[esp_lcd](https://docs.espressif.com/projects/esp-idf/en/latest/esp32/api-reference/peripherals/lcd.html) 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](https://docs.espressif.com/projects/esp-idf/en/latest/esp32/api-reference/system/esp_timer.html) 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](https://docs.lvgl.io/master/porting/index.html).
This example uses two kinds of **buffering mode** based on the number of frame buffers:
* 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](main/rgb_lcd_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](main/rgb_lcd_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)
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 the ESP Component Registry into `managed_components` folder.
See the [Getting Started Guide](https://docs.espressif.com/projects/esp-idf/en/latest/get-started/index.html) for full steps to configure and use ESP-IDF to build projects.
* 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**.
* 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.
