1. Overview

This application note explains how to run the LVGL GUI in uClinux running on the i.MX RT devices. LVGL is the most popular free and open-source embedded graphics library to create powerful UIs for any MCU, MPU and display type. Refer to LVGL — Light and Versatile Embedded Graphics Library for detailed information on the LVGL GUI.

The Emcraft BSP includes a port and integration of the LVGL GUI, specifically for the i.MX RT devices running uClinux. The LVGL GUI support is integrated on top of the Linux framebuffer device driver (for the display) and touch screen devices driver, providing seamless integration with the Linux display and /dev/input input devices frameworks.

Full sources of the LVGL GUI, as well as pre-built LVGL binaries are provided.

2. Understanding Implementation

2.1. Understanding Integration and Build Framework

The LVGL sources, as well as some pre-built binaries, are integrated to the Emcraft uCLinux distribution in the A2F directory. The following are the key LVGL directories available in the distribution:

• A2F/lvgl - source code of the LVGL library itself (clone of the original https://github.com/lvgl/lvgl / tag v8.3.6)

• A2F/lv_drivers- source code of the LVGL drivers (clone of the original https://github.com/lvgl/lv_driver / tag v8.3.0)

• A2F/lvgl_bins/IMXRT105X_NXPEVK/ - pre-built binaries for the NXP IMXRT10XX-EVKB board

• A2F/lvgl_bins/IMXRT105X_NXPEVK/liblvgl.so - LVGL shared library, pre-built for the NXP IMXRT10XX-EVKB board kit and display

• A2F/lvgl_bins/IMXRT105X_NXPEVK/benchmark - a standard LVGL demo application, pre-built for the NXP IMXRT10XX-EVKB board kit and display. Other pre-built LVGL demos included in this directory are: ebike, music, stress, widgets.

• A2F/lvgl_bins/IMXRT117X_NXPEVK/ - pre-built binaries for the NXP IMXRT1170-EVK board

• A2F/lvgl_bins/IMXRT117X_NXPEVK/liblvgl.so - LVGL shared library, pre-built for the NXP IMXRT1170-EVK board kit and display

• A2F/lvgl_bins/IMXRT117X_NXPEVK/benchmark - a standard LVGL demo application, pre-built for the NXP IMXRT1170-EVK board kit and display. Other pre-built LVGL demos included in this directory are: music, stress, widgets.

The LVGL is built by the user in context of the standard projects/rootfs/ project available in the Emcraft distribution. Specifically, the LVGL configuration and build files, along with some LVGL demo applications can be found in the projects/rootfs/lvgl directory. The key files in that directory are as follows:

• lv_drv_conf.h - header file with configuration parameters for the Linux framebuffer and touchscreen

• lv_conf.h - header file with configuration parameters for various LVGL options

• main.c - C source file containing a typical main() for an LVGL application

• Makefile - build file allowing to build it all together.

The core of the LVGL is built as a shared library and is available on the target board as /usr/lib/liblvgl.so. Any LVGL demos are built as separate Linux applications linked with the liblvgl.so library.

Even though, as noted above, the LVGL library as well as some standard LVGL demos prebuilt for a specific reference kit, are available from the Emcraft distribution, it is important to understand that in a typical situation one will have to rebuild the library and application binaries from scratch. The reason for that is that re-configuring the LVGL for a specific I/O configuration, as well as to port to a new display and / or a new input device, implies making custom changes to the lv_drv_conf.h and lv_conf.h configuration header files. Once an update has been made to those files, the LVGL library and any LVGL applications need to be rebuilt. One rebuild the library and applications by running the Makefile in the projects/rootfs/lvgl directory.

The Emcraft distribution includes 4 standard LVGL demos, which are built as part of the projects/rootfs/ project and included to the target root filesystem:

• benchmark_gui_demo

• music_gui_demo

• stress_gui_demo

• widgets_gui_demo.

2.2. Implementing I/O interactions

The ebike demo described below is available only in 480x272 resolution, so it is applicable to IMXRT10XX-EVKB board and display kit, but not to the IMXRT1170-EVK. Port to 720x1280 resolution is required to run the demo on the IMXRT1170-EVK target.

In addition to the 4 standard LVGL demos, Emcraft provides one more to demonstrate interactions of the GUI with various IO interfaces. The GUI is based on the Futuristic Ebike example form the SquareLine Studio (SquareLine Studio - Design and build UIs with ease ).

Open

The Original Futuristic Ebike project was modified by Emcraft in the SqureLine Studio to assign a pin_clicked C-function callback to the virtual keyboard button click events in the Group Pin component. Then the UI was then exported to the projects/rootfs/lvgl/ebike_ui directory. The pin_clicked callback was implemented in projects/rootfs/lvgl/ebike_ui/ui_event.c so that when the user enters a 4-digits PIN-code and presses the V button on the virtual keyboard the PIN-code in printed out to the Linux shell terminal from when the application is being run.

There is a separate thread implemented by Emcraft in the projects/rootfs/lvgl/main.c to monitor the state of the USER button (the SW8 button on the back side of the NXP IMXRT10XX-EVKB board). If the USER button is pressed the application assumes that the bike accelerates, and if the button is released the bike slows down. A dedicated LVGL timer was implemented to update the Speed, Trip, Odometer and other on-screen labels depending on the current button state. The user can see that the bike speed is increasing if the USER button is kept pressed, and decreasing if one releases the button.

The integration and build provisions for ebike_gui_demo are similar to the ones for the 4 standard LVGL demos described above. Once the demo is built, it is available on the target from the Emcraft standard rootfs project.

2.3. Understanding Interface to Linux I/O Frameworks

The LVGL implementation in the Emcraft BSP is configured to use the standard Linux framebuffer and input frameworks to support the display graphics and input for the LCD panel.

In case of the NXP i.MXRT10XX-EVKB board, this is the 4.3" LCD Panel RK043FN02H-CT.

Support for the framebuffer is provided by the mxs-lcdif driver (drivers/video/fbdev/mxsfb.c). The driver is configured to a 480x272-pixels resolution with a 16-bit color depth. Access to the display graphics is available via the /dev/fb0 device node:

Support for the touch screen is provided by the ft5x0x_ts driver (drivers/input/touchscreen/ft5x46_ts.c). The input is available via the /dev/input/event0 device node:

Both the Linux frame buffer and the LVGL internal rendering buffers are allocated in the on-chip SDRAM. This supports a 25 FPS, as measured with ebike and other standard LVGL demos.

3. Running LVGL Demos

3.1. Running Standard LVGL Demos

Step through the following procedure to run the standard LVGL demos:

1. From the Linux shell, perform the following command to power on the LCD backlight:

   / # echo 0 > /sys/class/backlight/backlight/bl_power

2. From the Linux shell, type the benchmark_gui_demo command to run the benchmark demo:

    

   Open

   

   Type Ctrl-C to finish the demo:

3. Type the music_gui_demo command to run the music demo. Click to widgets icons on the touch panel to navigate the demo:

    

   Type Ctrl-C to finish the demo:

4. Type the stress_gui_demo command to run the stress demo:

    

   Type Ctrl-C to finish the demo:

5. Type the widgets_gui_demo command to run the widgets demo. Click to widgets icons on the touch panel to navigate the demo:

    

   Type Ctrl-C to finish the demo:

3.2. Running Emcraft Ebike Demo

The ebike demo is not available on the IMXRT1170-EVK board + display kit, port to resolution 720x1280 is required.

Step through the following procedure to run the Emcraft Ebike demo.

1. From the Linux shell, type the ebike_gui_demo command to run the ebike demo:

2. Click to the padlock icon in the bottom right corner of the screen to switch to the Unlock Your Bike group.

    

3. Click 4 any digits and then v. Make sure the correct PIN-code in printed out to the Linux shell terminal:

4. Click to the bike icon to switch back to the Driving Information group.

5. Press and hold the SW8 button which resides on the opposite side of the i.MXR10XX-EVKB board to LCD. Make sure that the Speed, Trip, Odometer and other values are increasing on the corresponding widgets on the LCD. If release the SW8 button the Speed reading is decreasing:

4. Building LVGL

4.1. Secure Download Area

Emcraft support the LVGL port to the i.MX RT as a paid add-on.

Once you have purchased the add-on from Emcraft, you can obtain the LVGL patch from the following location:

• For the imxrt1050 BSP (release 3.0.4):

• For the imxrt1050 BSP (release 3.1.0):

• For the imxrt1060 BSP (release 3.1.0):

• For the imxrt1170 BSP:
  https://www.emcraft.com/imxrtaddon/imxrt1170/lvgl
  The page is protected as follows:

4.2. Building LVGL

Step through the following procedure to apply the LVGL add-on and build the LVGL binaries:

1. From the top of the Linux installation on the development host, go to the projects directory

2. Apply the patch:

3. Build the roofts project:

The built LVGL binaries will be included in the bootable target image. They can be run on the target as described in the previous sections.