Step-by-step Guide: How to use the LVGL v9 LCD drivers with STM32 devices¶

Introduction¶

This guide is intended to be a step-by-step instruction of how to configure the STM32Cube HAL with the new TFT-LCD display drivers introduced in LVGL v9.0. The example code has been tested on the STM32F746-based Nucleo-F746ZG board with an ST7789-based LCD panel connected via SPI. The application itself and the hardware configuration code were generated with the STM32CubeIDE 1.14.0 tool.

Tip

ST Micro provide their own TFT-LCD drivers in their X-CUBE-DISPLAY Software Extension Package. While these drivers can be used with LVGL as well, the LVGL LCD drivers do not depend on this package.

The LVGL LCD drivers are meant as an alternative, simple to use API to implement LCD support for your LVGL-based project on any platform. Moreover, even in the initial release we support more LCD controllers than X-CUBE-DISPLAY currently provides, and we plan to add support for even more LCD controllers in the future.

Please note however, that – unlike X-CUBE-DISPLAY – the LVGL LCD drivers do not implement the communication part, whether SPI, parallel i8080 bus or other. It is the user's responsibility to implement – and optimize – these on their chosen platform. LVGL will only provide examples for the most popular platforms.

By following the steps you will have a fully functional program, which can be used as the foundation of your own LVGL-based project. If you are in a hurry and not interested in the details, you can find the final project here. You will only need to configure LVGL to use the driver corresponding to your hardware (if it is other than the ST7789), and implement the function ui_init() to create your widgets.

Note

This example is not meant as the best possible implementation, or the recommended solution. It relies solely on the HAL drivers provided by ST Micro, which favor portability over performance. Despite of this the performance is very good, thanks to the efficient, DMA-based implementation of the drivers.

Note

Although the example uses FreeRTOS, this is not a strict requirement with the LVGL LCD display drivers.

You can find the source code snippets of this guide in the lv_port_lcd_stm32_template.c example.

Hardware configuration¶

In this example we'll use the SPI1 peripheral to connect the microcontroller to the LCD panel. Besides the hardware-controlled SPI pins SCK and MOSI we need some additional output pins for the chip select, command/data select, and LCD reset:

pin	configuration	LCD	user label
PA4	GPIO_Output	CS	LCD_CS
PA5	SPI1_SCK	SCK	–
PA7	SPI1_MOSI	SDI	–
PA15	GPIO_Output	RESET	LCD_RESET
PB10	GPIO_Output	DC	LCD_DCX

Step-by-step instructions¶

Create new project in File/New/STM32 Project.
Select target processor/board.
Set project name and location.
Set Targeted Project Type to STM32Cube and press Finish.
Say "Yes" to Initialize peripherals with their default Mode? After the project is created, the configuration file (.ioc) is opened automatically.
Switch to the Pinout & Configuration tab.
In the System Core category switch to RCC.
Set High Speed Clock to "BYPASS Clock Source", and Low Speed Clock to "Crystal/Ceramic Resonator".
In the System Core category select SYS, and set Timebase Source to other than SysTick (in our example, TIM2).
Switch to the Clock Configuration tab.
Set the HCLK clock frequency to the maximum value (216 MHz for the STM32F746).
Switch back to the Pinout & Configuration tab, and in the Middleware and Software Packs category select FREERTOS.
Select Interface: CMSIS_V1.
In the Advanced Settings tab enable USE_NEWLIB_REENTRANT. We are finished here.
In the Pinout view configure PA5 as SPI1_SCK, PA7 as SPI1_MOSI (right click the pin and select the function).
In the Pinout & Configuration/Connectivity category select SPI1.
Set Mode to Transmit Only Master, and Hardware NSS Signal to Disable.
In the Configuration subwindow switch to Parameter Settings.
Set Frame Format to Motorola, Data Size to 8 Bits, First Bit to MSB First.
Set the Prescaler to the maximum value according to the LCD controller's datasheet (e.g., 15 MBits/s). Set CPOL/CPHA as required (leave as default).
Set NSSP Mode to Disabled and NSS Signal Type to Software.
In DMA Settings add a new Request for SPI1_TX (when using SPI1).
Set Priority to Medium, Data Width to Half Word.
In NVIC Settings enable SPI1 global interrupt.
In GPIO Settings set SPI1_SCK to Pull-down and Very High output speed and set the User Label to LCD_SCK.
Set SPI1_MOSI to Pull-up and Very High, and name it LCD_SDI.
Select System Core/GPIO category. In the Pinout view configure additional pins for chip select, reset and command/data select. Name them LCD_CS, LCD_RESET and LCD_DCX, respectively. Configure them as GPIO Output. (In this example we will use PA4 for LCD_CS, PA15 for LCD_RESET and PB10 for LCD_DCX.)
Set LCD_CS to No pull-up and no pull-down, Low level and Very High speed.
Set LCD_RESET to Pull-up and High level.
Set LCD_DCX to No pull-up and no pull-down, High level and Very High speed.
Open the Project Manager tab, and select Advanced Settings. On the right hand side there is a Register Callback window. Select SPI and set it to ENABLE.
We are ready with the hardware configuration. Save the configuration and let STM32Cube generate the source.
In the project tree clone the LVGL repository into the Middlewares/Third_Party folder (this tutorial uses the release/v9.0 branch of LVGL):
git clone https://github.com/lvgl/lvgl.git -b release/v9.0
Cloning should create an 'lvgl' subfolder inside the 'Third_Party' folder. From the 'lvgl' folder copy 'lv_conf_template.h' into the 'Middlewares' folder, and rename it to 'lv_conf.h'. Refresh the project tree.
Open 'lv_conf.h', and in line 15 change #if 0 to #if 1.
Search for the string LV_USE_ST7735, and enable the appropriate LCD driver by setting its value to 1. This example uses the ST7789 driver:
#define LV_USE_ST7789 1
Right click the folder 'Middlewares/Third_Party/lvgl/tests', select Resource Configurations/Exclude from Build..., check both Debug and Release, then press OK.
Right click the project name and select "Properties". In the C/C++ Build/Settings panel select MCU GCC Compiler/Include paths. In the Configuration dropdown select [ All configurations ]. Add the following Include path:
../Middlewares/Third_Party/lvgl

Open Core/Src/stm32xxx_it.c (the file name depends on the processor variation). Add 'lv_tick.h' to the Private includes section:

/* Private includes ----------------------------------------------------------*/
/* USER CODE BEGIN Includes */
#include "./src/tick/lv_tick.h"
/* USER CODE END Includes */

Find the function TIM2_IRQHandler. Add a call to lv_tick_inc():

void TIM2_IRQHandler(void)
{
  /* USER CODE BEGIN TIM2_IRQn 0 */

  /* USER CODE END TIM2_IRQn 0 */
  HAL_TIM_IRQHandler(&htim2);
  /* USER CODE BEGIN TIM2_IRQn 1 */
  lv_tick_inc(1);
  /* USER CODE END TIM2_IRQn 1 */
}

Save the file, then open Core/Src/main.c. Add the following lines to the Private includes (if your LCD uses other than the ST7789, replace the driver path and header with the appropriate one):

/* Private includes ----------------------------------------------------------*/
/* USER CODE BEGIN Includes */
#include "lvgl.h"
#include "./src/drivers/display/st7789/lv_st7789.h"
/* USER CODE END Includes */

Add the following lines to Private defines (change them according to your LCD specs):

#define LCD_H_RES       240
#define LCD_V_RES       320
#define BUS_SPI1_POLL_TIMEOUT 0x1000U

Add the following lines to the Private variables:

osThreadId LvglTaskHandle;
lv_display_t *lcd_disp;
volatile int lcd_bus_busy = 0;

Add the following line to the Private function prototypes:

void ui_init(lv_display_t *disp);
void LVGL_Task(void const *argument);

Add the following lines after USER CODE BEGIN RTOS_THREADS:

osThreadDef(LvglTask, LVGL_Task, osPriorityIdle, 0, 1024);
LvglTaskHandle = osThreadCreate(osThread(LvglTask), NULL);

Copy and paste the hardware initialization and the transfer callback functions from the example code after USER CODE BEGIN 4:

/* USER CODE BEGIN 4 */

void lcd_color_transfer_ready_cb(SPI_HandleTypeDef *hspi)
{
        /* CS high */
        HAL_GPIO_WritePin(LCD_CS_GPIO_Port, LCD_CS_Pin, GPIO_PIN_SET);
        lcd_bus_busy = 0;
        lv_display_flush_ready(lcd_disp);
}

/* Initialize LCD I/O bus, reset LCD */
static int32_t lcd_io_init(void)
{
        /* Register SPI Tx Complete Callback */
        HAL_SPI_RegisterCallback(&hspi1, HAL_SPI_TX_COMPLETE_CB_ID, lcd_color_transfer_ready_cb);

        /* reset LCD */
        HAL_GPIO_WritePin(LCD_RESET_GPIO_Port, LCD_RESET_Pin, GPIO_PIN_RESET);
        HAL_Delay(100);
        HAL_GPIO_WritePin(LCD_RESET_GPIO_Port, LCD_RESET_Pin, GPIO_PIN_SET);
        HAL_Delay(100);

        HAL_GPIO_WritePin(LCD_CS_GPIO_Port, LCD_CS_Pin, GPIO_PIN_SET);
        HAL_GPIO_WritePin(LCD_DCX_GPIO_Port, LCD_DCX_Pin, GPIO_PIN_SET);

        return HAL_OK;
}

/* Platform-specific implementation of the LCD send command function. In general this should use polling transfer. */
static void lcd_send_cmd(lv_display_t *disp, const uint8_t *cmd, size_t cmd_size, const uint8_t *param, size_t param_size)
{
        LV_UNUSED(disp);
        while (lcd_bus_busy);   /* wait until previous transfer is finished */
        /* Set the SPI in 8-bit mode */
        hspi1.Init.DataSize = SPI_DATASIZE_8BIT;
        HAL_SPI_Init(&hspi1);
        /* DCX low (command) */
        HAL_GPIO_WritePin(LCD_DCX_GPIO_Port, LCD_DCX_Pin, GPIO_PIN_RESET);
        /* CS low */
        HAL_GPIO_WritePin(LCD_CS_GPIO_Port, LCD_CS_Pin, GPIO_PIN_RESET);
        /* send command */
        if (HAL_SPI_Transmit(&hspi1, cmd, cmd_size, BUS_SPI1_POLL_TIMEOUT) == HAL_OK) {
                /* DCX high (data) */
                HAL_GPIO_WritePin(LCD_DCX_GPIO_Port, LCD_DCX_Pin, GPIO_PIN_SET);
                /* for short data blocks we use polling transfer */
                HAL_SPI_Transmit(&hspi1, (uint8_t *)param, (uint16_t)param_size, BUS_SPI1_POLL_TIMEOUT);
                /* CS high */
                HAL_GPIO_WritePin(LCD_CS_GPIO_Port, LCD_CS_Pin, GPIO_PIN_SET);
        }
}

/* Platform-specific implementation of the LCD send color function. For better performance this should use DMA transfer.
 * In case of a DMA transfer a callback must be installed to notify LVGL about the end of the transfer.
 */
static void lcd_send_color(lv_display_t *disp, const uint8_t *cmd, size_t cmd_size, uint8_t *param, size_t param_size)
{
        LV_UNUSED(disp);
        while (lcd_bus_busy);   /* wait until previous transfer is finished */
        /* Set the SPI in 8-bit mode */
        hspi1.Init.DataSize = SPI_DATASIZE_8BIT;
        HAL_SPI_Init(&hspi1);
        /* DCX low (command) */
        HAL_GPIO_WritePin(LCD_DCX_GPIO_Port, LCD_DCX_Pin, GPIO_PIN_RESET);
        /* CS low */
        HAL_GPIO_WritePin(LCD_CS_GPIO_Port, LCD_CS_Pin, GPIO_PIN_RESET);
        /* send command */
        if (HAL_SPI_Transmit(&hspi1, cmd, cmd_size, BUS_SPI1_POLL_TIMEOUT) == HAL_OK) {
                /* DCX high (data) */
                HAL_GPIO_WritePin(LCD_DCX_GPIO_Port, LCD_DCX_Pin, GPIO_PIN_SET);
                /* for color data use DMA transfer */
                /* Set the SPI in 16-bit mode to match endianness */
                hspi1.Init.DataSize = SPI_DATASIZE_16BIT;
                HAL_SPI_Init(&hspi1);
                lcd_bus_busy = 1;
                HAL_SPI_Transmit_DMA(&hspi1, param, (uint16_t)param_size / 2);
                /* NOTE: CS will be reset in the transfer ready callback */
        }
}

Add the LVGL_Task() function. Replace the lv_st7789_create() call with the appropriate driver. You can change the default orientation by adjusting the parameter of lv_display_set_rotation(). You will also need to create the display buffers here. This example uses a double buffering scheme with 1/10th size partial buffers. In most cases this is a good compromise between the required memory size and performance, but you are free to experiment with other settings.

void LVGL_Task(void const *argument)
{
        /* Initialize LVGL */
        lv_init();

        /* Initialize LCD I/O */
        if (lcd_io_init() != 0)
                return;

        /* Create the LVGL display object and the LCD display driver */
        lcd_disp = lv_st7789_create(LCD_H_RES, LCD_V_RES, LV_LCD_FLAG_NONE, lcd_send_cmd, lcd_send_color);
        lv_display_set_rotation(lcd_disp, LV_DISPLAY_ROTATION_270);

        /* Allocate draw buffers on the heap. In this example we use two partial buffers of 1/10th size of the screen */
        lv_color_t * buf1 = NULL;
        lv_color_t * buf2 = NULL;

        uint32_t buf_size = LCD_H_RES * LCD_V_RES / 10 * lv_color_format_get_size(lv_display_get_color_format(lcd_disp));

        buf1 = lv_malloc(buf_size);
        if(buf1 == NULL) {
                LV_LOG_ERROR("display draw buffer malloc failed");
                return;
        }

        buf2 = lv_malloc(buf_size);
        if(buf2 == NULL) {
                LV_LOG_ERROR("display buffer malloc failed");
                lv_free(buf1);
                return;
        }
        lv_display_set_buffers(lcd_disp, buf1, buf2, buf_size, LV_DISPLAY_RENDER_MODE_PARTIAL);

        ui_init(lcd_disp);

        for(;;) {
                /* The task running lv_timer_handler should have lower priority than that running `lv_tick_inc` */
                lv_timer_handler();
                /* raise the task priority of LVGL and/or reduce the handler period can improve the performance */
                osDelay(10);
        }
}

All that's left is to implement ui_init() to create the screen. Here's a simple "Hello World" example:

    void ui_init(lv_display_t *disp)
    {
            lv_obj_t *obj;

            /* set screen background to white */
            lv_obj_t *scr = lv_screen_active();
            lv_obj_set_style_bg_color(scr, lv_color_white(), 0);
            lv_obj_set_style_bg_opa(scr, LV_OPA_100, 0);

            /* create label */
            obj = lv_label_create(scr);
lv_obj_set_align(widget, LV_ALIGN_CENTER);
lv_obj_set_height(widget, LV_SIZE_CONTENT);
lv_obj_set_width(widget, LV_SIZE_CONTENT);
lv_obj_set_style_text_font(widget, &lv_font_montserrat_14, 0);
lv_obj_set_style_text_color(widget, lv_color_black(), 0);
lv_label_set_text(widget, "Hello World!");
    }