How to Choose a Graphic LCD | Interface, Resolution & Temperature Range

When selecting a model, use I2C or SPI for low-speed interfaces, and RGB or Parallel interfaces for high-speed ones;

Select a resolution of 128x64 or higher based on display precision requirements;

For outdoor industrial control, you must lock in the -20°C to 70°C wide temperature parameters to ensure reliability.

Interface

Choosing a Graphic LCD interface requires directly calculating the number of available I/O pins on the MCU and the required data throughput.

Parallel Interfaces (8080/6800) consume 13 to 20 GPIOs, but in 8-bit or 16-bit bus modes, the time to write a frame of 320x240 pixel data can be less than 5ms, making them suitable for dynamic displays.

Serial Interfaces (SPI/I2C) reduce physical connections to 2-5 wires, greatly simplifying PCB routing, but are limited by bandwidth; a full-screen refresh of a 128x64 monochrome image requires more than 20ms.

If I/O resources are limited and only static parameters need to be displayed, prioritize SPI.

Parallel Interface

How to Wire

Although both are "parallel," they are divided into Intel 8080 and Motorola 6800 timing standards based on different historical control logic.

Most modern LCD driver chips (such as ILI9341, ST7789) support both modes, switched via the level configuration of hardware pins (usually marked as IM0-IMx).

• Intel 8080 Mode (Most Common):
  This mode has high logic separation and the code is easy to understand. It has independent "Read Enable" and "Write Enable" signals.
  • CS (Chip Select): Chip select, active low.
  • RS (Register Select) or D/C: Data/Command selection. Low level transmits commands (like setting coordinates), high level transmits pixel data.
  • WR (Write): Write signal. The MCU places data on the data lines, pulls the WR pin low, and the LCD reads the data on the rising edge (the instant it is pulled high).
  • RD (Read): Read signal. Used to read video memory data or ID numbers from the LCD.
  • DB0 - DB15: Bidirectional data bus.
• Motorola 6800 Mode:
  This mode uses one less wire, and the control logic is slightly different.
  • E (Enable): Enable signal, equivalent to a clock trigger.
  • R/W (Read/Write): Read/Write selection. High level represents read, low level represents write.
  • Data is considered valid during the high level period of the E signal.

Choosing 8-bit, 9-bit, or 16-bit Bus?

LCD pixels are usually stored in RGB565 format (16-bit), meaning one pixel occupies 2 bytes.

• 8-bit Bus (8-bit Data Bus):
  • Physical Connection: Uses DB0-DB7, saving GPIOs.
  • Transmission Logic: To transmit a 16-bit pixel (RGB565), the MCU needs to perform two operations. Send the high 8 bits first, then the low 8 bits.
  • Efficiency Impact: The number of writes is doubled. If your MCU clock speed is low (e.g., < 48MHz), you might see a visible "scan line" moving down during screen refreshes.
• 16-bit Bus (16-bit Data Bus):
  • Physical Connection: Uses DB0-DB15, occupying a large number of GPIOs.
  • Transmission Logic: A single write operation transmits a complete RGB565 pixel.
  • Efficiency Impact: The speed is twice that of an 8-bit bus. For a 480x320 resolution screen (153,600 pixels), an 8-bit bus requires toggling pins 307,200 times, while a 16-bit bus only needs 153,600 times.
• 9-bit and 18-bit Bus (Less Common):
  Mainly used for RGB666 format (262k colors).
  • 9-bit Bus: Transmitting one pixel takes 2 cycles (2 x 9bit = 18bit).
  • 18-bit Bus: One pixel is transmitted in one cycle. This configuration has extremely high requirements for the MCU interface, and usually only high-end MPUs have complete 18-bit or 24-bit interfaces.

Where is the Speed Bottleneck Here?

The upper limit of the refresh rate is determined not by the number of data lines, but by the Write Cycle Time.

• Minimum Write Cycle: Usually between 66ns and 100ns.
• MCU Coordination: If the MCU's GPIO toggling speed cannot keep up, the advantage of the parallel interface is canceled out by code execution efficiency.
• TE Signal (Tearing Effect): In parallel interfaces, the LCD often provides a TE output pin.

Hardware Wiring is Also a Major Hassle

Parallel interfaces are one of the biggest headaches for PCB layout engineers.

• Pin Occupancy and Fan-out:
  A standard 16-bit parallel interface LCD requires: 16 (Data) + 1 (CS) + 1 (RS) + 1 (WR) + 1 (RD) + 1 (RST) + 2 (Power) + 4 (Backlight) + 4 (Touch Screen) ≈ 31 wires. This takes up a lot of width on a 0.5mm pitch FPC cable, leading to larger connector sizes.
• Cable Length and Interference:
  Parallel signals are very sensitive to cable length.
  • Electromagnetic Interference (EMI): 16 data lines toggling levels simultaneously at 10MHz will generate huge radiated noise.
  • Crosstalk: If the FPC cable is too long (over 15cm), capacitance coupling between adjacent data lines causes signal distortion. For example, when DB0 to DB15 change from 0 to 1 simultaneously, the instantaneous current is huge, potentially causing Ground Bounce and leading to data transmission errors (screen corruption).

Serial Interface

How to Wire SPI and How Fast It Runs

Although standard SPI defines 4 wires, in LCD modules, you will often see 3-Wire and 4-Wire variants, which often confuses beginners.

• 4-Wire SPI (Most Common):
  This is the mode with the best balance of speed and control.
  • SCLK (Serial Clock): Clock line, generated by the MCU.
  • MOSI (Master Out Slave In): Data line (Master sends, Slave receives).
  • CS (Chip Select): Chip select line.
  • D/C (Data/Command): This line is very important. It is a physical pin; high level represents pixel data transmission, low level represents configuration command transmission.
  • Advantages: Because of the D/C pin, the MCU does not need to send extra command bits to distinguish data types, resulting in the highest transmission efficiency.
• 3-Wire SPI (9-bit SPI):
  To save that D/C line, this mode changes the data bit width to 9 bits.
  • Logic: The first bit of each transmission is the D/C bit (0 or 1), followed by 8 bits of actual data.
  • Disadvantages: The hardware SPI modules of most MCUs only support 8-bit or 16-bit data frames. To send 9-bit data, you usually need to use software simulation (Bit-banging), which consumes CPU performance extremely, or you must use an advanced MCU that supports 9-bit mode.

Speed Quantification Calculation:
Assume we want to drive a 240x240 resolution IPS color screen (common in smartwatches), with a pixel format of RGB565 (16-bit/pixel).

• Data per frame: 240 * 240 * 16 bit = 921,600 bits.
• 20 MHz Clock (General Microcontroller): 921,600 / 20,000,000 = 0.046 seconds. That is, 46ms to refresh a frame, approximately 21 FPS.
• 80 MHz Clock (High-end ESP32/STM32): 921,600 / 80,000,000 = 0.011 seconds. Approximately 11ms, i.e., 90 FPS.

Why DMA is Essential for Serial Screens

If DMA (Direct Memory Access) is not used, the CPU must constantly guard the SPI register: load a byte -> wait for transmission to finish -> load the next one.

To send a frame of 150KB data, the CPU has to run at full load for tens of milliseconds, during which it cannot handle button responses or sensor data.

How DMA Works:
You allocate a Frame Buffer array in memory, draw the image to be displayed, and then tell the DMA controller: "Move the data from this memory to the SPI transmit register, and call me when you are done."

The MCU can then go to sleep or process other logic, and the screen refresh is completed entirely by hardware.

For serial screens of 320x240 and above, DMA is standard equipment, otherwise the system will be too laggy to use.

I2C:

It only has two wires (SDA and SCL), saving pins drastically, but its protocol overhead is too large.

• Protocol Drags Down Speed:
  SPI can send data continuously as long as CS is pulled low. For I2C, for every byte (8-bit) sent, the receiver must send back an ACK bit (1-bit). Plus the Start signal and Stop signal, and the necessity to send the device Address first, the effective data transmission rate is greatly discounted.
• Bandwidth Limit:
  • Standard Mode: 100 kbps.
  • Fast Mode: 400 kbps.
  • High-Speed Mode: 3.4 Mbps.
• Actual Calculation:
  Taking the common 0.96-inch OLED (128x64 monochrome) as an example.
  • Data amount: 128 * 64 bit = 8,192 bits (approx. 1KB).
  • At 400 kbps speed, transmitting 8192 bits plus protocol overhead takes about 25ms to 30ms.
  • This is completely sufficient for displaying temperature, time, or battery level (30-40 FPS).
  • But if you want to use I2C to drive a 160x80 color screen (RGB565), the data volume instantly expands 16 times, and refreshing one frame takes 500ms (0.5 seconds).

FPC Cable and Hardware Layout Advantages

Choosing a serial interface is often not just because of a lack of pins, but also to reduce size.

• Connector Size:
  Parallel interfaces require a 0.5mm pitch 40Pin FPC socket, which is about 2cm wide.
  Serial interfaces (SPI/I2C) usually only need 8-12Pins, and the width is only 0.5cm.
• Anti-interference Ability:
  SPI and I2C have fewer traces and it is easier to perform GND Shielding.

Resolution

For monochrome graphic LCDs, a 128x64 screen requires a 1024-byte (1KB) Frame Buffer, while the QVGA specification (320x240) requires 9600 bytes (approx. 9.4KB).

If the MCU's RAM is less than 10KB, driving a high-resolution screen will cause a memory overflow.

In addition, pixel density affects interface selection: a standard I2C interface (400kHz) performs well when refreshing a 128x64 screen (approx. 30ms/frame), but when used for a 320x240 screen, the full-screen refresh time extends to over 200ms, causing visible lag to the naked eye.

Content Capacity

Calculating How Many Characters Can Be Displayed

On graphic LCDs, fonts are not fixed; you can draw them yourself, but to ensure readability, there are several common standard dot matrices in the industry.

Difference Between 5x7 and 6x8
The most basic English characters usually use a 5x7 dot matrix (5 pixels wide, 7 pixels high).

But you cannot calculate capacity directly based on this size because there must be gaps between characters and intervals between lines.

• Horizontal Spacing: Usually 1 blank pixel is left between characters, so the width becomes 6 pixels.
• Vertical Spacing: Usually 1 blank pixel is left between lines, so the height becomes 8 pixels.
• Actual Capacity (taking 128x64 screen as an example):
  • Width: 128 / 6 = 21.3. This screen can display at most 21 characters per line.
  • Height: 64 / 8 = 8. In full-screen plain text mode, it can display at most 8 lines.

Handling Letters with "Tails"
If your interface includes lowercase letters g, j, p, q, y, standard 5x7 fonts usually lift these letters up to align their "tails" with the baseline which looks messy.

If you need better typography, you must use 5x9 or 6x10 font structures to leave room for the Descender.

• In this case, the single line height needs to be at least 10 or 11 pixels.
• Capacity Change: 64 / 11 = 5.8. If you pursue beautiful typography, a 128x64 screen can actually only display 5 lines of text.

Impact of Embedded Font Chips
Some LCD modules (versions with Chinese character libraries) or controllers (such as T6963C) have built-in hardware character generators (CGROM).

These hardware fonts usually force the use of 8x8 or 8x16 block occupation.

• If you use hardware fonts, a 128x64 screen can only display 16 columns x 8 lines (8x8 font) or 16 columns x 4 lines (8x16 font). This is about 20% less capacity than software-drawn fonts, so you need to confirm during selection whether display density is sacrificed to save MCU computing power.

Pixel Overhead of Icons and UI Controls

On monochrome graphic screens, icons need enough pixels to outline the contours, otherwise they will turn into a black blob.

Icon Size Grading

• Status Bar Icons (8x8 pixels): This is the limit size. Usually used to display simple battery frames, Bluetooth symbols, or play/pause arrows.
• Function Icons (16x16 pixels): The most commonly used size. Can clearly express concepts like folders, floppy disks (save), gears (settings), etc.
  • Space Occupation: On a 64-pixel high screen, a 16x16 icon occupies 25% of the vertical space.
• Main Visual Icons (32x32 pixels): Used for boot screens or main operating states. This will occupy 50% of the height and 25% of the width of a 128x64 screen, leaving space for only two lines of short text next to it.

Margins for Inverse Cursors
A common menu selection method in graphical interfaces is "inverse display".

• To prevent the inverse cursor block (Highlight Bar) from sticking to the text of the lines above and below, 1-2 pixels of "breathing space" must be added to the top and bottom of each menu item.
• This results in a screen that was calculated to hold 8 lines of text effectively only holding 5 to 6 interactive menu items after adding the white space required for the inverse cursor.

Drawing and Waveform Display Area

Resolution directly limits the dynamic range and time span of data.

Vertical Resolution and Data Precision
Suppose you use a 128x64 screen to display a voltage waveform.

1. Remove Borders: Leave 8 pixels at the top for the title and 8 pixels at the bottom for coordinate axis values.
2. Plotting Area Height: Remaining 48 pixels (64 - 8 - 8).
3. Data Mapping: Your ADC might be 10-bit (0-1023) or 12-bit (0-4095). You need to compress values from 0-1023 into the 0-48 pixel range.
   • 1 pixel on the screen represents about 21 ADC counts.
   • Subtle signal fluctuations (like noise of 10 units) are completely invisible on the screen; only straight lines are displayed. If you need to observe minute fluctuations, you must choose a 240x128 or higher specification screen.

Horizontal Resolution and Time Window
The 128 pixel points in the horizontal direction determine how long of a history of data you can see.

• One-to-One Mapping: If each pixel represents a sampling point, the full screen can only display 128 data points.
• Sampling Rate Impact:
  • If you sample 10 times per second (10Hz), the screen can display 12.8 seconds of data.
  • If you sample 100 times per second (100Hz), the screen can only display 1.28 seconds of data, and the waveform will "run" very fast, making it impossible for the human eye to see the trend of change clearly.
• Solution: To display long-period waveforms on a low-resolution screen, it is usually necessary to take an average of every 10 collected points and draw it on the screen (downsampling), but this will lose the details of instantaneous glitch signals.

Layout Simulating Traditional Character Screens

Many projects use graphic screens to replace old character screens (Character LCDs, such as 1602 or 2004), aiming to support multiple languages or display Logos while maintaining the original interface logic.

The following table shows the correspondence between graphic resolution and standard character screen capacity:

Target Replacement Object	Original Character Count	Required Minimum Graphic Resolution	Layout Analysis
1602 LCD	16 chars x 2 lines	100 x 16	A 122x32 graphic screen is a perfect replacement and has extra space to display a battery icon.
2004 LCD	20 chars x 4 lines	120 x 32	A 128x64 graphic screen can easily accommodate it, and the line spacing is more spacious, improving readability.
4004 LCD	40 chars x 4 lines	240 x 32	Needs a 240x64 strip screen. If using a 128x64 screen, the 40 characters must be wrapped, complicating logic.

Hidden Overhead of UI Components
When planning the layout, not only the content must be calculated, but also the borders of the "containers".

• Scrollbar: Vertical scrollbars usually require a width of 3-5 pixels. So for a 128-width screen, the actual content area is only 123 pixels.
• Border Lines: If you want to add a box to a parameter, the border line itself takes 1 pixel, and the inner padding takes 1 pixel. Drawing a box costs you 4 pixels of available width and height. On a 64-pixel high screen, drawing two boxes loses 12% of the vertical space.

Hardware Resource Overhead

The True Bill of RAM Consumption

Many people only calculate the total amount of screen pixels but ignore the extra overhead of the driver library at runtime.

1. Static Frame Buffer Calculation
This is the most basic overhead. Unlike segment screens, every pixel state (on/off) of a graphic screen requires a bit in RAM to correspond.

• Calculation Formula: Width * Height / 8 = Bytes
• 128x64 Screen: Requires 1024 bytes (1 KB).
• 320x240 (QVGA) Screen: Requires 9600 bytes (9.375 KB).

This is not just a video memory issue.
If your MCU is an ATmega328P (the core of Arduino Uno), it has only 2 KB of SRAM in total.

• Driving a 128x64 screen (1 KB), your RAM is instantly reduced by 50%.
• The remaining 1 KB has to be responsible for the program stack (Stack), global variables, and communication buffers.

2. The Invisible Cost of Double Buffering
If you want to create smooth animations (such as a spinning fan icon or a scrolling waveform), a single buffer is not enough.

• Single Buffer Problem: When the MCU is writing a new image to RAM, the screen might just be refreshing the display.
• Cost of Double Buffering: To solve this problem, you need to allocate two areas in RAM:
  • Back Buffer: The MCU draws silently here.
  • Front Buffer: The screen is responsible for displaying data from here.
  • After drawing, the two buffers are instantly swapped.
• Result: A 128x64 screen now requires 2 KB RAM.

Flash Storage

The advantage of graphic screens is that they can display images and various fonts, but this data must be stored in the MCU's non-volatile memory (Flash).

1. Full Screen Image Size
Many engineers like to design a full-screen boot Logo.

• Uncompressed Bitmap:
  • 128x64 screen: One image takes 1 KB.
  • 320x240 screen: One image takes 9.4 KB.
• Reality: If your MCU has only 32 KB Flash (like STM32F030K6), putting 3 QVGA resolution boot images will eat up 90% of the Flash space, leaving no room for the main program code.
• Solution: Simple compression algorithms like RLE (Run-Length Encoding) must be used, or an external SPI Flash chip must be attached.

2. Storage Pressure of Font Libraries

• ASCII Characters: A standard 5x7 font set (96 characters) occupies about 480 bytes, which is usually not a problem.
• Multi-language Support: If the product is to be sold globally, you need to support the Unicode character set. Even if it only contains a few thousand commonly used characters, the font library size will easily exceed 200 KB. Most MCUs with internal Flash less than 256 KB cannot store a complete font library on-chip.
• Large Digit Fonts: The 32x48 pixel large digits (0-9) commonly used in industrial meters, just these 10 characters plus a colon and unit, will occupy about 2 KB of Flash.

CPU Computing Power and Bus Bandwidth Occupation

The higher the resolution, the greater the workload for the MCU to "move bricks". This directly competes for CPU time needed to process sensor data or control motors.

1. Refresh Rate vs SPI/I2C Bandwidth Bottleneck
Assume we need to refresh the screen at a speed of 20 FPS (20 times per second) to ensure numerical changes do not stutter.

Resolution	Bytes Per Frame	Net Bandwidth Required for 20 FPS	I2C (400kHz) Feasibility	SPI (10MHz) Feasibility
128x64	1 KB	20 KB/s (160 kbps)	Barely (Bus occupancy approx. 50%)	Extremely Fast (Bus occupancy < 2%)
240x128	3.8 KB	76 KB/s (608 kbps)	Infeasible (Exceeds physical limit)	Easy (Bus occupancy ~6%)
320x240	9.6 KB	192 KB/s (1.5 Mbps)	Completely Infeasible	Feasible (Requires DMA)

• Trap of I2C: Although I2C is rated at 400kHz, adding device addresses, ACK bits, and start/stop signals means the actual effective data throughput rarely exceeds 30 KB/s. Using I2C to drive a 128x64 screen for a full refresh, you will visually see the screen refreshing line by line like blinds (taking about 30-50ms).
• Advantage of SPI: SPI has no complex protocol overhead. At 10MHz clock, refreshing a 128x64 screen takes less than 1ms.

2. CPU Overhead of Software Rendering
They are only responsible for displaying dots in RAM, not for drawing.

• Circle Drawing Algorithm: When you call DrawCircle(x, y, r), the MCU must run the Bresenham algorithm, performing a large number of integer addition/subtraction and judgment operations.
• Fill Operation: Executing FillRect() to fill a rectangular area requires the MCU to perform thousands of memory write operations within that area.
• Measured Data: On an 8MHz AVR microcontroller, clearing a 128x64 screen buffer (writing all 0s) takes about 1-2ms. If the interface is extremely complex (containing many rounded rectangles and dashed lines), just the "computing graphics" step may consume more than 20ms, causing the interface response to slow down.
• Cost of Misaligned Memory Mapping: Many LCD video memories are arranged by "Page" (8 vertical pixels as 1 byte), but your coordinate system is X/Y. Every time a dot is drawn (SetPixel), the CPU has to read the original byte, perform Bit Shift and Mask operations, modify a certain bit, and then write it back.

"Hidden Tax" of I/O Pins

When choosing resolution and interface, you also need to check how many "hands" (pins) your MCU has available.

• Serial Interface (SPI/I2C): Whether it is 128x64 or 320x240, usually only 4-5 wires are needed (SCK, SDA, CS, DC, RST). Suitable for 20-pin or 32-pin MCUs with tight pins.
• Parallel Interface (8080/6800): For large data volume screens like 320x240, parallel is usually recommended for speed. This requires:
  • 8 data lines (D0-D7)
  • 4-5 control lines (RD, WR, RS, CS, RST)
  • Total: 12-13 GPIOs.
• Outcome: If you choose a high-resolution screen with a parallel interface, you may be forced to upgrade the MCU from a cheap 48-pin package to an expensive 64-pin or 100-pin package, just for those extra pins.

Physical Size and Dot Pitch

Big Screen Does Not Mean Many Pixels

In graphic LCD selection, there is a common intuitive misconception: thinking that a 3-inch screen definitely displays more content than a 2-inch one.

In fact, Physical Size and Resolution are completely decoupled.

You can buy a 1.3-inch 128x64 screen, and you can also buy a 3.5-inch or even 5-inch 128x64 screen.

Their total number of pixels is exactly the same, both being 8,192 dots.

The difference is that large screens just make each pixel (Pixel) bigger and the gaps between pixels wider.

• Small Size High Resolution (High PPI): The image is extremely fine, with no graininess. Suitable for handheld medical devices or high-end audio players viewed at close range (within 30cm).
• Large Size Low Resolution (Low PPI): The image has obvious blockiness, like pixel games from the 80s. But when viewed from a long distance (more than 1 meter), its character outlines are easier to capture than fine screens, commonly used for forklift meters or factory automation control boxes.

Dot Pitch Determines Image Fineness

Dot Pitch is one of the most important data in hardware specification sheets. It usually refers to the distance from the center of one pixel to the center of the adjacent pixel (Unit: mm).

Dot pitch consists of two parameters:

1. Pixel Size (Dot Size): The size of the actual light-emitting or color-displaying block.
2. Pixel Gap (Dot Gap): The blank area between two blocks.

Dot Pitch = Pixel Size + Pixel Gap

Typical Dot Pitch (mm)	Visual Effect	Applicable Scenario
0.15 - 0.25	Extremely fine, difficult to distinguish pixels with naked eye	High-end handheld testers, laboratory equipment
0.30 - 0.45	Standard definition, slight graininess when viewed closely	Coffee machine panels, smart home thermostats
0.50 - 0.80	Obvious graininess, strong blockiness	Industrial weighing scales, fuel pump displays

If the dot pitch is too large (e.g., exceeding 0.8mm), serious "aliasing" phenomena will occur when displaying diagonal lines or arcs.

For applications that need to display Brand Logos or exquisite icons, the dot pitch must be controlled below 0.4mm.

Quantified Calculation of Pixel Density PPI

PPI (Pixels Per Inch) intuitively reflects the image quality of the screen. The method to calculate PPI is as follows:

1. Calculate diagonal pixels: Using Pythagorean theorem, Width² + Height².
2. Divide by diagonal physical dimension (Inches).

Case Comparison:

• Option A: 0.96-inch 128x64 screen.
• Option B: 2.4-inch 128x64 screen.

Data Reference:
In industrial UI design, to prevent operator visual fatigue, it is recommended to maintain PPI between 80 and 120.

If PPI is lower than 50, unless displaying extra-large numbers, the interface will look very cheap and difficult to read.

Relationship Between Viewing Distance, Text Height, and Physical Size

When choosing physical size, the distance of the user's eyes from the screen must be considered.

This follows a simple optical principle: for every doubling of distance, the physical height of the text must also double to maintain the same clarity.

1. Handheld Operation (Viewing Distance 30-50 cm)

• Minimum Character Height: 2.5 mm to 3.0 mm.
• Selection Suggestion: If you use 128x64 resolution and hope to display 8 lines of text, each line height is about 8 pixels. On a 2.0-inch screen, each pixel is about 0.3mm.

2. Desktop/Device Panel Operation (Viewing Distance 0.8-1.2 meters)

• Minimum Character Height: 5 mm to 7 mm.
• Selection Suggestion: If displaying on a 2.0-inch screen, you need to occupy 20 pixels in height to draw a 6mm high character. A 128x64 screen can only display 3 lines of text. If you need to display more information, you must expand the physical screen size to over 3.5 inches, or increase the dot pitch.

3. Indoor Far View (Viewing Distance 2-3 meters)

• Minimum Character Height: Above 12 mm.
• Case: Number calling screens in hospital corridors. At this distance, even on a 5-inch screen with 128x64 resolution, the strokes will appear too thin because the pixel points are too fine. In this case, Segment LCD or LED Dot Matrix Modules are usually used instead.

Avoiding the "Mechanical Trap" of Case Design

Physical size is not only about vision but also determines your PCB layout and Bezel Opening size.

• VA (Viewing Area) vs AA (Active Area):
  • AA (Active Area): This is the area where pixels actually exist. Your UI design must be strictly limited within this range.
  • VA (Viewing Area): This is the area on the glass panel not covered by the frame. Usually, VA will be larger than AA by 1mm to 2mm on each side.
• Structural Hidden Danger: If you refer to the AA size when designing the case opening, pixels on the edge will be blocked by the case when the user views the screen from the side. The correct practice is to base the opening size on the VA size and reserve a 0.5mm assembly tolerance.
• Glass Thickness and Mechanical Strength:
  As physical size increases, LCD glass becomes thinner and more fragile.
  • Screens under 1.5 inches usually use 0.7mm or even thinner glass.
  • Industrial-grade screens over 3.0 inches usually require 1.1mm thick glass for shock resistance.
    If you use large-size, thin-glass screens in environments with high vibration, additional support brackets must be added behind the PCB, otherwise the glass will suffer stress fracture under mid-frequency vibration.

Temperature Range

Standard commercial screens usually can only work in the range of 0°C to 50°C, while industrial grades extend to -20°C to 70°C.

Low temperatures cause liquid crystal viscosity to increase, delaying pixel response time from 200ms to several seconds, causing ghosting;

High temperatures exceeding the Clearing Point cause the material to become an isotropic liquid, turning the screen completely black.

For every 1°C change, the driving voltage needs to be adjusted by about 13mV to maintain normal contrast.

The storage temperature range is usually 10°C wider than the operating temperature to prevent polarizer peeling or bubble generation.

Operating vs Storage Differences

Physical Boundaries During Powered Operation

Operating temperature defines the range in which the LCD module, when powered on, meets all nominal photoelectric indicators (such as contrast, response time, viewing angle).

• Heat Generated by Internal Power Consumption: Although LCD is a low-power device, its backlight (LED Backlight) and integrated circuit (Driver IC) generate heat during operation. For example, a typical side-emitting backlight for a 128x64 resolution screen generates about 60mW to 100mW of heat at 20mA current.
• Dynamic Balance of Liquid Crystal Molecules: Within the operating temperature, liquid crystal molecules can maintain a specific deflection speed driven by an electric field. Once below the lower limit (e.g., below -20°C), the viscosity of the liquid crystal increases exponentially. Experimental data shows that for every 10°C drop in temperature, viscosity increases by about 2 to 3 times.

Physical Durability When Powered Off and Stored

Usually, this range is 10°C to 15°C wider than the operating temperature.

• Phase Transition Point of Liquid Crystal Materials: The storage lower limit is usually limited by the crystallization temperature (Crystallization Point) of the liquid crystal. If the storage environment is below -30°C (for standard industrial screens), the liquid crystal will transform from liquid to crystalline state.
• Mismatch of Material Thermal Expansion/Contraction: LCD is a composite structure composed of glass, plastic polarizers, metal frames, and PCB. The coefficient of thermal expansion for each material is different. In storage upper limit environments (e.g., +80°C), the pressure-sensitive adhesive (PSA) of the polarizer may soften and generate bubbles, damage which will not disappear after returning to room temperature.
• Chemical Stability: Under long-term high-heat storage, iodine molecules in the polarizer will undergo a fading reaction, causing a permanent drop in screen contrast. Data indicates that after continuous storage for 500 hours in an environment exceeding 70°C, the transmittance of the polarizer may decrease by 3% to 5%.

Why Storage Range is Wider

This design logic is based on different stress tolerances under static and dynamic conditions.

1. No Electric Field Intervention: In the storage state, liquid crystal molecules do not need to undergo frequent orientation flips. As long as no permanent phase change (crystallization or vaporization) occurs, the material can maintain its original state.
2. Protection of Driver Chips: When semiconductor driver ICs are not working, there is no current flowing internally, so no electromigration occurs, thus they have higher tolerance to high temperatures.
3. Sealant Pressure: When powered on, the alignment of liquid crystal molecules generates tiny internal pressure. Under extreme temperatures, this internal pressure combined with thermal expansion and contraction is more likely to cause seal failure.

Hidden Costs in Transport and Deployment

Ignoring the difference between these two parameters will directly increase after-sales and maintenance costs.

• Ocean Shipping Container Effect: During transoceanic transport, the temperature inside containers can easily reach over 65°C due to heat absorption by the metal shell, and humidity also fluctuates significantly. If the chosen LCD storage upper limit is only 60°C, polarizer peeling may have already occurred before the product reaches its destination.
• Power-off Storage in Cold Zones: In winter in North America or Northern Europe, outdoor equipment (such as charging piles) may be powered off due to grid maintenance. At this time, the equipment enters storage mode.

How Engineers Select Models to Avoid Risks

Before determining specifications, data quantification is needed for the actual exposure environment.

Scenario Requirements	Recommended Operating Range	Recommended Storage Range	Precautions
Indoor Constant Temperature Lab	0 ~ +50°C	-10 ~ +60°C	Pay attention to heat accumulation from long-time backlight use
Car Center Console	-40 ~ +85°C	-40 ~ +90°C	Temperature rise under direct sunlight can reach 100°C+
Outdoor Handheld Terminal	-20 ~ +70°C	-30 ~ +80°C	Consider battery voltage drop at low temperatures

Temperature Difference Between Glass Substrate and Circuit Board

In practical applications, the temperature of the LCD screen surface is often not equal to the ambient air temperature.

• Backlight Heat Conduction: LED beads are arranged at the bottom or side of the screen. For high-brightness (>500 nits) screens, the temperature of the backlight part may be 10°C higher than the distal glass. When the ambient temperature is 40°C, the local screen may have already reached the operating upper limit of 50°C.
• Driver IC Hot Spots: Driver chips with COG (Chip on Glass) packaging are pasted directly on the glass. If the refresh rate is set too high, the power consumption of the driver IC will increase, and the heat generated will directly change the viscosity of the surrounding liquid crystal, causing the local color of the screen to be darker than other areas. When working in a -10°C environment, pixels around the IC may respond quickly, while areas far from the IC have obvious ghosting.

Low Temperature Lag Phenomenon

Physical Fact of Liquid Crystal Thickening

Liquid crystal molecules need to physically rotate or twist under the action of an electric field to change the polarization direction of light. The resistance to this rotation is called Rotational Viscosity.

At room temperature (25°C), the viscosity of typical nematic liquid crystals is approximately between 15 cSt and 25 cSt (centistokes), which is similar to the fluidity of whole milk.

However, viscosity follows an exponential relationship with temperature.

• At 0°C: Viscosity usually increases to 2 to 3 times that at 25°C.
• At -20°C: Viscosity will surge to 10 to 15 times that at 25°C.
• At -30°C: The fluid approaches a semi-solid state, and viscosity may reach over 1000 cSt, close to the state of thick syrup or engine oil.

In this high-viscosity state, the torque generated by the electric field struggles to overcome the internal friction between molecules. Even if voltage is applied, the molecules take a long time to slowly rotate into position.

Data Collapse of Response Time

Response Time is usually defined as the sum of the time for a pixel to change from 10% brightness to 90% brightness and the reverse change.

For standard monochrome STN graphic dot matrix screens, the impact of low temperature on these two parameters is asymmetric and severe.

Here is a comparison of measured data for a standard STN module:

Ambient Temperature	Rise Time	Fall Time	Total Response Time	Performance Decay Multiplier
+25°C	80 ms	120 ms	200 ms	1x (Benchmark)
0°C	200 ms	350 ms	550 ms	2.75x
-10°C	450 ms	800 ms	1250 ms	6.25x
-20°C	900 ms	1800 ms	2700 ms	13.5x
-30°C	2500 ms	4500 ms	7000 ms	35x

Disconnect Between Frame Rate and Physical Action

The speed at which the Display Controller sends data and the speed of the physical action of liquid crystal molecules will have a severe timing misalignment at low temperatures.

Usually, the LCD refresh rate is set to 60Hz to 70Hz, and the controller refreshes the pixel data every 16.6ms.

• Scenario Reconstruction: Suppose the screen needs to display text scrolling from left to right.
• Controller Behavior: Sends the next frame of image every 16.6ms, requiring pixel points to change state.
• Physical Behavior (-20°C): Pixel points receive the instruction and start to turn slowly. It takes 2700ms to complete this turn.
• Conflict Result: When the pixel point has not even completed 1% of the first frame image, data for the second, third, or even hundredth frame has already arrived.

This disconnect results in severe Ghosting and Smearing.

Originally clear text leaves a long gray trail when scrolling, and the contrast between background and foreground drops sharply.

Voltage Not Enough to Move Molecules

In addition to increased viscosity, low temperatures also change the threshold voltage of liquid crystal materials.

Simply put, frozen liquid crystal molecules become "harder" and require greater electric field force to twist them.

• Temperature Coefficient: Most liquid crystal materials have a negative temperature coefficient, approximately -0.05% / °C to -0.15% / °C.
• Consequences of Insufficient Power: If you set VOP to 9.0V at 25°C, when the temperature drops to -20°C, the actual optimal voltage required by the screen may have risen to 11.5V.
• Visual Performance: If the system power supply does not adjust accordingly, the 9.0V voltage supplied at this time is too weak for the cold liquid crystal molecules to be fully twisted to the light-blocking angle.

Technical Cost of Forced Heating

To overcome low-temperature physical limitations, the most effective engineering method is not to change the liquid crystal, but to change the environment.

Common heating solutions include:

1. ITO Glass Heating: Coating a transparent conductive film (Indium Tin Oxide) on the LCD glass substrate.
2. Resistance Wire Heating: Attaching a resistance heating film to the backlight plate or metal frame.

Power Consumption Calculation Example:
A 3.5-inch graphic LCD module typically requires 1.5 Watts to 2.5 Watts of continuous heating power to maintain a surface temperature of 0°C in an ambient temperature of -30°C.

• Comparison: The display power consumption of the LCD module itself may be only 0.05 Watts.
• Battery Impact: After adding the heater, the total power consumption of the system increases by 30 to 50 times. For battery-powered portable devices, power that could originally last 100 hours can only support 2 hours after turning on heating.

Fluid Formula Differences in Selection

Panel manufacturers usually provide different Fluid formulas to deal with this problem, but this is usually a Trade-off.

• Normal Fluid: Moderate viscosity, high contrast at room temperature, good viewing angle, but fails below -10°C.
• Low-Temp Fluid: By adding solvents that reduce viscosity, it maintains lower viscosity at -20°C.
  • Cost: This fluid has poor stability at high temperatures (e.g., +50°C), and the Contrast Ratio is usually lower than that of normal fluid. Moreover, the cost of this special formula is usually 15% to 30% higher than the standard formula.

High Temperature Black Screen Effect

Liquid Crystal is No Longer "Liquid Crystal"

Liquid Crystal is a special state of matter (Mesophase) between solid crystal and liquid.

Its ability to display images relies entirely on the ordered arrangement of its molecules at the microscopic level (Nematic Phase).

This arrangement gives it Optical Anisotropy, allowing it to twist the polarization direction of light passing through it.

• Thermal Motion Defeats Van der Waals Forces: As heat increases, the Brownian Motion of molecules intensifies. When the energy exceeds the intermolecular forces maintaining molecular orientation, all liquid crystal molecules no longer arrange neatly but become chaotic like water or alcohol.
• Phase Change Data:
  • Ordinary Commercial Fluid: The Clearing Point is usually between +60°C and +70°C.
  • Wide Temperature Industrial Fluid: By adjusting the chemical formula, the Clearing Point can be raised to +100°C to +110°C.
• Optical Consequences: Once entering the Isotropic liquid state, the birefringence difference of the liquid crystal drops to zero. It no longer rotates the polarization angle of light. Light passes straight through the liquid crystal layer just like passing through ordinary glass.

Why It Turns Black Specifically

The color displayed by the screen after reaching the Clearing Point depends on the bonding angle of the Polarizer, not the circuit signal. For the vast majority of graphic dot matrix screens, an orthogonal polarizer design is used.

1. During Normal Operation: Light emitted from the backlight passes through the lower polarizer and becomes linearly polarized light. The orderly arranged liquid crystal molecules, like a spiral staircase, rotate the polarization direction of the light by 90 degrees (or 180 degrees/240 degrees, depending on the STN type). The rotated light can pass exactly through the upper polarizer which is perpendicular in direction.
2. During High Temperature Failure: Liquid crystals become ordinary liquid and do not rotate light. Linearly polarized light maintains its original direction and reaches the upper polarizer. Since the upper polarizer is perpendicular to the lower polarizer (90-degree angle), the light is completely blocked.

At this time, no matter what data you send to the driver IC or how you adjust the voltage, the screen will not have any reaction because it has lost all optical modulation capabilities.

Temperature Rise Trap Under Sunlight Exposure

A mistake engineers often make is looking at the weather forecast saying "maximum temperature 35°C" and thinking a 50°C limit screen is safe enough. The fact is quite the opposite.

• Greenhouse Effect and Heat Accumulation: LCD modules are usually stacked with multiple layers of glass and plastic sheets and encapsulated inside the device case. Infrared radiation (IR) in sunlight passes through the transparent front panel and is absorbed by the black LCD pixel matrix or frame, converting into heat energy.
• Quantification of Temperature Difference:
  • Under standard solar radiation of 1000 W/㎡ (typical sunny noon).
  • The heat absorption coefficient of dark cases or screen surfaces can reach 0.8 to 0.9.
  • Measured Data: When the ambient air temperature is 30°C, the surface temperature of an LCD without active cooling can easily rise to 65°C to 75°C within 20 minutes.
  • Even in temperate summers, an outdoor device can easily cause a standard commercial screen to exceed the Clearing Point and turn black directly.

Irreversible Damage to Polarizers

Polarizers are mainly made of PVA stretched film adsorbing Iodine molecules.

• Iodine Sublimation and Fading: Long-term exposure to high-temperature environments above +60°C causes iodine molecules in the PVA film to become unstable, starting to migrate or even sublime.
• Drift in Transmittance: With the loss of iodine molecules, the Polarizing Efficiency of the polarizer decreases. The "black character" part that should have blocked light starts to leak light, turning yellow or brown.
• Lifespan Data:
  • The lifespan of standard iodine-based polarizers in an +80°C environment may be less than 500 hours.
  • Once Bleaching occurs, the contrast will be permanently reduced and cannot be repaired. Even if the temperature drops back, the screen looks like a stale yellow smoked by smoke, and characters are blurred.
• Alternative Solution: For equipment that needs to work at high temperatures for a long time (such as car dashboards), Dye-based Polarizers must be specified. Although their optical contrast is slightly lower than iodine-based ones, they can remain unfaded for thousands of hours at +90°C or even +105°C.

Recovery Risk After Cooling

When the temperature falls back below the Clearing Point, the liquid crystal material will re-crystallize back to the Nematic Phase, and optical activity will recover.

• Thermal Hysteresis: The phase change process often has thermal hysteresis. If the Clearing Point is 70°C, the liquid crystal may need to cool down to 60°C or even lower to fully restore ordered arrangement.
• Alignment Layer Damage: The arrangement of liquid crystal molecules relies on the friction grooves of the Polyimide (PI) layer coated on the inner side of the glass. Extreme high temperatures (especially exceeding +90°C) may cause microscopic deformation or chemical decomposition of the PI layer.
• Bubble Generation: High temperatures cause the volume of liquid crystal material to expand. If the expansion pressure exceeds the endurance of the Sealant, or causes trace volatile impurities to vaporize, vacuum bubbles will be generated inside the screen.

High Temperature Leakage of Driver Circuits

In addition to optical materials, high temperatures also directly hit the stability of driver circuits.

• Shutdown Current Leakage: At high temperatures, the Leakage Current of TFTs or MOSFETs increases exponentially.
• Aggravated Ghosting: Leakage current causes the pixel voltage that should be maintained to fail to hold (voltage drop), and pixels that should be off get charged.
• Electrochemical Corrosion: If high temperature is accompanied by high humidity, the insulation resistance between electrodes drops. DC Components may be applied to the liquid crystal due to circuit drift.