Micropython

What is Micropython?

Micropython is Python for microcontrollers. Using Micropython, you can write Python3 code and run it even on a bare metal architecture with limited resources.

Highlights of Micropython

  • Compact: Fits and runs within just 256k of code space and 16k of RAM. No OS is needed, although you can also run it with an OS, if you want.

  • Compatible: Strives to be as compatible as possible with normal Python (known as CPython).

  • Versatile: Supports many architectures (x86, x86-64, ARM, ARM Thumb, Xtensa).

  • Interactive: No need for the compile-flash-boot cycle. With the REPL (interactive prompt) you can type commands and execute them immediately, run scripts, etc.

  • Popular: Many platforms are supported. The user base is growing bigger. Notable forks:

  • Embedded Oriented: Comes with modules specifically for embedded systems, such as the machine module for accessing low-level hardware (I/O pins, ADC, UART, SPI, I2C, RTC, Timers etc.)

Why Micropython + LVGL?

Micropython does not have a good native high-level GUI library. LVGL is an Object-Oriented Component Based high-level GUI library, which seems to be a natural candidate to map into a higher level language, such as Python. LVGL is implemented in C and its APIs are in C.

Here are some advantages of using LVGL in Micropython:

  • Develop GUI in Python, a very popular high level language. Use paradigms such as Object-Oriented Programming.

  • Usually, GUI development requires multiple iterations to get things right. With C, each iteration consists of ``Change code`` > ``Build`` > ``Flash`` > ``Run``. In Micropython it's just ``Change code`` > ``Run`` ! You can even run commands interactively using the REPL (the interactive prompt)

Micropython + LVGL could be used for:

  • Fast prototyping GUI.

  • Shortening the cycle of changing and fine-tuning the GUI.

  • Modelling the GUI in a more abstract way by defining reusable composite objects, taking advantage of Python's language features such as Inheritance, Closures, List Comprehension, Generators, Exception Handling, Arbitrary Precision Integers and others.

  • Make LVGL accessible to a larger audience. No need to know C to create a nice GUI on an embedded system. This goes well with CircuitPython vision. CircuitPython was designed with education in mind, to make it easier for new or inexperienced users to get started with embedded development.

  • Creating tools to work with LVGL at a higher level (e.g. drag-and-drop designer).

So what does it look like?

It's very much like the C API, but Object-Oriented for LVGL components.

Let's dive right into an example!

A simple example

import lvgl as lv
lv.init()
scr = lv.obj()
btn = lv.btn(scr)
btn.align(lv.ALIGN.CENTER, 0, 0)
label = lv.label(btn)
label.set_text('Hello World!')
lv.screen_load(scr)

How can I use it?

Online Simulator

If you want to experiment with LVGL + Micropython without downloading anything - you can use our online simulator! It's a fully functional LVGL + Micropython that runs entirely in the browser and allows you to edit a python script and run it.

Click here to experiment on the online simulator

Many LVGL examples are available also for Micropython. Just click the link!

PC Simulator

Micropython is ported to many platforms. One notable port is "unix", which allows you to build and run Micropython (+LVGL) on a Linux machine. (On a Windows machine you might need Virtual Box or WSL or MinGW or Cygwin etc.)

Click here to know more information about building and running the unix port

Embedded Platforms

In the end, the goal is to run it all on an embedded platform. Both Micropython and LVGL can be used on many embedded architectures. lv_micropython is a fork of Micropython+LVGL and currently supports Linux, ESP32, STM32 and RP2. It can be ported to any other platform supported by Micropython.

  • You would also need display and input drivers. You can either use one of the existing drivers provided with lv_micropython, or you can create your own input/display drivers for your specific hardware.

  • Drivers can be implemented either in C as a Micropython module, or in pure Python!

lv_micropython already contains these drivers:

  • Display drivers:

    • SDL on Linux

    • ESP32 specific:

      • ILI9341

      • ILI9488

      • GC9A01

      • ST7789

      • ST7735

    • Generic (pure Python):

      • ILI9341

      • ST7789

      • ST7735

  • Input drivers:

    • SDL

    • XPT2046

    • FT6X36

    • ESP32 ADC with resistive touch

Where can I find more information?

The Micropython Binding is auto generated!

  • LVGL is a git submodule inside lv_micropython (LVGL is a git submodule of lv_binding_micropython which is itself a submodule of lv_micropython).

  • When building lv_micropython, the public LVGL C API is scanned and Micropython API is auto-generated. That means that lv_micropython provides LVGL API for any LVGL version, and generally does not require code changes as LVGL evolves.

LVGL C API Coding Conventions

To support the auto-generation of the Python API, the LVGL C API must follow some coding conventions:

  • Use enums instead of macros. If inevitable to use defines export them with LV_EXPORT_CONST_INT(defined_value) right after the define.

  • In function arguments use type name[] declaration for array parameters instead of type * name

  • Use typed pointers instead of void* pointers

  • Widget constructor must follow the lv_<widget_name>_create(lv_obj_t * parent) pattern.

  • Widget members function must start with lv_<module_name> and should receive lv_obj_t* as first argument which is a pointer to widget object itself.

  • struct APIs should follow the widgets' conventions. That is to receive a pointer to the struct as the first argument, and the prefix of the struct name should be used as the prefix of the function name too (e.g. lv_disp_set_default(lv_disp_t * disp))

  • Functions and structs which are not part of the public API must begin with underscore in order to mark them as "private".

  • Argument must be named in H files too.

  • Do not malloc into a static or global variables. Instead declare the variable in lv_global_t structure in lv_global.h and mark the variable with (LV_GLOBAL_DEFAULT()->variable) when it's used. See Memory Management

  • To register and use callbacks one of the following needs to be followed. See Callbacks

    • Pass a pointer to a struct as the first argument of both the registration function and the callback. That struct must contain void * user_data field.

    • The last argument of the registration function must be void * user_data and the same user_data needs to be passed as the last argument of the callback.

Most of these rules are simple and straightforward but there are two related concepts that worth a deeper look: Memory Management and Callbacks.

Memory Management

When LVGL runs in Micropython, all dynamic memory allocations (lv_malloc()) are handled by Micropython's memory manager which is garbage-collected (GC).
To prevent GC from collecting memory prematurely, all dynamic allocated RAM must be reachable by GC.
GC is aware of most allocations, except from pointers on the Data Segment:

  • Pointers which are global variables

  • Pointers which are static global variables

  • Pointers which are static local variables

Such pointers need to be defined in a special way to make them reachable by GC

Identify The Problem

Problem happens when an allocated memory's pointer (return value of lv_malloc()) is stored only in either global, static global or static local pointer variable and not as part of a previously allocated struct or other variable.

Solve The Problem

  • Replace the global/static local var with (LV_GLOBAL_DEFAULT()->_var)

  • Include lv_global.h on files that use LV_GLOBAL_DEFAULT

  • Add _var to lv_global_t on lv_global.h

Example

More Information

Callbacks

In C a callback is just a function pointer. But in Micropython we need to register a Micropython callable object for each callback. Therefore in the Micropython binding we need to register both a function pointer and a Micropython object for every callback.

Therefore we defined a callback convention for the LVGL C API that expects lvgl headers to be defined in a certain way. Callbacks that are declared according to the convention would allow the binding to register a Micropython object next to the function pointer when registering a callback, and access that object when the callback is called.

  • The basic idea is that we have void * user_data field that is used automatically by the Micropython Binding to save the Micropython callable object for a callback. This field must be provided when registering the function pointer, and provided to the callback function itself.

  • Although called "user_data", the user is not expected to read/write that field. Instead, the Micropython glue code uses user_data to automatically keep track of the Micropython callable object. The glue code updates it when the callback is registered, and uses it when the callback is called in order to invoke a call to the original callable object.

There are a few options for defining a callback in LVGL C API:

  • Option 1: user_data in a struct

    • There's a struct that contains a field called void * user_data

      • A pointer to that struct is provided as the first argument of a callback registration function

      • A pointer to that struct is provided as the first argument of the callback itself

  • Option 2: user_data as a function argument

    • A parameter called void * user_data is provided to the registration function as the last argument

      • The callback itself receives void * as the last argument

  • Option 3: both callback and user_data are struct fields

    • The API exposes a struct with both function pointer member and user_data member

      • The function pointer member receives the same struct as its first argument

In practice it's also possible to mix these options, for example provide a struct pointer when registering a callback (option 1) and provide user_data argument when calling the callback (options 2), as long as the same ``user_data`` that was registered is passed to the callback when it's called.

Examples

  • lv_anim_t contains user_data field. lv_anim_set_path_cb() registers path_cb callback. Both lv_anim_set_path_cb and lv_anim_path_cb_t receive lv_anim_t as their first argument

  • path_cb field can also be assigned directly in the Python code because it's a member of lv_anim_t which contains user_data field, and lv_anim_path_cb_t receive lv_anim_t as its first argument.

  • lv_imgfont_create() registers path_cb and receives user_data as the last argument. The callback lv_imgfont_get_path_cb_t also receives the user_data as the last argument.

More Information