esp-idf/examples/peripherals/lcd/rgb_panel/README.md

Supported Targets	ESP32-S3

RGB LCD Panel Example

esp_lcd supports RGB interfaced LCD panel, with one or two frame buffer(s) managed by the driver itself.

This example shows the general process of installing an RGB panel driver, and displays a scatter chart on the screen based on the LVGL library.

This example uses the esp_timer to generate the ticks needed by LVGL and uses a dedicated task to run the lv_timer_handler(). Since the LVGL APIs are not thread-safe, this example uses a mutex which be invoked before the call of lv_timer_handler() and released after it. The same mutex needs to be used in other tasks and threads around every LVGL (lv_...) related function call and code. For more porting guides, please refer to LVGL porting doc.

This example uses two kinds of buffering mode based on the number of frame buffers:

Number of Frame Buffers	LVGL buffering mode	Way to avoid tear effect
1	Two buffers	Extra synchronization mechanism is needed, e.g. using semaphore.
2	Full refresh	There's no intersection between writing to an offline frame buffer and reading from an online frame buffer.

How to use the example

Hardware Required

• An ESP development board, which has RGB LCD peripheral supported and Octal PSRAM onboard
• A general RGB panel, 16 bit-width, with HSYNC, VSYNC and DE signal
• An USB cable for power supply and programming

Hardware Connection

The connection between ESP Board and the LCD is as follows:

       ESP Board                           RGB  Panel
+-----------------------+              +-------------------+
|                   GND +--------------+GND                |
|                       |              |                   |
|                   3V3 +--------------+VCC                |
|                       |              |                   |
|                   PCLK+--------------+PCLK               |
|                       |              |                   |
|             DATA[15:0]+--------------+DATA[15:0]         |
|                       |              |                   |
|                  HSYNC+--------------+HSYNC              |
|                       |              |                   |
|                  VSYNC+--------------+VSYNC              |
|                       |              |                   |
|                     DE+--------------+DE                 |
|                       |              |                   |
|               BK_LIGHT+--------------+BLK                |
+-----------------------+              |                   |
                               3V3-----+DISP_EN            |
                                       |                   |
                                       +-------------------+

The GPIO number used by this example can be changed in lvgl_example_main.c.

Especially, please pay attention to the level used to turn on the LCD backlight, some LCD module needs a low level to turn it on, while others take a high level. You can change the backlight level macro EXAMPLE_LCD_BK_LIGHT_ON_LEVEL in lvgl_example_main.c.

If the RGB LCD panel only supports DE mode, you can even bypass the HSYNC and VSYNC signals, by assigning EXAMPLE_PIN_NUM_HSYNC and EXAMPLE_PIN_NUM_VSYNC with -1.

Configure

Run idf.py menuconfig and go to Example Configuration:

1. Choose whether to Use double Frame Buffer
2. Choose whether to Avoid tearing effect (available only when step 1 was chosen to false)
3. Choose whether to Use bounce buffer (available only when step 1 was chosen to false)

Build and Flash

Run idf.py -p PORT build flash monitor to build, flash and monitor the project. A scatter chart will show up on the LCD as expected.

The first time you run idf.py for the example will cost extra time as the build system needs to address the component dependencies and downloads the missing components from registry into managed_components folder.

(To exit the serial monitor, type Ctrl-].)

See the Getting Started Guide for full steps to configure and use ESP-IDF to build projects.

Example Output

...
I (0) cpu_start: Starting scheduler on APP CPU.
I (856) esp_psram: Reserving pool of 32K of internal memory for DMA/internal allocations
I (856) example: Create semaphores
I (866) example: Turn off LCD backlight
I (866) gpio: GPIO[4]| InputEn: 0| OutputEn: 1| OpenDrain: 0| Pullup: 0| Pulldown: 0| Intr:0
I (876) example: Install RGB LCD panel driver
I (906) example: Register event callbacks
I (906) example: Initialize RGB LCD panel
I (906) example: Turn on LCD backlight
I (906) example: Initialize LVGL library
I (916) example: Allocate separate LVGL draw buffers from PSRAM
I (916) example: Register display driver to LVGL
I (926) example: Install LVGL tick timer
I (926) example: Create LVGL task
I (926) example: Starting LVGL task
I (926) example: Display LVGL Scatter Chart
...

Troubleshooting

• Why the LCD doesn't light up?
  • Check the backlight's turn-on level, and update it in EXAMPLE_LCD_BK_LIGHT_ON_LEVEL
• No memory for frame buffer
  • The frame buffer of RGB panel is located in ESP side (unlike other controller based LCDs, where the frame buffer is located in external chip). As the frame buffer usually consumes much RAM (depends on the LCD resolution and color depth), we recommend to put the frame buffer into PSRAM (like what we do in this example). However, putting frame buffer in PSRAM will limit the maximum PCLK due to the bandwidth of SPI0.
• LCD screen drift
  • Slow down the PCLK frequency
  • Adjust other timing parameters like PCLK clock edge (by pclk_active_neg), sync porches like VBP (by vsync_back_porch) according to your LCD spec
  • Enable CONFIG_SPIRAM_FETCH_INSTRUCTIONS and CONFIG_SPIRAM_RODATA, which can saves some bandwidth of SPI0 from being consumed by ICache.
• LCD screen tear effect
  • Using double frame buffers
  • Or adding an extra synchronization mechanism between writing (by Cache) and reading (by EDMA) the frame buffer.
• Low PCLK frequency
  • Enable CONFIG_EXAMPLE_USE_BOUNCE_BUFFER, which will make the LCD controller fetch data from internal SRAM (instead of the PSRAM), but at the cost of increasing CPU usage.
  • Enable CONFIG_SPIRAM_FETCH_INSTRUCTIONS and CONFIG_SPIRAM_RODATA can also help if the you're not using the bounce buffer mode. These two configurations can save some SPI0 bandwidth from being consumed by ICache.
• Why the RGB timing is correct but the LCD doesn't show anything?
  • Please read the datasheet of the IC used by your LCD module, and check if it needs a special initialization sequence. The initialization is usually done by sending some specific SPI commands and parameters to the IC. After the initialization, the LCD will be ready to receive RGB data. For simplicity, this example only works out of the box for those LCD modules which don't need extra initialization.

For any technical queries, please open an issue on GitHub. We will get back to you soon.