Usage:

import machine

adc = machine.ADC()             # create an ADC object
apin = adc.channel(pin='GP3')   # create an analog pin on GP3
val = apin()                    # read an analog value

.. method:: adc.channel(id, \*, pin)

   Create an analog pin. If only channel ID is given, the correct pin will
   be selected. Alternatively, only the pin can be passed and the correct
   channel will be selected. Examples::

      # all of these are equivalent and enable ADC channel 1 on GP3
      apin = adc.channel(1)
      apin = adc.channel(pin='GP3')
      apin = adc.channel(id=1, pin='GP3')

.. method:: adc.init()

   Enable the ADC block.

.. method:: adc.deinit()

   Disable the ADC block.

ADC channels can be connected to internal points of the MCU or to GPIO pins. ADC channels are created using the ADC.channel method.

.. method:: adcchannel()

   Fast method to read the channel value.

.. method:: adcchannel.value()

   Read the channel value.

.. method:: adcchannel.init()

   Re-init (and effectively enable) the ADC channel.

.. method:: adcchannel.deinit()

   Disable the ADC channel.