.array`` for example. The ADC values
   have 12-bit resolution and are stored directly into the buffer if its element
   size is 16 bits or greater.  If buffers have only 8-bit elements (eg a
   ``bytearray``) then the sample resolution will be reduced to 8 bits.

   *timer* must be a Timer object. The timer must already be initialised
   and running at the desired sampling frequency.

   Example reading 3 ADC's::

       adc0 = pyb.ADC(pyb.Pin.board.X1)    # Create ADC's
       adc1 = pyb.ADC(pyb.Pin.board.X2)
       adc2 = pyb.ADC(pyb.Pin.board.X3)
       tim = pyb.Timer(8, freq=100)        # Create timer
       rx0 = array.array('H', (0 for i in range(100))) # ADC buffers of
       rx1 = array.array('H', (0 for i in range(100))) # 100 16-bit words
       rx2 = array.array('H', (0 for i in range(100)))
       # read analog values into buffers at 100Hz (takes one second)
       pyb.ADC.read_timed_multi((adc0, adc1, adc2), (rx0, rx1, rx2), tim)
       for n in range(len(rx0)):
           print(rx0[n], rx1[n], rx2[n])

   This function does not allocate any heap memory. It has blocking behaviour:
   it does not return to the calling program until the buffers are full.

   The function returns ``True`` if all samples were acquired with correct
   timing. At high sample rates the time taken to acquire a set of samples
   can exceed the timer period. In this case the function returns ``False``,
   indicating a loss of precision in the sample interval. In extreme cases
   samples may be missed.

   The maximum rate depends on factors including the data width and the
   number of ADC's being read. In testing two ADC's were sampled at a timer
   rate of 210kHz without overrun. Samples were missed at 215kHz.  For three
   ADC's the limit is around 140kHz, and for four it is around 110kHz.
   At high sample rates disabling interrupts for the duration can reduce the
   risk of sporadic data loss.

The ADCAll Object
-----------------

Instantiating this changes all masked ADC pins to analog inputs. The preprocessed MCU temperature,
VREF and VBAT data can be accessed on ADC channels 16, 17 and 18 respectively.
Appropriate scaling is handled according to reference voltage used (usually 3.3V).
The temperature sensor on the chip is factory calibrated and allows to read the die temperature
to +/- 1 degree centigrade. Although this sounds pretty accurate, don't forget that the MCU's internal
temperature is measured. Depending on processing loads and I/O subsystems active the die temperature
may easily be tens of degrees above ambient temperature. On the other hand a pyboard woken up after a
long standby period will show correct ambient temperature within limits mentioned above.

The ``ADCAll`` ``read_core_vbat()``, ``read_vref()`` and ``read_core_vref()`` methods read
the backup battery voltage, reference voltage and the (1.21V nominal) reference voltage using the
actual supply as a reference. All results are floating point numbers giving direct voltage values.

``read_core_vbat()`` returns the voltage of the backup battery. This voltage is also adjusted according
to the actual supply voltage. To avoid analog input overload the battery voltage is measured
via a voltage divider and scaled according to the divider value. To prevent excessive loads
to the backup battery, the voltage divider is only active during ADC conversion.

``read_vref()`` is evaluated by measuring the internal voltage reference and backscale it using
factory calibration value of the internal voltage reference. In most cases the reading would be close
to 3.3V. If the pyboard is operated from a battery, the supply voltage may drop to values below 3.3V.
The pyboard will still operate fine as long as the operating conditions are met. With proper settings
of MCU clock, flash access speed and programming mode it is possible to run the pyboard down to
2 V and still get useful ADC conversion.

It is very important to make sure analog input voltages never exceed actual supply voltage.

Other analog input channels (0..15) will return unscaled integer values according to the selected
precision.

To avoid unwanted activation of analog inputs (channel 0..15) a second parameter can be specified.
This parameter is a binary pattern where each requested analog input has the corresponding bit set.
The default value is 0xffffffff which means all analog inputs are active. If just the internal
channels (16..18) are required, the mask value should be 0x70000.

Example::

    adcall = pyb.ADCAll(12, 0x70000) # 12 bit resolution, internal channels
    temp = adcall.read_core_temp()