Boards
======

Support for a board is loaded by including ``feather/feather.h`` - this
will pull in support for the appropriate board. The boards that are 
currently supported are

* Feather M0-based boards; this has been tested with the Feather M0 basic
  and the Feather M0 with RFM95 LoRa radio.
* The Feather M4 Express.

It may work on other M0 or M4-based boards, but these haven't been tested.


The ``Board`` abstract base class
---------------------------------

The ``Board`` class defines a few virtual methods common to all Feathers:

* ``double voltage()`` returns the current battery voltage as read from
  the onboard voltage divider.
* ``setup(int baudrate, bool wait)`` starts the serial console at the given
  baudrate; if ``wait`` is ``true``, it will wait for a serial connection
  before continuing with the boot process. It will also load support for the
  ``%f`` verb in ``printf``, and seed the random number generator.
* ``uint32_t random()`` returns a random number; for boards that support a
  true random number generator, it will be a cryptographically valid random
  number generator; otherwise, it's a best-effort random number generated by
  repeatedly sampling the unused analog pin.
* ``seed`` uses a random number from ``Board::random`` to seed the Arduino
  random number generator.


Feather M0
----------

The ``FeatherM0`` class is instantiated using one of three constructors:

+ The default constructor, ``FeatherM0()``, is a wrapper for 
  ``FeatherM0(INPUT, UNUSED_ANALOG)``.
+ ``FeatherM0(int pin9Mode)`` is a wrapper for ``FeatherM0(pin9Mode, UNUSED_ANALOG)``.
+ ``FeatherM0(int pin9Mode, int unusedAnalog)`` explicitly defines the pin 9 mode
  and the unused analog port.

The Feather M0's voltage divider is on pin 9; in order to be read, it has to be
put in the ``INPUT`` mode. Afterwards, it will be reset to whatever the ``pin9Mode``
is. For example, when using the OLED featherwing (described later), it should be
set to ``INPUT_PULLUP``.

The unused analog pin is used for the random number generator as described above.


Feather M4
----------

The ``FeatherM4`` class is instantiated using a default constructor. It has a
true random number generator that is used for the seeding process and for
returning random numbers.