Guides and videos are available through the [Adafruit Learning System](https://learn.adafruit.com/) under the [CircuitPython category](https://learn.adafruit.com/category/circuitpython) and [MicroPython category](https://learn.adafruit.com/category/micropython). An API reference is also available on [Read the Docs](http://circuitpython.readthedocs.io/en/latest/?).
* The order that files are run and the state thats shared between them. The goal is to clarify the role of each file and make them independent from each other.
*`boot.py` (or `settings.py`) runs only once on start up before USB is initialized. This lays the ground work for configuring USB at startup rather than it being fixed. Since serial is not available, output is written to boot_out.txt.
*`code.py` (or `main.py`) is run after every reload until it finishes or is interrupted. After its done the vm and hardware is reinitialized. **This means you cannot read state from code.py in the REPL anymore.** This was changed to reduce confusion about pins and memory being in use.
* After code.py the REPL can be entered by pressing any key. It no longer shares state with code.py so its a fresh vm.
* Autoreload state will be maintained across reload.
* Adds a safe mode that does not run user code after a hard crash or brown out. The hope is that this will make it easier to fix code that causes nasty crashes by making it available through mass storage after the crash. A reset (the button) is needed after its fixed to get back into normal mode.
* Unified hardware APIs:
[`audioio`](https://circuitpython.readthedocs.io/en/latest/shared-bindings/audioio/__init__.html), [`analogio`](https://circuitpython.readthedocs.io/en/latest/shared-bindings/analogio/__init__.html), [`busio`](https://circuitpython.readthedocs.io/en/latest/shared-bindings/busio/__init__.html), [`digitalio`](https://circuitpython.readthedocs.io/en/latest/shared-bindings/digitalio/__init__.html), [`pulseio`](https://circuitpython.readthedocs.io/en/latest/shared-bindings/pulseio/__init__.html), [`touchio`](https://circuitpython.readthedocs.io/en/latest/shared-bindings/touchio/__init__.html), [`microcontroller`](https://circuitpython.readthedocs.io/en/latest/shared-bindings/microcontroller/__init__.html), [`board`](https://circuitpython.readthedocs.io/en/latest/shared-bindings/board/__init__.html), [`bitbangio`](https://circuitpython.readthedocs.io/en/latest/shared-bindings/bitbangio/__init__.html) (Only available on atmel-samd21 and ESP8266 currently.)
* No module aliasing. (`uos` and `utime` are not available as `os` and `time` respectively.)
* Modules with a CPython counterpart, such as `time`, are strict [subsets](https://circuitpython.readthedocs.io/en/latest/shared-bindings/time/__init__.html) of their [CPython version](https://docs.python.org/3.4/library/time.html?highlight=time#module-time). Therefore, code from CircuitPython is runnable on CPython but not necessarily the reverse.
* tick count is available as [`time.monotonic()`](https://circuitpython.readthedocs.io/en/latest/shared-bindings/time/__init__.html#time.monotonic)
-`shared-bindings` Shared definition of Python modules, their docs and backing C APIs. Ports must implement the C API to support the corresponding module.
-`tools` Various tools, including the pyboard.py module.
### Ports
Ports include the code unique to a microcontroller line and also variations
based on the board.
-`atmel-samd` Support for SAMD21 based boards such as [Arduino Zero](https://www.arduino.cc/en/Main/ArduinoBoardZero), [Adafruit Feather M0 Basic](https://www.adafruit.com/products/2772), and
-`bare-arm` A bare minimum version of MicroPython for ARM MCUs.
-`cc3200` Support for boards based [CC3200](http://www.ti.com/product/CC3200) from TI such as the [WiPy 1.0](https://www.pycom.io/solutions/py-boards/wipy1/).
-`esp8266` Support for boards based on ESP8266 WiFi modules such as the [Adafruit Feather HUZZAH](https://www.adafruit.com/products/2821).
-`minimal` A minimal MicroPython port. Start with this if you want
-`pic16bit` Support for 16-bit PIC microcontrollers.
-`qemu-arm` Support for ARM emulation through [QEMU](https://qemu.org).
-`stmhal` Support for boards based on STM32 microcontrollers including the MicroPython flagship [PyBoard](https://store.micropython.org/store/#/products/PYBv1_1).
-`teensy` Support for the Teensy line of boards such as the [Teensy 3.1](https://www.pjrc.com/teensy/teensy31.html).
-`unix` Support for UNIX.
-`windows` Support for [Windows](https://www.microsoft.com/en-us/windows/).
-`zephyr` Support for [Zephyr](https://www.zephyrproject.org/), a real-time operating system by the Linux Foundation.
CircuitPython only maintains the `atmel-samd` and `esp8266` ports. The rest are here to maintain compatibility with the [MicroPython](https://github.com/micropython/micropython) parent project.
