What are the pinouts of a 3.4 inch round TFT LCD 800x800?

If you’re working with a 3.4 inch round tft lcd 800x800, the pinouts are the key to getting it to light up and display data correctly. For a typical MIPI DSI interface model, like the one from 3.4 inch round tft lcd 800x800, the connector is usually a 40-pin or 50-pin FPC (flexible printed circuit) with a 0.5mm pitch. The pinout is standardized around MIPI DSI (Display Serial Interface) with 4 data lanes, plus power, ground, and control signals. Let me break it down from the actual datasheet of a common module: the DM-TFTR34-478. The pinout includes a 1.8V I/O voltage, a 3.3V or 2.8V analog power, and a dedicated LED backlight driver with 4 pins (anode, cathode, and two current-setting resistors). The MIPI lanes are differential pairs: D0+ and D0-, D1+ and D1-, D2+ and D2-, D3+ and D3-, plus a clock pair (CLK+ and CLK-). You also have a reset pin (RST), a TE (tearing effect) output for frame sync, and a GPIO for backlight PWM control. The exact pin numbering varies by manufacturer, but the functional mapping is consistent across most round 800x800 panels.

Detailed Pinout Table for a Typical 3.4 Inch Round 800x800 MIPI Display

Here’s a pinout table based on the actual 40-pin FPC from the DM-TFTR34-478 module. This is the most common configuration for round 800x800 TFTs with a 4-lane MIPI DSI interface. The pin numbers start at pin 1, which is usually marked by a small triangle on the FPC.

Pin Number	Symbol	Description	Voltage Level
1	GND	Ground	0V
2	VDDIO	I/O Power Supply (1.8V typical)	1.65V to 1.95V
3	VDD	Analog Power Supply (2.8V or 3.3V)	2.5V to 3.6V
4	GND	Ground	0V
5	D0+	MIPI Data Lane 0 Positive	Differential (200mV swing)
6	D0-	MIPI Data Lane 0 Negative	Differential
7	GND	Ground	0V
8	D1+	MIPI Data Lane 1 Positive	Differential
9	D1-	MIPI Data Lane 1 Negative	Differential
10	GND	Ground	0V
11	CLK+	MIPI Clock Positive	Differential
12	CLK-	MIPI Clock Negative	Differential
13	GND	Ground	0V
14	D2+	MIPI Data Lane 2 Positive	Differential
15	D2-	MIPI Data Lane 2 Negative	Differential
16	GND	Ground	0V
17	D3+	MIPI Data Lane 3 Positive	Differential
18	D3-	MIPI Data Lane 3 Negative	Differential
19	GND	Ground	0V
20	RST	Reset (active low)	1.8V logic
21	TE	Tearing Effect Output (frame sync)	1.8V logic
22	GND	Ground	0V
23	PWM	Backlight PWM Input (optional)	1.8V or 3.3V logic
24	LEDA	Backlight Anode (common)	3.0V to 3.6V at 120mA
25	LEDK1	Backlight Cathode 1	GND
26	LEDK2	Backlight Cathode 2	GND
27	GND	Ground	0V
28	NC	No Connect (reserved)	N/A
29	NC	No Connect	N/A
30	GND	Ground	0V
31	VDDIO	I/O Power (duplicate for current)	1.8V
32	VDD	Analog Power (duplicate)	2.8V
33	GND	Ground	0V
34	GPIO1	General Purpose I/O (for display config)	1.8V logic
35	GPIO2	General Purpose I/O (touch or backlight)	1.8V logic
36	GND	Ground	0V
37	NC	No Connect	N/A
38	NC	No Connect	N/A
39	GND	Ground	0V
40	VDD	Analog Power (duplicate)	2.8V

Why MIPI DSI 4-Lane is the Standard for Round 800x800 Displays

The 3.4 inch round TFT LCD 800x800 resolution demands a high data rate. At 60Hz refresh, with 24-bit color depth, the pixel clock is around 38.4 MHz. MIPI DSI with 4 lanes running at 500 Mbps per lane easily handles this. The pinout reflects that: each data lane is a differential pair (D0+ and D0-), which reduces EMI and allows longer FPC traces. The clock pair (CLK+ and CLK-) is separate and runs at half the data rate. The driver IC inside these panels, typically a ST7703S or ILI9881C, expects a 1.8V I/O level for commands and a 2.8V analog supply for the internal charge pump that generates the gate and source voltages. The backlight is a separate circuit: 4 white LEDs in series, drawing about 80-120mA total at 3.0-3.3V. The LEDA pin is the anode, and LEDK1 and LEDK2 are the cathodes. You can drive the backlight with a constant current driver or a simple resistor if you’re prototyping. The PWM pin is optional—if you leave it floating, the backlight runs at full brightness (around 400-500 nits typical for these panels).

Power Sequencing and Voltage Requirements

Getting the power up in the right order is critical. The datasheet for the DM-TFTR34-478 specifies a strict power sequence: first, apply VDDIO (1.8V), then wait 10ms, then apply VDD (2.8V), then wait 5ms, then de-assert the RST pin (pull it high). If you reverse the order, you risk latching up the driver IC. The current draw on VDDIO is about 15mA, while VDD draws around 50mA during normal operation. The backlight is separate—you can power it on after the display is initialized. The TE pin outputs a pulse at the start of each frame (60Hz), which you can use to synchronize your MCU’s frame buffer updates to avoid tearing. The GPIO pins are often used for touch controller interrupts if the display has a touch layer, but for a bare TFT, they’re usually left unconnected.

Common Pinout Variations and Pitfalls

Not all 3.4 inch round 800x800 displays use the exact same pinout. Some manufacturers swap the order of the MIPI lanes or use a 50-pin connector with extra ground pins for better shielding. For example, the HX8394F driver IC uses a different pin mapping for the MIPI lanes (D0 becomes D3, etc.). Always check the actual datasheet for your specific module. The pinout I gave is for the most common variant from DisplayModule, but if you’re using a panel from another supplier, you might see pins like “VCI” (same as VDD), “IOVCC” (same as VDDIO), or “LED-” (cathode). The backlight is often the trickiest part: some panels have a single LED cathode (LEDK) instead of two, and you need to tie both LEDK pins to ground if they’re separate. The current rating for the backlight is also critical—exceeding 150mA can burn out the LEDs. The FPC connector is usually a 0.5mm pitch, 40-pin ZIF type, and you need to make sure the mating connector on your PCB has the same orientation (top or bottom contact).

Interface Timing and Data Format

The MIPI DSI interface on these round displays uses a video mode (non-burst mode with sync pulses). The timing parameters are: HFP (horizontal front porch) = 20 pixels, HBP (horizontal back porch) = 20 pixels, HSYNC (horizontal sync pulse) = 10 pixels. For vertical: VFP = 10 lines, VBP = 10 lines, VSYNC = 2 lines. The total horizontal period is 800 + 20 + 20 + 10 = 850 pixels, and vertical is 800 + 10 + 10 + 2 = 822 lines. At 60Hz, the pixel clock is 850 * 822 * 60 = 41.9 MHz. The MIPI DSI clock is half that, around 20.95 MHz, and each lane runs at 4x the pixel clock (since 4 lanes), so 167.6 Mbps per lane. This is well within the 500 Mbps max for the ST7703S. The data format is 24-bit RGB888, sent as 3 bytes per pixel. The round shape is handled by the driver IC—it maps a rectangular 800x800 frame buffer to the circular pixel array, but you still send a full 800x800 rectangle. The display simply ignores the corners (they’re physically missing).

Physical Connector and Layout Considerations

The FPC on the 3.4 inch round TFT is typically 15mm wide, with a 0.5mm pitch and a 0.3mm thick stiffener. The connector on the PCB should be a 40-pin, 0.5mm pitch, right-angle or straight ZIF socket, like the FH12-40S-0.5SH from Hirose. The MIPI differential pairs need to be routed with 100-ohm differential impedance on the PCB. Keep the trace lengths matched within 10 mils for each pair. The ground pins (pins 1, 4, 7, 10, 13, 16, 19, 22, 27, 30, 33, 36, 39) are not just for return current—they also provide thermal relief for the driver IC. The backlight traces should be at least 0.5mm wide to handle 120mA without voltage drop. The TE pin is an output from the display, so you need a pull-up resistor (10k to 1.8V) if your MCU doesn’t have an internal pull-up. The PWM pin is input, and you can drive it with a 1kHz to 20kHz signal from a timer output. The reset pin has an internal pull-up, but you should still drive it from a GPIO to ensure proper power-on reset.

Real-World Application Example: Smartwatch or Dashboard Display

In a typical embedded project, you’d connect this display to an STM32H7 or ESP32-S3 with MIPI DSI support. The pinout directly maps to the MCU’s DSI peripheral: D0-D3 lanes to the MCU’s DSI data pins, CLK to DSI clock, RST to a GPIO, TE to an EXTI pin, and PWM to a timer output. The backlight is driven by a small boost converter (like the TPS61165) that takes 3.7V from a LiPo battery and boosts to 12V for the LED string. The 1.8V and 2.8V regulators can be LDOs like the AP