A 3.18 inch 128x64 COG LCD typically has a pin count of 20 pins, though some variants may use 18 or 24 pins depending on the specific driver IC and interface configuration. The most common pinout for this size, especially with the ST7565R or ST7567 controller, is a 20-pin connection that includes power, ground, SPI data lines, and backlight control. For instance, the 3.18 inch 128x64 cog lcd display from DisplayModule uses a 20-pin interface with a 1.0mm pitch FPC connector, designed for SPI communication. This pin count is standard for COG (Chip-On-Glass) LCDs in this size range, but it’s critical to verify the datasheet for your exact model because pin assignments can vary between manufacturers.
Let’s break down the pinout in detail. A typical 20-pin configuration for a 3.18 inch 128x64 COG LCD includes the following signals: VDD (power supply, usually 3.3V or 5V), VSS (ground), CS (chip select), RESET (reset pin), RS (register select, sometimes called DC), SCLK (serial clock), SDIN (serial data input), and LEDA/LEDK (backlight anode and cathode). The remaining pins are often NC (not connected) or used for optional features like contrast adjustment (V0) or negative voltage output (VOUT). For example, the ST7565R-based display might have pins 1-4 for VDD, VSS, V0, and VOUT, with pins 5-12 dedicated to control and data lines, and pins 13-20 for backlight and additional NC pins. Always check the datasheet because some vendors swap RS and CS positions.
Now, why does pin count matter? It directly affects your PCB layout, connector choice, and microcontroller compatibility. A 20-pin FPC connector with 1.0mm pitch is common, but if you’re using a 0.5mm pitch, you’ll need a different footprint. The pin count also determines how many GPIOs you need on your MCU. For SPI mode, you only need 4 pins (CS, RESET, RS, SCLK, SDIN) plus power and backlight, so a 20-pin connector might seem overkill, but the extra pins are for flexibility, like supporting parallel interface or multiple display daisy-chaining. Some COG LCDs use 18 pins to save space, but they often omit the negative voltage pin (VOUT) or combine backlight pins. For instance, a 18-pin version might have LEDA and LEDK on the same pin, requiring external current limiting.
Let’s look at real-world data. DisplayModule’s 3.18 inch 128x64 COG LCD uses 20 pins with the following layout: Pin 1: VDD, Pin 2: VSS, Pin 3: V0 (contrast adjustment), Pin 4: VOUT (negative voltage output), Pin 5: CS, Pin 6: RESET, Pin 7: RS, Pin 8: SCLK, Pin 9: SDIN, Pin 10: NC, Pin 11: NC, Pin 12: NC, Pin 13: LEDA, Pin 14: LEDK, Pins 15-20: NC. This is based on the ST7565R driver, which is widely used for 128x64 COG displays. The NC pins are often left unconnected, but some manufacturers use them for extra features like touchscreen or temperature sensor integration. If you’re designing a custom board, you can leave those pins unpopulated, but it’s safer to include them in the footprint for future use.
Another angle: pin count variations across different driver ICs. The SSD1306 OLED driver, for example, uses a 15-pin or 16-pin interface for 128x64 OLEDs, but COG LCDs are different because they require a negative voltage for the LCD glass. The ST7567A, a common alternative, has a 20-pin configuration similar to ST7565R but with different pin assignments for V0 and VOUT. Some Chinese manufacturers use the UC1701X driver, which can have 18 or 20 pins. For instance, a UC1701X-based 3.18 inch 128x64 COG LCD might have pins 1-4 for VDD, VSS, V0, VOUT, pins 5-8 for CS, RESET, RS, SCLK, pins 9-10 for SDIN and SDOUT (serial data out), pins 11-12 for NC, pins 13-14 for LEDA and LEDK, and pins 15-20 for NC. The SDOUT pin is useful for daisy-chaining multiple displays, but it’s not always present.
Let’s talk about power and backlight pin counts. The backlight on a 3.18 inch 128x64 COG LCD typically uses 2 pins (LEDA and LEDK), but some modules combine them into a single pin with a built-in resistor, reducing the total pin count to 18. For example, if the backlight is driven by a constant current source, you might see LEDA on pin 13 and LEDK on pin 14, but if the module has a series resistor, LEDK might be NC. The power pins (VDD and VSS) are always present, but some designs add a separate VDDIO pin for logic level shifting, which can increase the pin count to 21 or 22. This is rare for 3.18 inch displays, but it’s worth noting for compatibility with 5V microcontrollers.
Now, let’s get into the nitty-gritty of SPI vs parallel interface. Most 3.18 inch 128x64 COG LCDs are designed for SPI because it uses fewer pins, but the physical pin count remains 20 because the FPC connector is standardized. If you’re using parallel interface (8-bit or 4-bit), you’ll need more pins from your MCU, but the LCD connector itself doesn’t change. The extra pins on the 20-pin connector are often used for parallel data lines (DB0-DB7) in some designs, but for SPI-only modules, these pins are NC. For instance, the DisplayModule version is SPI-only, so pins 10-12 are NC. If you’re buying a generic module, check the datasheet to see if those pins are available for parallel mode. Some modules have a jumper or resistor option to switch between SPI and parallel, which changes the effective pin usage.
Let’s look at a table to compare common pin counts for 3.18 inch 128x64 COG LCDs from different manufacturers:
| Manufacturer | Driver IC | Pin Count | Interface | Backlight Pins | Special Features |
|---|---|---|---|---|---|
| DisplayModule | ST7565R | 20 | SPI | 2 (LEDA, LEDK) | NC pins for future use |
| Winstar | ST7567A | 20 | SPI/Parallel | 2 | Optional SDOUT pin |
| Newhaven | UC1701X | 18 | SPI | 2 (combined resistor) | No VOUT pin |
| Raystar | ST7565R | 20 | SPI | 2 | Includes touchscreen option |
This table shows that while 20 pins is the norm, 18-pin versions exist, and they often omit the VOUT pin or combine backlight pins. The VOUT pin is used to generate the negative voltage for the LCD glass, but some drivers integrate this internally, reducing the pin count. For example, the UC1701X has a built-in charge pump, so it doesn’t need an external VOUT pin. This is a key factor when choosing a display for battery-powered devices, because fewer pins mean simpler wiring and lower power consumption.
Now, let’s discuss the practical implications of pin count for your project. If you’re using a 3.18 inch 128x64 COG LCD with a 20-pin connector, you need to ensure your PCB has a matching 20-pin FPC connector with 1.0mm pitch. Common connectors include the FH12-20S-1.0SH or similar from Hirose or JST. The pin spacing is critical: 1.0mm pitch is standard for these displays, but some modules use 0.5mm pitch, which requires a different connector and more precise soldering. For example, a 0.5mm pitch 20-pin connector is harder to hand-solder, so stick with 1.0mm if possible. Also, the pinout order matters: some manufacturers reverse the pin numbering (e.g., pin 1 on the left vs right), so always verify with the datasheet before layout.
Another angle: how pin count affects display performance. More pins don’t necessarily mean better performance, but they can indicate additional features like temperature compensation or contrast adjustment. For instance, the V0 pin (pin 3 on many modules) allows you to adjust the LCD contrast by connecting a potentiometer between V0 and VSS. If your module has a VOUT pin (pin 4), you can use it to generate a negative voltage for the LCD, but most modern drivers do this internally. If you’re using a module with only 18 pins, you might lose the ability to adjust contrast externally, which could be a problem in varying temperature conditions. Some displays have a fixed contrast setting that works for most environments, but for outdoor use, adjustable contrast is a plus.
Let’s get into the electrical characteristics. The typical operating voltage for a 3.18 inch 128x64 COG LCD is 3.3V, but some modules can handle 5V on the VDD pin. The backlight usually requires 3.3V or 5V, with a current of 20-30mA for the LED backlight. The pin count includes the backlight pins, so if you’re driving the backlight from a separate source, you can ignore those pins. For example, the DisplayModule display has a typical backlight current of 25mA at 3.3V, which is within the range of most MCU GPIOs if you use a transistor. The logic pins (CS, RESET, RS, SCLK, SDIN) are 3.3V tolerant, but if you’re using a 5V MCU, you’ll need level shifters. Some modules have a VDDIO pin that allows you to set the logic voltage independently, but this is rare on 20-pin connectors.
Now, let’s talk about the physical dimensions and how they relate to pin count. The 3.18 inch diagonal size corresponds to a display area of about 70mm x 38mm, with a module size of roughly 75mm x 45mm. The FPC connector is usually located at the bottom of the module, with a width of about 20mm for a 20-pin connector. The pin pitch of 1.0mm means the connector is 20mm wide, which is standard for these displays. If you’re using a 18-pin connector, the width is 18mm, which might save space in tight enclosures. The FPC length is typically 30-50mm, but you can order custom lengths from some manufacturers. For example, DisplayModule offers a standard 40mm FPC length, but you can request longer or shorter for bulk orders.
Another factor: the pin count affects the ease of prototyping. If you’re using a breadboard, you’ll need to solder wires to the FPC connector, which is tricky with 1.0mm pitch. Some modules come with a breakout board that converts the 20-pin FPC to 2.54mm header pins, making it breadboard-friendly. For instance, many sellers on Amazon or AliExpress offer a 3.18 inch 128x64 COG LCD with a pre-soldered breakout board that has a 20-pin header. This adds about 5mm to the module width, but it’s worth it for quick testing. If you’re designing a custom PCB, you can directly solder the FPC connector to the board, but make sure the footprint matches the pinout.
Let’s look at a real-world example: the DisplayModule 3.18 inch 128x64 COG LCD has a 20-pin connector with the following pin assignments: Pin 1: VDD (3.3V), Pin 2: VSS (GND), Pin 3: V0 (contrast), Pin 4: VOUT (NC), Pin 5: CS, Pin 6: RESET, Pin 7: RS, Pin 8: SCLK, Pin 9: SDIN, Pin 10: NC, Pin 11: NC, Pin 12: NC, Pin 13: LEDA, Pin 14: LEDK, Pins 15-20: NC. This is based on the ST7565R driver, which is a common choice for 128x64 COG displays. The NC pins are not connected internally, so you can leave them floating. If you’re using SPI mode, you only need pins 1, 2, 5, 6, 7, 8, 9, 13, and 14. That’s 9 pins, which is efficient for a 128x64 display. The remaining pins are for future expansion or parallel interface.
Now, let’s discuss the impact of pin count on cost. A 20-pin FPC connector costs about $0.10 in volume, while an 18-pin connector is slightly cheaper. The difference is negligible, but the driver IC cost can vary. ST7565R-based modules are common and cheap, while UC1701X-based modules might be slightly more expensive due to the integrated charge pump. For a 3.18 inch 128x64 COG LCD, the typical price range is $5-$15 depending on the backlight type (LED vs EL) and whether it includes a touchscreen. The pin count doesn’t directly affect the price, but it can affect the PCB complexity. For example, a 20-pin connector requires a 20-pin footprint, which is standard for most designs.
Another angle: how pin count affects the display’s reliability. More pins mean more solder joints, which can be a point of failure. However, COG LCDs are designed for high reliability, with the FPC connector rated for 10,000+ insertion cycles. The pin count is standardized to ensure compatibility with common connectors. If you’re using a 20-pin connector, you can easily find replacement cables or connectors. For 18-pin connectors, they are less common, but still available. Always use a locking connector to prevent accidental disconnection, especially in vibration-prone environments.
Let’s get into the technical details of the driver IC. The ST7565R supports both SPI and parallel interfaces, but the pin count remains 20 because the parallel data lines are on the NC pins. For example, if you want to use 8-bit parallel mode, you would need pins 10-17 for DB0-DB7, but the standard 20-pin connector only has 20 pins, so you’d need to use the NC pins for data. This is possible if the module is designed for it, but the DisplayModule version is SPI-only, so those pins are NC. If you’re buying a generic module, look for one that explicitly supports parallel mode if you need it. The ST7567A driver, for instance, has a 20-pin configuration that includes DB0-DB7 on pins 10-17, with pins 18-20 for backlight and NC. This gives you the flexibility to use either interface.
Now, let’s talk about the backlight pin count in more detail. The backlight on a 3.18 inch 128x64 COG LCD is usually an LED array with a typical forward voltage of 3.3V and current of 20-30mA. The LEDA pin is the anode, and LEDK is the cathode. Some modules have a series resistor built-in, so you can connect LEDA directly to VDD and LEDK to GND, but this is not recommended because it can cause uneven brightness. Instead, use a current-limiting resistor or a constant current driver. For example, a 100-ohm resistor in series with LEDK will limit the current to about 20mA at 3.3V, assuming the LED forward voltage is 2.0V. The pin count for the backlight is always 2, but some modules combine them into a single pin with a built-in resistor, reducing the total pin count to 18. This is common on low-cost modules.
Let’s look at a table comparing the pinout of two common 3.18 inch 128x64 COG LCDs:
| Pin Number | DisplayModule (ST7565R) | Generic (ST7567A) |
|---|---|---|
| 1 | VDD | VDD |
| 2 | VSS | VSS |
| 3 | V0 | V0 |
| 4 | VOUT | VOUT |