NuttX RTOS

What is NuttX?

NuttX is a mature and secure real-time operating system (RTOS) with an emphasis on technical standards compliance and small size. It is scalable from 8-bit to 64-bit microcontrollers and microprocessors and compliant with the Portable Operating System Interface (POSIX) and the American National Standards Institute (ANSI) standards and with many Linux-like subsystems. The best way to think about NuttX is to think of it as a small Unix/Linux for microcontrollers.

Highlights of NuttX

  • Small - Fits and runs in microcontrollers as small as 32 kB Flash and 8 kB of RAM.

  • Compliant - Strives to be as compatible as possible with POSIX and Linux.

  • Versatile - Supports many architectures (ARM, ARM Thumb, AVR, MIPS, OpenRISC, RISC-V 32-bit and 64-bit, RX65N, x86-64, Xtensa, Z80/Z180, etc.).

  • Modular - Its modular design allows developers to select only what really matters and use modules to include new features.

  • Popular - NuttX is used by many companies around the world. Probably you already used a product with NuttX without knowing it was running NuttX.

  • Predictable - NuttX is a preemptible Realtime kernel, so you can use it to create predictable applications for realtime control.

Why NuttX + LVGL?

Although NuttX has its own graphic library called NX, LVGL is a good alternative because users could find more eye-candy demos and they can reuse code from previous projects. LVGL is an Object-Oriented Component Based high-level GUI library, that could fit very well for a RTOS with advanced features like NuttX. LVGL is implemented in C and its APIs are in C.

Here are some advantages of using LVGL in NuttX

  • Develop GUI in Linux first and when it is done just compile it for NuttX. Nothing more, no wasting of time.

  • Usually, GUI development for low level RTOS requires multiple iterations to get things right, where each iteration consists of Change code > Build > Flash > Run. Using LVGL, Linux and NuttX you can reduce this process and just test everything on your computer and when it is done, compile it on NuttX and that is it.

NuttX + LVGL could be used for

  • GUI demos to demonstrate your board graphics capacities.

  • Fast prototyping GUI for MVP (Minimum Viable Product) presentation.

  • visualize sensor data directly and easily on the board without using a computer.

  • Final products with a GUI without a touchscreen (i.e. 3D Printer Interface using Rotary Encoder to Input data).

  • Final products with a touchscreen (and all sorts of bells and whistles).

How to get started with NuttX and LVGL?

There are many boards in the NuttX mainline with support for LVGL. Let's use the STM32F429IDISCOVERY as an example because it is a very popular board.

First you need to install the pre-requisites on your system

Let's use the Windows Subsystem for Linux

$ sudo apt-get install automake bison build-essential flex gcc-arm-none-eabi gperf git libncurses5-dev libtool libusb-dev libusb-1.0.0-dev pkg-config kconfig-frontends openocd

Now let's create a workspace to save our files

$ mkdir ~/nuttxspace
$ cd ~/nuttxspace

Clone the NuttX and Apps repositories

$ git clone https://github.com/apache/incubator-nuttx nuttx
$ git clone https://github.com/apache/incubator-nuttx-apps apps

NuttX Simulator

The NuttX simulator allows you to run NuttX and LVGL on your PC. This can be especially useful for developing LVGL UIs with immediate visual feedback, and also developing platform independent NuttX apps.

Configure NuttX to use the sim board and the LVGL Demo

$ ./tools/configure.sh sim:lvgl_fb
$ make

If everything went fine you should have now the file nuttx

$ ls -l nuttx
-rwxr-xr-x 1 root root 2819920 May 12 15:01 nuttx

Running the NuttX Simulator

$ ./nuttx

Using the 'NSH>' terminal start the LVGL demo:

nsh> lvgldemo

NuttX On-Device

If you don't have an STM32F429I-DISCO, this process will be similar for other boards.

Configure NuttX to use the stm32f429i-disco board and the LVGL Demo

$ ./tools/configure.sh stm32f429i-disco:lvgl
$ make

If everything went fine you should have now the file nuttx.bin to flash on your board:

$ ls -l nuttx.bin
-rwxrwxr-x 1 alan alan 287144 Jun 27 09:26 nuttx.bin

Flashing the firmware in the board using OpenOCD

$ sudo openocd -f interface/stlink-v2.cfg -f target/stm32f4x.cfg -c init -c "reset halt" -c "flash write_image erase nuttx.bin 0x08000000"

Reset the board and using the 'NSH>' terminal start the LVGL demo:

nsh> lvgldemo

Custom Usage

The example app called lvgldemo uses LVGL's NuttX integration to run the demos in NuttX.

https://github.com/apache/nuttx-apps/tree/master/examples/lvgldemo

It is used by all LVGL defconfigs. See sim:lvgl_fb and sim:lvgl_lcd.

You can use it as a reference for adding LVGL to your own NuttX app. Start by copying the contents of lvgldemo.c. It handles fb and lcd based display drivers, the touchscreen input driver, and libuv.

NuttX Driver Support

  • fbdev - LVGL can create a display for /dev/fb* devices.

  • lcd - LVGL can create a display for /dev/lcd* devices.

  • input/uinput touchscreen - LVGL can create an indev(s) for touch devices that use the NuttX input/uinput system.

  • libuv - LVGL has support for using libuv as its event loop manager. libuv apps can integrate seamlessly.

There is no support for input/uinput keyboards yet. The NuttX X11 sim keyboard driver emits keycodes that are not standard so the LVGL integration with it would not be generic, if it were added.

Other Integrations

  • LVGL+NuttX profiler integration

  • Optional dedicated image cache heap

Configurations

The LVGL library is a NuttX "app". You can configure LVGL in menuconfig.

$ ./tools/configure.sh stm32f429i-disco:lvgl
$ make menuconfig

The LVGL Kconfig file provides the available config options to the NuttX build system. The LVGL configs in menuconfig can be found under Application Configuration > Graphics Support > Light and Versatile Graphic Library (LVGL) > LVGL configuration.

To find the location of a specific config, Press the "/" key to open a search interface.

Here are some configurations that you can use to customize your NuttX and LVGL setup:

  • LV_USE_NUTTX_INDEPENDENT_IMAGE_HEAP - You can enable or disable the LVGL image heap in NuttX. By default, it is disabled. If you enable it, LVGL will use the NuttX heap instead.

Using LV_STDLIB_CLIB for LV_USE_STDLIB_MALLOC, LV_USE_STDLIB_STRING, or LV_USE_STDLIB_SPRINTF is reasonable as NuttX fully implements these standard library APIs. Whether or not you choose to use NuttX's malloc depends on whether you want LVGL to allocate from the NuttX global heap or use its own.

Where is LV_OS_NUTTX?

NuttX tries to be POSIX compliant where possible, meaning it supports pthreads (POSIX threads). To enable OS features in LVGL on NuttX, set LV_USE_OS to LV_OS_PTHREAD. The main reason for enabling OS features is for multi-core rendering. See LV_DRAW_SW_DRAW_UNIT_CNT. Otherwise there is no case for setting it.

Using a Specific Version of LVGL

Here are the steps to use the latest (or any) version of LVGL in NuttX.

First ensure the build artifacts have been cleared. It will remove the existing unpacked LVGL and ZIP if there is one.

$ make distclean

Now you can git clone the latest LVGL into the NuttX apps tree. It will persist even after make distclean because the build system will not remove lvgl if it is a Git repo.

$ cd /path/to/nuttxspace/apps/graphics/lvgl
$ git clone https://github.com/lvgl/lvgl.git

Finally, you must copy the content of apps/graphics/lvgl/lvgl/Kconfig into the middle of apps/graphics/lvgl/Kconfig. See the "PASTE THE CONTENTS ..." section below.

apps/graphics/lvgl/Kconfig:

#
# For a description of the syntax of this configuration file,
# see the file kconfig-language.txt in the NuttX tools repository.
#

menuconfig GRAPHICS_LVGL
    bool "Light and Versatile Graphic Library (LVGL)"
    default n
    ---help---
            Enable support for the LVGL GUI library.

if GRAPHICS_LVGL

(PASTE THE CONTENTS OF apps/graphics/lvgl/lvgl/Kconfig HERE)

config LV_OPTLEVEL
    string "Customize compilation optimization level"
    default ""

endif # GRAPHICS_LVGL

Where can I find more information?