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docs/esp8266/general: Add "Boot process" section.

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Paul Sokolovsky 2016-05-02 17:41:08 +03:00
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@ -56,3 +56,38 @@ For your convinience, some of technical specifications are provided below:
* I2S: 1.
* Programming: using BootROM bootloader from UART. Due to external FlashROM
and always-available BootROM bootloader, ESP8266 is not brickable.
Boot process
------------
On boot, MicroPython EPS8266 port executes ``_boot.py`` script from internal
frozen modules. It mounts filesystem in FlashROM, or if it's not available,
performs first-time setup of the module and creates the filesystem. This
part of boot process is considered fixed, and not available for customization
for end users (even if you build from source, please refrain from changes to
it; customization of early boot process is available only to advanced users
and developers, who can diagnose themselves any issues arising from
modifying the standard process).
Once filesystem is mounted, ``boot.py`` is executed from it. The standard
version of this file is created during first-time module set up and by
defaults starts up a WebREPL daemon to handle incoming connections. This
file is customizable by end users (for example, you may want to disable
WebREPL for extra security, or add other services which should be run on
module start-up). But keep in mind that incorrect modifications to boot.py
may still lead to boot loops or lock ups, requiring to reflash a module
from scratch.
As a final step of boot procedure, ``main.py`` is executed from filesystem,
if exists. This file is a hook to start up a user application each time
on boot (instead of going to REPL). For small test applications, you may
name them directly as ``main.py``, and upload to module, but instead it's
recommended to keep your application(s) in separate files, and have just
the following in ``main.py``::
import my_app
my_app.main()
This will allow to keep structure of your application clear, as well as
allow to install multiple applications on a board, and switch among them.