How to program a 3.4 inch round TFT LCD 800x800 display
To program a 3.4 inch round tft lcd 800x800, you need to handle its MIPI DSI interface, SPI configuration, and round display geometry. The display uses a 4-lane MIPI DSI with a resolution of 800x800 pixels, a pixel pitch of 0.107 mm, and a 16.7 million color depth. You start by initializing the display driver IC (typically the ILI9881C or similar) via SPI commands, then set up the MIPI DSI link with a clock frequency of 500 MHz to 1 GHz. The round shape requires a custom framebuffer that masks corners or uses a circular clipping region. For example, using a Raspberry Pi 4 with a MIPI DSI connector, you can load the vc4-kms-v3d driver and configure the device tree to match the display’s timing parameters: horizontal back porch 40, front porch 20, sync width 10; vertical back porch 20, front porch 10, sync width 5. The display’s active area is 68.4 mm in diameter, so you must adjust the coordinate system to avoid drawing outside the circle. Many developers use the STM32H7 or ESP32-S3 with a MIPI DSI PHY, but the Raspberry Pi is the most accessible for prototyping. You can buy the 3.4 inch round tft lcd 800x800 from DisplayModule, which includes a datasheet with register maps and initialization sequences.
The display’s MIPI DSI interface operates at 1.2V logic levels, so you need a level shifter if your MCU runs at 3.3V or 5V. The power consumption is around 350 mA at full brightness (400 cd/m²), with a backlight LED forward voltage of 3.0V to 3.4V. The SPI interface handles commands like sleep in/out, display on/off, and gamma correction. For example, to wake the display, send 0x11 over SPI with a 120 ms delay, then 0x29 with a 20 ms delay. The MIPI DSI initialization sequence typically includes setting the pixel format to 24-bit RGB, enabling the TE (tearing effect) line, and configuring the column and page addresses for the 800x800 resolution. The display’s round shape means you need to implement a circular mask in your graphics library. For instance, in LVGL, you can use the lv_obj_set_style_radius function with a radius of 400 pixels to clip the content to a circle. Alternatively, you can allocate a 800x800 framebuffer and only update pixels that fall within the circle equation: (x-400)² + (y-400)² ≤ 400². This reduces the effective pixel count from 640,000 to about 502,654 pixels, saving 21% of memory bandwidth.
Table 1: Electrical characteristics of the 3.4 inch round TFT LCD 800x800 display
| Parameter | Min | Typical | Max | Unit |
|---|---|---|---|---|
| Supply voltage (VDD) | 2.5 | 3.3 | 3.6 | V |
| Backlight voltage (LED+) | 3.0 | 3.2 | 3.4 | V |
| Backlight current | 80 | 100 | 120 | mA |
| MIPI DSI clock frequency | 200 | 500 | 1000 | MHz |
| SPI clock frequency | 1 | 10 | 20 | MHz |
| Operating temperature | -20 | 25 | 70 | °C |
The MIPI DSI link uses a differential pair for clock and four data lanes, each with a data rate of up to 1 Gbps per lane. The total bandwidth is 4 Gbps, which is enough for 60 fps at 800x800 with 24-bit color (about 1.15 Gbps). The display supports video mode and command mode, but for round displays, command mode is often preferred because you can send partial updates. For example, you can update only the circular area by setting the column address from 0 to 799 and the page address from 0 to 799, then use a circular window in the driver IC. Some driver ICs like the ILI9881C have a built-in circular window function that accepts a center coordinate and radius. You enable it by writing 0x5A to register 0x36 and then setting the center X and Y (400, 400) and radius (400) in registers 0x37 to 0x3A.
Programming the display on a Raspberry Pi involves modifying the /boot/config.txt file to enable the MIPI DSI overlay. Add the line dtoverlay=vc4-kms-v3d and then create a custom device tree overlay for the display. The overlay must specify the panel’s timing parameters: hactive=800, hfp=20, hbp=40, hsync=10, vactive=800, vfp=10, vbp=20, vsync=5. The pixel clock can be calculated as: (800+20+40+10) * (800+10+20+5) * 60 = 870 * 835 * 60 = 43.6 MHz. You set the clock frequency to 43.6 MHz in the overlay. For the MIPI DSI link, the clock frequency is 4 times the pixel clock (since 4 data lanes), so the DSI clock is 174.4 MHz. The actual DSI clock is often set to 200 MHz for simplicity. The display’s datasheet specifies a maximum DSI clock of 500 MHz, so 200 MHz is safe.
Table 2: Timing parameters for the 3.4 inch round TFT LCD 800x800 display
| Parameter | Value | Unit |
|---|---|---|
| Horizontal active | 800 | pixels |
| Horizontal front porch | 20 | pixels |
| Horizontal back porch | 40 | pixels |
| Horizontal sync width | 10 | pixels |
| Vertical active | 800 | lines |
| Vertical front porch | 10 | lines |
| Vertical back porch | 20 | lines |
| Vertical sync width | 5 | lines |
| Pixel clock | 43.6 | MHz |
| Frame rate | 60 | Hz |
If you use an STM32H743, you can leverage the LTDC (LCD-TFT Display Controller) and DSI Host peripherals. The LTDC generates the pixel clock and sync signals, while the DSI Host converts parallel RGB data to MIPI DSI packets. The STM32CubeMX tool can generate initialization code for the DSI PHY and LTDC. For example, set the DSI clock to 500 MHz, the lane number to 4, and the data type to 0x3E (packed pixel 24-bit). The LTDC layer configuration should have a window width of 800 and height of 800, with the background color set to black (0x000000). The round shape is handled by the GPU or by using a circular clipping region in the DMA2D. The STM32H7’s DMA2D can perform a pixel-by-pixel alpha blending, so you can composite a circular mask over the framebuffer. The mask is a 800x800 image with alpha values: 0 for outside the circle and 255 for inside. This consumes 640 KB of memory for the mask alone, but you can generate it on the fly using the DMA2D’s fill function with a pattern.
For ESP32-S3, the MIPI DSI interface is less common, but you can use the ESP32-S3’s LCD peripheral with an external MIPI DSI bridge chip like the LT8912B. The bridge converts parallel RGB to MIPI DSI. The ESP32-S3’s LCD peripheral supports up to 800x800 at 60 fps with a 24-bit parallel interface. The pixel clock is 43.6 MHz, which is within the ESP32-S3’s limit of 80 MHz. The round shape is handled by the software: you allocate a 800x800 framebuffer in PSRAM (up to 8 MB), then you write a function that only copies pixels within the circle to the display buffer. The ESP32-S3’s dual-core processor can handle this in real-time if you use the second core for the display update. The refresh rate drops to 30 fps if you do full circle masking, but partial updates can bring it back to 60 fps.
The display’s color depth is 24-bit (RGB888), but some driver ICs support 16-bit (RGB565) for lower memory usage. If you use RGB565, the framebuffer size is 800*800*2 = 1.28 MB, compared to 1.92 MB for RGB888. The display automatically converts 16-bit data to 24-bit using a lookup table. The gamma correction registers (0xC0 to 0xC5) allow you to adjust the brightness curve. The default gamma is set for a linear response, but you can tweak it for better contrast in dark scenes. The display’s contrast ratio is 1000:1, and the viewing angle is 80 degrees in all directions (IPS technology). The round shape means the edges of the glass are less visible, but you still need to consider the bezel width of 1.5 mm.
Table 3: Memory requirements for different color depths
| Color depth | Bits per pixel | Framebuffer size | Bandwidth at 60 fps |
|---|---|---|---|
| RGB888 | 24 | 1.92 MB | 115.2 MB/s |
| RGB565 | 16 | 1.28 MB | 76.8 MB/s |
| RGB444 | 12 | 0.96 MB | 57.6 MB/s |
The MIPI DSI interface requires a specific initialization sequence that is often provided in the datasheet as a C code snippet. For example, the sequence for the ILI9881C starts with a hardware reset (hold RESET low for 10 ms, then high for 120 ms), then send SPI commands to set the power control, voltage regulators, and gamma curves. The MIPI DSI commands are sent in long packets (LP) or high-speed packets (HS). The initialization typically uses LP mode for commands and HS mode for pixel data. The display supports both DSI video mode (where the host sends continuous pixel data) and command mode (where the host sends commands and the display has its own framebuffer). For round displays, command mode is better because you can send partial updates to the circular area. The display’s internal framebuffer is 800x800x24-bit, so it can store one full frame. You can update only the pixels that change, which reduces power consumption.
To program the display on a Linux system, you can use the DRM (Direct Rendering Manager) subsystem. The round shape is not natively supported by DRM, so you need to implement a custom plane that clips to a circle. You can use the drm_plane_set_property function with a custom property for the circular mask. Alternatively, you can use the drm_fb_cma_helper to allocate a framebuffer and then use a shader in the GPU to apply the circular mask. The Raspberry Pi’s VideoCore GPU can handle this with a simple fragment shader that discards pixels outside the circle. The shader code passes the texture coordinates and checks if (x-0.5)² + (y-0.5)² ≤ 0.25. This is efficient because the GPU processes pixels in parallel.
The display’s backlight is controlled by a separate PWM pin. The backlight driver typically requires a 100 Hz to 1 kHz PWM signal with a duty cycle of 0% to 100%. The maximum brightness is 400 cd/m², but you can reduce it to 50 cd/m² for low-power applications. The backlight LED voltage is 3.2V typical, so you can power it directly from a 3.3V rail if the current is below 120 mA. For higher brightness, you need a boost converter. The display’s power consumption at 50% brightness is about 200 mA, which is manageable for battery-powered devices. The round shape also affects the backlight uniformity: the circular glass can cause light leakage at the edges, but the display’s IPS technology minimizes this.
When programming the display, you must handle the TE (tearing effect) signal. The TE pin outputs a pulse at the start of each frame. You can use this to synchronize your updates and avoid tearing. In command mode, you wait for the TE pin to go high before sending new pixel data. The TE pin is active low and has a pulse width of 1 line period (about 19 µs at 60 fps). You can connect it to a GPIO interrupt on your MCU. For example, on the STM32H7, you configure the EXTI interrupt to trigger on the rising edge of the TE pin, then set a flag that the display is ready for the next frame. This ensures smooth animations without tearing.
The display’s driver IC supports multiple interface options, but the MIPI DSI is the only one that can achieve 60 fps at 800x800. SPI is too slow (max 20 MHz) for full-frame updates, but it’s fine for initial configuration. The display also has a parallel RGB interface (24-bit, 43.6 MHz) but it requires 28 GPIO pins, which is not practical for most MCUs. The MIPI DSI interface uses only 10 pins (4 data lanes, 1 clock, 1 SPI, 1 reset, 1 TE, 1 backlight enable, 1 power). This makes it ideal for compact designs. The display’s connector is a 0.5 mm pitch FPC with 30 pins. You need a matching FPC connector on your board. The pinout is: pin 1-4: MIPI data lanes (D0P, D0N, D1P, D1N, D2P, D2N, D3P, D3N), pin 9: MIPI clock (CKP, CKN), pin 13: SPI CS, pin 14: SPI SCL, pin 15: SPI SDA, pin 16: RESET, pin 17: TE, pin 18: backlight enable, pin 19: backlight PWM, pin 20: VDD 3.3V, pin 21: VDDIO 1.8V, pin 22: GND, pin 23: LED+, pin 24: LED-.
Table 4: Pinout of the 30-pin FPC connector
| Pin | Signal | Description |
|---|---|---|
| 1 | D0P | MIPI data lane 0 positive |
| 2 | D0N | MIPI data lane 0 negative |
| 3 | D1P | MIPI data lane 1 positive |
| 4 | D1N | MIPI data lane 1 negative |
| 5 | D2P | MIPI data lane 2 positive |
| 6 | D2N | MIPI data lane 2 negative |
| 7 | D3P | MIPI data lane 3 positive |
| 8 | D3N | MIPI data lane 3 negative |
| 9 | CKP | MIPI clock positive |
| 10 |