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* atmel-samd: Remove ASF3. This will break builds. * atmel-samd: Add ASF4 for the SAMD21 and SAMD51. * Introduce the supervisor concept to facilitate porting. The supervisor is the code which runs individual MicroPython VMs. By splitting it out we make it more consistent and easier to find. This also adds very basic SAMD21 and SAMD51 support using the supervisor. Only the REPL currently works. This begins the work for #178.
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We love CircuitPython and would love to see it come to more microcontroller
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platforms. With 3.0 we've reworked CircuitPython to make it easier than ever to
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add support. While there are some major differences between ports, this page
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covers the similarities that make CircuitPython what it is and how that core
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fits into a variety of microcontrollers.
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Architecture
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============
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There are three core pieces to CircuitPython:
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The first is the Python VM that the awesome MicroPython devs have created.
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These VMs are written to be portable so there is not much needed when moving to
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a different microcontroller, especially if it is ARM based.
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The second is the infrastructure around those VMs which provides super basic
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operating system functionality such as initializing hardware, running USB,
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prepping file systems and automatically running user code on boot. In
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CircuitPython we've dubbed this component the supervisor because it monitors
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and facilitates the VMs which run user Python code. Porting involves the
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supervisor because many of the tasks it does while interfacing with the
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hardware. Once its going though, the REPL works and debugging can migrate to a
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Python based approach rather than C.
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The third core piece is the plethora of low level APIs that CircuitPython
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provides as the foundation for higher level libraries including device drivers.
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These APIs are called from within the running VMs through the Python interfaces
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defined in ``shared-bindings``. These bindings rely on the underlying
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``common_hal`` C API to implement the functionality needed for the Python API.
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By splitting the two, we work to ensure standard functionality across which
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means that libraries and examples apply across ports with minimal changes.
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Porting
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=======
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Step 1: Getting building
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------------------------
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The first step to porting to a new microcontroller is getting a build running.
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The primary goal of it should be to get ``main.c`` compiling with the assistance
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of the ``supervisor/supervisor.mk`` file. Port specific code should be isolated
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to the port's directory (in the top level until the ``ports`` directory is
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present). This includes the Makefile and any C library resources. Make sure
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these resources are compatible with the MIT License of the rest of the code!
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Step 2: Init
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--------------
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Once your build is setup, the next step should be to get your clocks going as
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you expect from the supervisor. The supervisor calls `port_init` to allow for
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initialization at the beginning of main. This function also has the ability to
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request a safe mode state which prevents the supervisor from running user code
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while still allowing access to the REPL and other resources.
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The core port initialization and reset methods are defined in
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``supervisor/port.c`` and should be the first to be implemented. Its required
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that they be implemented in the ``supervisor`` directory within the port
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directory. That way, they are always in the expected place.
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The supervisor also uses three linker variables, ``_ezero``, ``_estack`` and
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``_ebss`` to determine memory layout for stack overflow checking.
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Step 3: REPL
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------------
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Getting the REPL going is a huge step. It involves a bunch of initialization to
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be done correctly and is a good sign you are well on your porting way. To get
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the REPL going you must implement the functions and definitions from
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``supervisor/serial.h`` with a corresponding ``supervisor/serial.c`` in the port
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directory. This involves sending and receiving characters over some sort of
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serial connection. It could be UART or USB for example.
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