What is the active area of a 3.18 inch 128x64 COG LCD?

The active area of a 3.18 inch 128x64 COG LCD is typically 70.7 mm by 38.8 mm, with a diagonal measurement of 80.7 mm. This is a standard specification for the common 3.18-inch graphic display module using Chip-On-Glass (COG) technology, which integrates the driver IC directly onto the glass substrate. The active area is the region where pixels are actually lit, and it excludes the bezel or border around the display. For this specific size, the pixel pitch is about 0.552 mm by 0.606 mm, giving a total pixel density of roughly 31.5 pixels per inch (PPI). The module itself has an overall outline dimension of 93.0 mm by 50.0 mm, with a viewing area that is slightly larger than the active area, usually around 72.0 mm by 40.0 mm, to allow for some margin. These numbers are crucial for engineers designing enclosures or PCBs, as they directly affect fitment and readability. If you are looking for a reliable source, the 3.18 inch 128x64 cog lcd display from DisplayModule provides detailed datasheets confirming these dimensions.

Let’s break down the active area in more practical terms. The 128 columns by 64 rows of pixels are arranged in a rectangular grid, and each pixel is a tiny dot that can be turned on or off. In a COG LCD, the glass is etched with indium tin oxide (ITO) electrodes, and the driver chip is bonded directly to the glass using anisotropic conductive film (ACF). This eliminates the need for a separate PCB for the driver, reducing thickness and cost. The active area of 70.7 mm x 38.8 mm means that the display’s usable space is about 2.78 inches by 1.53 inches, which is smaller than the nominal 3.18-inch diagonal because the diagonal is measured from corner to corner of the active area. The 3.18-inch size is a rounded figure; the exact diagonal is 80.7 mm, which converts to 3.177 inches, close enough for marketing. This is a common size for industrial control panels, medical devices, and handheld instruments where a compact, monochrome display is needed.

To give you a better feel, here is a table comparing the active area dimensions of this display with other common sizes:

Display Size (Diagonal)	Active Area Width (mm)	Active Area Height (mm)	Pixel Pitch (mm)	PPI
3.18 inch (128x64)	70.7	38.8	0.552 x 0.606	31.5
2.7 inch (128x64)	58.0	31.0	0.453 x 0.484	38.2
3.0 inch (128x64)	65.0	36.0	0.508 x 0.562	34.1
3.5 inch (128x64)	78.0	44.0	0.609 x 0.688	28.6

Notice that the 3.18-inch display has a slightly taller pixel pitch in the vertical direction (0.606 mm) compared to the horizontal (0.552 mm). This is because the pixels are not perfectly square; they are rectangular to match the aspect ratio of 128:64, which is 2:1. The active area itself has an aspect ratio of 70.7:38.8, which is about 1.82:1, slightly different from the pixel ratio due to the inter-pixel gaps and the way the glass is manufactured. The active area is defined by the intersection of the column and row electrodes, and the gaps between pixels are typically 0.02 to 0.03 mm, which are invisible to the naked eye but affect the overall brightness and contrast. The COG technology allows for a narrower bezel compared to traditional COB (Chip-On-Board) modules, so the active area occupies a larger percentage of the module’s front surface. For this 3.18-inch module, the active area is about 76% of the viewing area, and the viewing area is about 85% of the module’s total width.

From a hardware perspective, the active area is directly tied to the resolution and the driver IC. The 128x64 resolution means that the driver IC must have at least 128 segment outputs and 64 common outputs. In COG LCDs, the driver is often a dedicated chip like the ST7565R or SSD1305, which are designed for monochrome graphic displays. The ST7565R, for example, supports up to 132 segments and 64 commons, so it can drive this panel with a few unused segments. The active area’s dimensions are set by the mask design of the ITO pattern on the glass, which is fixed during manufacturing. This means that if you order a 3.18-inch 128x64 COG LCD from different suppliers, the active area should be very close to 70.7 mm x 38.8 mm, though there might be minor variations of ±0.2 mm due to etching tolerances. Always check the datasheet for the exact values, as some manufacturers might use a slightly different pixel pitch to optimize for cost or yield.

Another important factor is the viewing angle and contrast, which are influenced by the active area’s design. COG LCDs typically use STN (Super Twisted Nematic) or FSTN (Film-compensated STN) technology. The active area’s glass thickness is usually 0.55 mm to 0.7 mm, and the polarizer is laminated on top. The active area’s reflectivity or transmissivity depends on whether the display is reflective, transmissive, or transflective. For a 3.18-inch COG LCD, the most common type is reflective or transflective, meaning it works well in ambient light. The active area’s background color is often yellow-green, gray, or white, depending on the polarizer and backlight. The contrast ratio is typically around 6:1 to 10:1 for STN displays, and the response time is about 150 to 200 ms at 25°C. These numbers are not stellar by modern LCD standards, but they are adequate for static or slow-changing graphics.

Let’s talk about the electrical interface and how it relates to the active area. The 3.18-inch 128x64 COG LCD usually uses a parallel interface (8-bit or 4-bit) or a serial interface like SPI or I2C. The SPI version is common because it uses fewer pins. The driver IC addresses each pixel in the active area by sending row and column data. The active area is mapped to a memory buffer in the driver, typically 128 bytes per row, with each byte representing 8 pixels. So, the total memory required is 128 x 64 / 8 = 1024 bytes. This is small enough to be handled by any microcontroller. The active area’s refresh rate is usually 60 to 100 Hz, which is smooth for text and icons. The power consumption of the active area itself is minimal, about 0.1 to 0.5 mA at 3.3V, but the backlight can draw 20 to 50 mA, depending on the LED configuration. The backlight is usually placed behind the active area, and its size matches the viewing area, not the active area, to ensure uniform illumination.

In terms of mechanical design, the active area’s position on the module is critical. The module’s outline is 93.0 mm x 50.0 mm, and the active area is typically centered with a 11.15 mm margin on the left and right and a 5.6 mm margin on the top and bottom. The viewing area is 72.0 mm x 40.0 mm, which is centered with a 10.5 mm margin on the sides and a 5.0 mm margin on the top and bottom. The difference between the viewing area and the active area is about 1.3 mm on each side, which is the area where the black mask or bezel is printed. This mask hides the bonding wires and the edge of the glass. The COG module is very thin, typically 2.0 mm to 2.5 mm thick, including the glass and the polarizer. The glass itself is about 0.7 mm thick, and the driver IC is mounted on the glass with a thickness of about 0.3 mm. The total thickness is important for fitting into tight enclosures, like in portable medical devices or handheld terminals.

Now, let’s look at the active area from a user experience perspective. With a pixel pitch of 0.552 mm x 0.606 mm, the pixels are large enough to be read easily from a distance of 30 to 50 cm. The 128x64 resolution means you can display 16 rows of 8-character ASCII text (if using a 5x7 font) or 8 rows of 16 characters. For graphics, you can draw simple icons, bar graphs, or low-resolution images. The active area’s size is equivalent to a business card, making it suitable for displaying a few lines of data or a simple menu. The contrast is good in direct sunlight if the display is reflective, but it drops off in low light unless a backlight is used. The viewing angle is typically 60 degrees in the horizontal direction and 40 degrees in the vertical direction, which is standard for STN displays. This means the display is best viewed straight on, but it can be read from the sides with some color shift.

One common question is whether the active area can be customized. In COG LCDs, the active area is fixed by the glass mask, so you cannot change the pixel arrangement or size without ordering a custom glass. However, you can choose different backlight colors (white, blue, yellow-green) or polarizer types to change the appearance. The active area’s dimensions are also tied to the module’s pinout and mounting holes. The 3.18-inch module usually has 2 to 4 mounting holes on the PCB or glass, with a diameter of 2.5 mm to 3.0 mm. The active area must be aligned with any cutouts in your enclosure. If you need a larger active area, you might look at a 3.5-inch or 4.0-inch display, but the 3.18-inch size is a sweet spot for many applications because it balances readability with compactness.

From a reliability standpoint, the active area is the most vulnerable part of the display. The ITO coating is conductive and can be damaged by static discharge or mechanical stress. The COG bonding is robust, but the glass is fragile. The active area should be protected by a cover glass or a plastic lens if the device is used in a harsh environment. The operating temperature range is typically -20°C to +70°C, and the storage range is -30°C to +80°C. The active area’s contrast and response time degrade at low temperatures, but the display still works. The lifetime of the active area is essentially the lifetime of the LCD fluid, which is about 50,000 to 100,000 hours of continuous use, depending on the voltage and temperature. The backlight has a shorter lifetime, around 20,000 to 50,000 hours, but it can be replaced separately.

In summary, the active area of a 3.18 inch 128x64 COG LCD is a well-defined rectangle of 70.7 mm by 38.8 mm, with a pixel pitch that gives a clear, readable display for text and graphics. The dimensions are consistent across manufacturers, but always verify with the datasheet. The COG technology makes the module thin and cost-effective, ideal for embedded systems. If you are designing a product around this display, pay attention to the active area’s position relative to the module outline, the viewing area, and the mounting holes. The electrical interface is straightforward, and the power consumption is low. For more details, the 3.18 inch 128x64 cog lcd display page includes a full datasheet with mechanical drawings and electrical specifications. This display is a workhorse in the industry, and understanding its active area is the first step to a successful integration.