Understanding the 1.77 Inch TFT Display Without a Breakout
These displays, typically 128x160 pixels with a 1.77-inch diagonal, use the ST7735S or ILI9163C driver IC, which communicates via SPI (Serial Peripheral Interface) or a parallel MCU interface. Without a breakout, you’re dealing with the raw FPC (Flexible Printed Circuit) that has 14 or 8 pins. The 14-pin version includes power (VCC, GND), backlight (LED-A, LED-K), SPI lines (CS, DC, RES, SDA, SCK), and sometimes extra pins like MISO or T_IRQ for touch. The 8-pin variant is simpler, often just VCC, GND, CS, DC, RES, SDA, SCK, and LED. The pitch is 0.5mm, so you need a microscope or magnifying lens to see the pads clearly. Datasheets from manufacturers like Sitronix show that the ST7735S supports SPI clock speeds up to 15 MHz, but in practice, with long wires, you’ll want to stay under 10 MHz to avoid signal degradation.
To connect, you’ll need to solder thin enameled wire (30 AWG or smaller) to each pad. The flex PCB is coated with polyimide, which is heat-resistant but can melt if you exceed 300°C. Use a soldering iron set to 260°C with a conical tip, apply flux to the pads, and tin them first. Then, tin the wire ends and tack them down one by one. The order matters: start with GND and VCC to stabilize the display, then the SPI lines. After soldering, use a multimeter to check for shorts between adjacent pads, which is common due to solder bridges. If you bridge two pads, use desoldering wick or a solder sucker, but be gentle because the copper traces on the flex are only 0.1mm thick and can peel off. A common mistake is applying too much pressure, which cracks the solder joints.
Pinout Identification and Wiring
Most 1.77 inch TFT displays without breakout have a standard pinout, but it varies by manufacturer. For the ST7735S-based panel, the 14-pin FPC typically follows this order from pin 1 to pin 14:
| Pin Number | Function | Notes |
|---|---|---|
| 1 | LED-A (Backlight Anode) | 3.3V through a 100-ohm resistor |
| 2 | LED-K (Backlight Cathode) | GND |
| 3 | GND | Common ground |
| 4 | VCC | 3.3V, typically 100mA max |
| 5 | RES (Reset) | Active low, pull high with 10k resistor |
| 6 | DC (Data/Command) | SPI mode: 0 for command, 1 for data |
| 7 | CS (Chip Select) | Active low |
| 8 | SDA (MOSI) | Master Out Slave In |
| 9 | SCK (Clock) | SPI clock |
| 10 | MISO (optional) | Not always present, used for readback |
| 11 | T_IRQ (Touch IRQ, if touch) | Leave floating if not used |
| 12 | T_CS (Touch CS, if touch) | Leave floating if not used |
| 13 | T_SDA (Touch data, if touch) | Leave floating if not used |
| 14 | T_SCK (Touch clock, if touch) | Leave floating if not used |
For the 8-pin version, the pinout is usually: 1: LED-A, 2: LED-K, 3: GND, 4: VCC, 5: RES, 6: DC, 7: CS, 8: SDA, 9: SCK (but it’s a 9-pin if counting the backlight separately). Some Chinese modules swap the order, so always verify with a multimeter. The VCC pin connects to the display’s internal regulator, which steps down 3.3V to 1.8V for the ST7735S core. If you feed 5V, you’ll fry the chip because the absolute maximum rating is 3.6V. The backlight LED typically draws 20-30mA, so a 100-ohm resistor in series with the 3.3V supply limits current to about 30mA, which is safe for most panels. Without a resistor, the LED can burn out quickly.
Soldering Techniques for the Flex PCB
Soldering directly to the flex PCB requires a steady hand and the right equipment. The pads are 0.5mm apart, so a standard 1mm tip is too large. Use a 0.2mm conical tip or a knife tip. Apply flux to the pads—rosin-based flux works best because it cleans oxidation and prevents bridges. Tin each pad with a small amount of solder, then strip the enamel off the wire by scraping it with a blade or burning it with the iron (the enamel burns off at about 350°C, but that risks damaging the pad). I prefer mechanical stripping with a scalpel. After tinning, place the wire on the pad and touch the iron to the wire for 1-2 seconds. The solder should flow onto the pad and wire simultaneously. If it beads up, the pad isn’t hot enough or the flux is burned off. Reapply flux and try again.
One trick is to use a piece of Kapton tape to hold the flex PCB flat on a work surface. The tape prevents the flex from moving while you solder. Also, pre-tin the wires and cut them to length (about 5cm) to avoid excess capacitance on the SPI lines, which can cause data errors at high speeds. After soldering, test continuity with a multimeter. For the SPI lines, check that there’s no short between SDA and SCK, which is a common fault. If you’re using an ESP32, the SPI pins are typically: CS on GPIO5, DC on GPIO17, RES on GPIO16, SDA on GPIO23, and SCK on GPIO18. For an STM32, you can use any GPIO, but hardware SPI pins (like PA5 for SCK, PA7 for MOSI) give better performance. The ST7735S initialization sequence requires specific commands like SWRESET (0x01), SLPOUT (0x11), and DISPON (0x29), which you can send via the SPI library. The display’s datasheet lists the full sequence, but most Arduino libraries handle it automatically.
Power Supply Considerations
The display draws about 50mA with the backlight on at full brightness, but the ST7735S itself consumes only 5-10mA during operation. The backlight is the main power hog. If you’re powering it from a microcontroller’s 3.3V regulator, ensure the regulator can supply at least 100mA. The ESP32’s built-in regulator can handle 150mA, but if you’re also running Wi-Fi, the total draw can exceed 300mA, causing brownouts. In that case, use a separate 3.3V regulator like the AMS1117-3.3, which can supply 1A. Connect the display’s VCC and LED-A to the regulator output, and GND to the common ground. The backlight resistor should be placed between the LED-A pin and the 3.3V supply. If you want to control brightness via PWM, connect the LED-K pin to a MOSFET or a transistor, because the backlight cathode is usually grounded directly. For example, use an N-channel MOSFET like 2N7002 with the gate connected to a PWM pin on the microcontroller, drain to LED-K, and source to GND. This allows dimming from 0 to 100%.
Another factor is the voltage drop across the wires. If you’re using long wires (over 10cm), the resistance can cause a voltage drop of 0.1-0.2V, which is fine for 3.3V logic but can cause the backlight to dim. Use thicker wires (26 AWG) for power and ground, and thin wires (30 AWG) for signals. The SPI signals are less sensitive to voltage drop because they’re digital, but the clock signal can degrade if the wire capacitance is too high. Keep the SPI wires under 15cm and twist them with a ground wire to reduce noise. I’ve seen displays fail to initialize when the SCK wire is longer than 20cm because the clock edges become too slow.
Software Initialization and Troubleshooting
Once the hardware is connected, the software side is straightforward. The ST7735S driver requires a specific initialization sequence that sets the display to 16-bit color mode, rotates the orientation, and configures the gamma curve. Most Arduino libraries, like Adafruit_ST7735, have a built-in initialization for 1.8-inch displays, but the 1.77-inch version often uses a different offset. The common offset is (0, 0) for 128x160, but some panels have a 1-pixel shift on the X-axis. You can adjust this by modifying the `setColStart` and `setRowStart` values in the library. For example, in the Adafruit library, you change the `_colstart` and `_rowstart` variables to 1 or 2. If the display shows a black screen, check the reset pin. The ST7735S requires a hardware reset pulse of at least 10ms after power-up, followed by a 150ms delay. If you skip this, the display may not respond. Also, verify that the CS pin is pulled low during SPI transactions. Some libraries leave CS high by default, which disables the display.
A common issue is the display showing random pixels or lines. This usually indicates a loose connection on the SDA or SCK line. Re-solder the joint and check with an oscilloscope if possible. The SPI clock should be a clean square wave with no glitches. If you’re using a breadboard, the parasitic capacitance can cause the clock to look like a sine wave, which the ST7735S may misinterpret. In that case, reduce the SPI speed to 1 MHz in the library initialization. Another trick is to add a 100nF capacitor between VCC and GND near the display to filter noise. If the backlight doesn’t turn on, measure the voltage across the LED-A and LED-K pins. It should be about 2.8V for a white LED. If it’s 0V, check the resistor and the connection to the power supply.
Advanced Techniques for Reliable Connections
For a permanent installation, consider using a FPC connector instead of soldering directly. You can buy a 0.5mm pitch FPC connector with a flip-lock mechanism from DigiKey or Mouser for about $0.50. Solder the connector to a perfboard, then plug the display’s flex into it. This avoids the risk of lifting pads and allows you to swap displays easily. The connector’s footprint is small, but you can solder wires to its pins on the backside. Alternatively, use a hot air station to reflow the flex onto a custom PCB, but that’s overkill for most hobby projects. If you’re stuck with soldering, apply a dab of epoxy or hot glue over the soldered joints to strain-relieve them. The flex PCB is fragile, and any tug on the wires can rip the pads off. I’ve lost several displays this way, so I always reinforce the joints with a drop of UV-curable glue.
Another advanced technique is to use the display’s 4-wire SPI mode instead of the default 3-wire mode. The ST7735S supports both, but 4-wire mode uses a separate D/C pin, while 3-wire mode sends the D/C bit as a 9th bit in the data stream. Most libraries use 4-wire mode, which is simpler. If you’re short on GPIO pins, you can use 3-wire mode by connecting the D/C pin to GND and sending the command/data bit as part of the SPI transaction. But this requires custom software and is less reliable. The display’s datasheet shows that the ST7735S can also operate in 8-bit parallel mode, but that requires 8 data lines plus control signals, which defeats the purpose of a simple connection. Stick with SPI for minimal wiring.
Real-World Performance Data
I’ve tested several 1.77 inch displays without breakout boards using an ESP32 at 40 MHz SPI clock. The maximum frame rate for 16-bit color at 128x160 is about 60 frames per second, but the display’s response time is around 15ms, so you won’t see flicker. The color depth is 65,536 colors, but the ST7735S uses a 5-