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Display Connection

The STM32F429I-DISCO board uses both SPI and LTDC for TFT Dispaly, (but not simultaneously for the same purpose.)

✔️ 1. SPI is used for Register Configuration (ILI9341 control)

• SPI5 (with pins PF7 = SCK, PF9 = MOSI) is used to send commands/data to configure the ILI9341 registers.

• This includes initialization commands such as:

  • Display ON
  • Memory access mode
  • Pixel format
  • Frame rate, etc.

• GPIOs like PD13 (WRX_DCX), PC2 (CSX), PF10 (RESET) are also used.

✔️ 2. LTDC is used for Framebuffer-Based Pixel Streaming

• The LTDC controller streams actual pixel data (framebuffer) to the display using RGB interface (parallel RGB signals + control lines).
• The pixel data is fetched from a memory location (FBStartAddress = 0xD0000000 (used onboard SDRAM)) and sent to the display without CPU intervention.
• This is hardware-accelerated and gives smooth graphics performance.

💡 So Why Use SPI If LTDC Is Doing the Pixel Job?

The ILI9341 has two interface roles:

1. Register Access (Control) – via SPI
2. Pixel Data Streaming – via RGB (parallel) through LTDC

The ILI9341 supports this dual role, where registers are configured through SPI, and pixel data is streamed through parallel RGB interface once it’s properly configured.

📌 Key Takeaway

• SPI (4-wire, IM[3:0] = 0110) is used only to configure the ILI9341 registers.
• LTDC uses the RGB parallel data interface (DB0–DB17 mapped to color signals: RGB) to continuously push pixel data.
• This is a hybrid configuration:
  • 🔧 SPI = command/control
  • 📺 LTDC = framebuffer image output

In the Code:

• ili9341_Init() is called before LTDC init — this sets up the controller via SPI.
• LTDC_Init() sets up the pixel streaming path using a framebuffer.
• GPIOs for WRX, CSX, etc., are toggled manually during initialization.
• SPI5_Init() configures the SPI peripheral for ILI9341 communication.

📊 System-Level Diagram: STM32F429 ↔ ILI9341

+-------------------+          +---------------------------+
|                   |   SPI    |                           |
|   STM32F429 MCU   +---------->  ILI9341 TFT Controller   |
|                   |          |                           |
|   (SPI5: PF7, PF9)|          |  Control Interface (SPI)  |
|   PD13 = WRX/DCX  +---------->  CMD/DATA                 |
|   PC2  = CSX      |          |                           |
|   PF10 = RESET    +---------->  RESET                    |
|                   |          |                           |
|                   |          |                           |
|     (AHB Master)  |   RGB    |      Display Data (RGB)   |
|    LTDC Hardware  +=========> R[5:0], G[5:0], B[5:0]     |
|    via DB0–DB17   |          |  (RGB565 Format)          |
|                   |          |                           |
+-------------------+          +---------------------------+
                                      |
                                      v
                             +-------------------+
                             |  240x320 TFT LCD   |
                             +-------------------+

🔧 How It Works

1. Startup Phase (Using SPI)

• SPI commands are sent to:

  • Reset the ILI9341
  • Configure orientation, pixel format (e.g. RGB565), inversion, gamma
  • Enable memory write mode for LTDC streaming

• Example SPI command:

  • WriteReg(0x36, 0x48); → sets Memory Access Control (row/column order, RGB/BGR)

2. Display Phase (Using LTDC)

• LTDC reads from a framebuffer (e.g., 0xD0000000) in external RAM
• It sends pixel data to the display using parallel RGB signals plus:
  • HSYNC
  • VSYNC
  • DOTCLK
  • DE (Data Enable)

The ILI9341, now in RGB interface mode, behaves like a passive display that only needs timing + data signals. It no longer needs SPI during normal operation.

🖼️ Example Framebuffer Flow (LTDC)

#define FRAMEBUFFER_ADDR  ((uint32_t)0xD0000000)

// Framebuffer format: RGB565 = 16 bits per pixel

// Fill screen with red
uint16_t* fb = (uint16_t*)FRAMEBUFFER_ADDR;
for (int y = 0; y < 320; ++y)
{
    for (int x = 0; x < 240; ++x)
    {
        fb[y * 240 + x] = 0xF800; // RGB565: Red
    }
}

✅ Summary of Signal Roles

Signal	STM32 Pin	Purpose	Used In
SPI5_SCK	PF7	Clock for SPI	Init phase
SPI5_MOSI	PF9	Data out to ILI9341	Init phase
WRX_DCX	PD13	Data/Command select	Init phase
RESET	PF10	Hardware reset	Init phase
CSX	PC2	Chip select	Init phase
DB0–DB17	Multiple	RGB data	Display phase
DOTCLK	PG7	Pixel clock	Display phase
HSYNC	PC6	Horizontal sync	Display phase
VSYNC	PC7	Vertical sync	Display phase
ENABLE	PF10	Data Enable	Display phase