MIPI-DSI Command Mode

This scenario describes how to configure MIPI DSI interface in Command Mode and TouchGFX Generator when using a display with a Display Serial Interface (DSI) and GRAM. The example used in this article will be for 16-bit RGB565 frame buffer format and goes through configurations in STM32CubeMX.

Note

This scenario assumes that a working display driver has been developed during the Board Bringup phase. The driver must be able to transfer pixels to the display, and to control the memory writing position of the display. Check the datasheet for your display for further details.

Configuration

LTDC Configuration

• Mode
  • Set Display Type to RGB565 (16 bits) - DSI Mode
• Layer Settings
  • Set Number of layers to 1 layer
  • Set the screen resolution in Windows Position and Frame Buffer Line Length and Frame Buffer Number of Lines
  • Set Layer 0 - Pixel Format to RGB565
  • Set Layer 0 - Alpha constant for blending to 255
    

    LTDC Configuration

• NVIC Settings
  • Both LTDC global interrupt and LTDC global error interrupt are not needed, and should be disabled.
    

    LTDC NVIC Settings

DSIHOST Configuration

• Mode
  • Set DSIHost to Adapted Command Mode with TE Pin
• Display Interface
  • Set Color Coding to RGB565 (16 bits) - DSI mode
  • Set Maximum Command Size to a number on the order of the width of the display
  • Set The Refresh of the Display Frame Buffer is Triggered to manually by Enabling the LTDC
  • Remaining configurations depends on the selected LCD HW
    

    DSIHOST Configuration

• NVIC Settings
  • Enable DSI global interrupt
    

    DSIHOST NVIC Settings

TouchGFX Generator

• Mode
  • Enable Graphics Application
• TouchGFX Generator
  • Set Display / Interface to Parallel RGB (LTDC) since this is still the controller the application needs to communicate with.
  • Set Application Tick Source to Custom
    

    TouchGFX Generator Configuration

User Code

The TouchGFX Generator can only generate a partial TouchGFX AL that configures the LTDC to transfer pixels through the DSI Host controller from the framebuffer memory to the display and synchronize the display with the TouchGFX Engine. However, all necessary handles to accomplish this are generated by the TouchGFX Generator.

Generally, for displays with embedded GRAM, the implementation of the generated TouchGFX HAL handles in TouchGFXHAL.cpp should perform the following steps to transfer pixels to the display and synchronize the display with the TouchGFX Engine:

1. Wait for "VSYNC" (sometimes called Tearing Effect (TE) signal) to signal the TouchGFX Engine.
2. Based on the area of the framebuffer to be redrawn, move the "display cursor" and "active window" (the region of the display being updated) to a place in GRAM that matches this area.
3. Prepare to write incoming pixel data to GRAM. Depending on the framebuffer strategy and display interface used, this could be swapping framebuffer pointers, signaling TouchGFX Engine, or waiting for previous transfers to complete.
4. Send pixel data.

Depending on the display used and the framebuffer strategy, the implementation of the above steps will vary.

Some of the adjustments that may be needed specifcally for DSI Command Mode interfaces are described below.

DSIHOST / LTDC Initialization sequence

The call to MX_DSIHOST_DSI_Init() must be done before MX_LTDC_Init(). This should be handled by STM32CubeMX. If this is not correct, take care to fix the order in a user code section.

After calling HAL_DSI_Start(), switch DSIHOST clock to the DSIPHY source:

static void MX_DSIHOST_DSI_Init(void)
{
  ...
  /* Switch to DSI PHY PLL clock */
  RCC_PeriphCLKInitTypeDef PeriphClkInit;
  PeriphClkInit.PeriphClockSelection = RCC_PERIPHCLK_DSI;
  PeriphClkInit.DsiClockSelection    = RCC_DSICLKSOURCE_DSIPHY;
  HAL_RCCEx_PeriphCLKConfig(&PeriphClkInit);
  /* USER CODE END DSIHOST_Init 2 */
  ...
}

User has to add the required initialization code specific to the used LCD controller at the end of the MX_LTDC_Init() function. That code will be based on the DSI HAL APIs HAL_DSI_ShortWrite() and HAL_DSI_LongWrite():

  static void MX_LTDC_Init(void)
  {
    ...
    /* USER CODE BEGIN LTDC_Init 2 */
    // Specific LCD controller's initialization code
    ...

    // Exit Sleep Mode
    if (HAL_DSI_ShortWrite(&hdsi, 0, DSI_DCS_SHORT_PKT_WRITE_P0, DSI_EXIT_SLEEP_MODE, 0x00) != HAL_OK)
    {
      Error_Handler();
    }

    HAL_Delay(120);
    /* USER CODE END LTDC_Init 2 */
    ...
  }

Updated TouchGFXHAL class for DSI Command Mode

One way to prevent the MIPI DSI display from turning on until the first frame in the application has beem rendered is to guard the function TouchGFXHAL::endFrame to keep the display off until first frame is rendered by TouchGFX. The TouchGFXHAL::endFrame() could be updated as below, to enable the LCD and its Backlight through a HW Timer configured for PWM output.

void TouchGFXHAL::endFrame()
{
    if (!display_on)
    {
        display_on = true;
        /* Enable the LCD, Send Display on DCS command to display */
        HAL_DSI_ShortWrite(&hdsi, 0, DSI_DCS_SHORT_PKT_WRITE_P1, DSI_SET_DISPLAY_ON, 0x00);
        /* Start PWM Timer channel */
        (void)HAL_TIM_PWM_Start(&htim8, TIM_CHANNEL_2);
        /* Enable Backlight by setting Brightness to 100% */
        __HAL_TIM_SET_COMPARE(&htim8, TIM_CHANNEL_2, 2U * 100);
    }

    TouchGFXGeneratedHAL::endFrame();
}

Supported Framebuffer Strategies

• Single
• Double
• Partial - GRAM display

Further reading

See article Framebuffer Strategies for a general introduction to framebuffer strategies in TouchGFX.

DSI Command Mode usually has a high enough bandwidth to transfer pixels to GRAM fast than the display scans the pixels. This is very similar to to the FMC Parallel display interface. Therefore, the implementation steps for a working TouchGFX AL for DSI Command Mode is very similar to the FMC Parallel display interface.

Further reading

See article FMC Display Interface for more information about the TouchGFX AL implementation.

Single

Currently, no TouchGFX Board Support have a reference implementation for Single buffering with DSI Command Mode. The setup would be similar to FMC Single buffer setup, but using DSI Command Mode driver functions instead of FMC.

Double

Currently, no TouchGFX Board Support have a reference implementation for Double buffering with DSI Command Mode. The setup would be similar to FMC Double buffer setup, but using DSI Command Mode driver functions instead of FMC.

Partial - GRAM display

Currently, no TouchGFX Board Support have a reference implementation for Partial - GRAM display with DSI Command Mode. The setup would be similar to FMC Partial - GRAM display setup, but using DSI Command Mode driver functions instead of FMC.