* Always init SPI to 250k to start for SD cards.
* Add ability to configure byte written during read.
* Add ability to read and write to portions of buffers like existing I2C API.
You can either set it once up front, or set variable_frequency on custruction to
indicate that the frequency must be able to change. This informs whether a timer
can be shared amongst pins.
This also adds persistent clock calibration on atmel-samd. Once the device has
synced its clock frequency over USB it will remember that config value until USB
is used again. This helps ensure the clock frequency is similar on and off USB.
Lastly, this also corrects time.sleep() when on USB by correcting the tick counter.
This prevents corrupting previous functional objects by stealing their pins
out from under them. It prevents this by ensuring that pins are in default
state before claiming them. It also verifies pins are released correctly and
reset on soft reset.
Fixes#4, instantiating a second class will fail.
Fixes#29, pins are now reset too.
Docs are here: http://tannewt-micropython.readthedocs.io/en/microcontroller/
It differs from upstream's machine in the following ways:
* Python API is identical across ports due to code structure. (Lives in shared-bindings)
* Focuses on abstracting common functionality (AnalogIn) and not representing structure (ADC).
* Documentation lives with code making it easy to ensure they match.
* Pin is split into references (board.D13 and microcontroller.pin.PA17) and functionality (DigitalInOut).
* All nativeio classes claim underlying hardware resources when inited on construction, support Context Managers (aka with statements) and have deinit methods which release the claimed hardware.
* All constructors take pin references rather than peripheral ids. Its up to the implementation to find hardware or throw and exception.
This commit also introduces a new shared-bindings directory which is used to store the common Python -> C binding code. By having a shared directory we can ensure that the Python API across ports is the same. Each port will have a corresponding common-hal directory which provides definitions for the C api used in the shared-bindings code. That way the compiler can enforce the C api.
To migrate to this new shared API create a common-hal directory within your port and change the Makefile to compile both the shared-bindings and common-hal files. See atmel-samd/Makefile SRC_BINDINGS for an example.
