2026년 8월 10일8분 읽기

Round OLED Display Module: How to Match Size, Brightness, and Interface for Industrial Use

For industrial use, select a round OLED in this order: mechanical fit, readable UI, optical performance, interface bandwidth, RAM, power, lifetime, environment, and supply control. A 240 × 240 RGB565 frame is 112.5 KiB and needs 27.6 Mbit/s of raw pixel data at 30 full-screen fps; a 454 × 454 frame is about 403 KiB and needs 98.9 Mbit/s. Also separate PMOLED from AMOLED, because the controller, color format, interface, power rails, and available resolution can be different.

Mechanical Dimensions to Lock Before PCB and Enclosure Design

Item	What to Record
Active area	Pixel area diameter or X/Y size
Viewing area	Supplier-defined visible area; do not replace it with the enclosure opening
Module outline	Full glass, frame, PCB, or module width and height
Z-height	OLED + touch + adhesive + cover glass + gasket + connector + PCB
FPC	Exit direction, width, length, connector position, bend area, minimum bend radius
Rotation	Allowed mounting angle after FPC, touch FPC, and connector positions are fixed

A 1.18-inch 128 × 128 round PMOLED still needs rectangular module and FPC space behind the circular opening. Check the mechanical drawing, not only the front diameter.

Include centering tolerance in the stack-up. If the display center shifts 0.3 mm to one side, the visible border becomes about 0.3 mm narrower on one side and 0.3 mm wider on the other, producing a 0.6 mm side-to-side border difference.

Resolution, Pixel Load, and Safe Round UI Area

Resolution	Total Pixels	Pixel Load vs. 240 × 240	Practical UI Use
128 × 128	16,384	0.28×	Large numbers, status icons, simple menus
240 × 240	57,600	1.00×	Gauges, several values, labels, simple charts
390 × 390	152,100	2.64×	Fine text, detailed icons, smoother graphics
454 × 454	206,116	3.58×	Dense UI, fine graphics, frequent animation

Moving from 240 × 240 to 454 × 454 increases the pixel count from 57,600 to 206,116, or 3.58×. With the same color depth and frame rate, framebuffer size and raw transfer demand also increase by 3.58×.

Physical size does not determine resolution. Current round display modules include combinations such as a 1.2-inch 390 × 390 AMOLED and a 1.39-inch 454 × 454 AMOLED.

If a round display uses a square D × D address space and rectangular content must stay fully inside the visible circle, the largest centered square is approximately D ÷ √2. For a 240-pixel circle, that is about 170 × 170 pixels; for a 454-pixel circle, about 321 × 321 pixels. This is useful for fixed text blocks and rectangular touch controls that must never be clipped by the round edge.

Brightness and Reflection Values to Check

Record brightness in cd/m² together with the test temperature, supply condition, test image, brightness setting, and whether the value is minimum or typical. Numerically, 1 nit equals 1 cd/m²; CIE lists “nit” as a deprecated unit name.^[1] IEC 62341-6-1 defines OLED optical and electro-optical measurement methods, including defined picture-loading conditions.^[2]

Diffuse Reflectance	Ambient Illuminance	Calculated Reflected Luminance
1%	100,000 lux	about 318 cd/m²
2%	100,000 lux	about 637 cd/m²
4%	100,000 lux	about 1,273 cd/m²

The table uses reflected luminance ≈ illuminance × diffuse reflectance ÷ π and assumes a purely diffuse Lambertian surface. CIE defines the Lambertian relation as M = πL.^[3] Real cover glass can include diffuse, mirror-like, and haze reflection, so use the table only as a scale check; NIST measurements show that simplified reflection models can be inaccurate for flat-panel displays.^[4]

Measure the finished stack: OLED + touch + adhesive/air gap + cover glass + coating.
Check both maximum brightness and the lowest stable brightness needed in dark conditions.
For outdoor products, record both display luminance and front-surface reflection; do not approve the display from luminance alone.

SPI, MIPI, and Full-Frame Bandwidth

240 × 240 RGB565: 240 × 240 × 2 = 115,200 bytes = 112.5 KiB per frame. At 30 full-screen fps, raw pixel data is 27.648 Mbit/s.

454 × 454 RGB565: 454 × 454 × 2 = 412,232 bytes ≈ 403 KiB per frame. At 30 full-screen fps, raw pixel data is 98.9 Mbit/s.

SPI Clock	240 × 240 RGB565	454 × 454 RGB565
20 MHz	21.7 fps theoretical	6.1 fps theoretical
30 MHz	32.6 fps theoretical	9.1 fps theoretical
40 MHz	43.4 fps theoretical	12.1 fps theoretical
80 MHz	86.8 fps theoretical	24.3 fps theoretical

These are theoretical pixel-only limits. Commands, addressing, chip-select gaps, DMA setup, RTOS scheduling, and other bus traffic reduce the measured frame rate. Use the display controller's supported clock limit and measure the actual transfer time on the target MCU.

A 100 × 40 RGB565 partial update contains 8,000 bytes. Compared with a 115,200-byte 240 × 240 full frame, that is a 93.1% reduction in transferred pixel data.

Use SPI or QSPI when measured update time meets the UI requirement. Use a faster path when it does not. MIPI DSI is designed for high-speed display links with reduced pin count.^[5] D-PHY is commonly used for MIPI display links.^[6] For a round AMOLED implementation, the MIPI setup and graphic rendering guide covers lane setup, initialization, and rendering steps.

Framebuffer RAM by Resolution and Color Depth

Resolution	RGB565 One Frame	RGB565 Two Frames	RGB888 One Frame
128 × 128	32 KiB	64 KiB	48 KiB
240 × 240	112.5 KiB	225 KiB	168.75 KiB
360 × 360	about 253 KiB	about 506 KiB	about 380 KiB
390 × 390	about 297 KiB	about 594 KiB	about 446 KiB
454 × 454	about 403 KiB	about 805 KiB	about 604 KiB

RGB888 uses 50% more pixel memory and raw transfer data than RGB565. For 454 × 454, one framebuffer rises from about 403 KiB to about 604 KiB.

Add RAM for fonts, decoded images, GUI objects, DMA buffers, RTOS tasks, communications, touch data, and application code.
If SRAM is tight, use one framebuffer, tile/line rendering, partial updates, external PSRAM/SDRAM, or display-side GRAM.
Do not apply RGB565/RGB888 calculations to a monochrome PMOLED; calculate from its real bits per pixel.

Power and Reset Checks

Check	Required Result
Logic and I/O voltage	MCU GPIO levels match the display input limits
OLED drive rail	Voltage and peak current stay within module limits
Normal UI current	Measure on the real product
Brightest allowed UI current	Measure separately and use it for peak-load checks
Power-up / power-down sequence	Matches the controller or module specification
Brownout / fast cycling	Display returns to normal operation without removing power
MCU off, display on	No unwanted back-powering through interface pins

Do not connect 3.3 V GPIO directly to a 1.8 V display input unless the input is explicitly 3.3 V tolerant. For power sizing, use the measured current of the brightest screen the product is allowed to show, not only the normal dark UI.

OLED Lifetime by Daily Operating Hours

Compare supplier lifetime data only when the starting luminance, temperature, test pattern, color, and end-of-life definition are known. IEC 62341-5-3 covers OLED image-sticking and lifetime measurement methods.^[7]

Display Use	5 Years	10 Years
8 h/day	14,600 h	29,200 h
12 h/day	21,900 h	43,800 h
16 h/day	29,200 h	58,400 h
24 h/day	43,800 h	87,600 h

A 50,000-hour supplier figure does not automatically cover a 10-year industrial product: 16 h/day for 10 years already equals 58,400 hours, while continuous 24/7 operation reaches 87,600 hours.

Static bright areas also need their own check. Logos, borders, scales, status bars, and fixed white text can age differently from dark areas. Peer-reviewed OLED research links uneven aging with image sticking and shows stronger aging at higher current and temperature.^[8]

Dim the panel when full brightness is not required.
Use dark backgrounds where the UI allows it.
Turn the display off during long idle periods.
Reduce always-bright logos, borders, and status areas.
Use small pixel shifts for fixed graphics when practical.

Temperature, Touch, Cleaning, and EMC Test Items

Use the temperature measured at the display, not only room temperature. If ambient is 45°C and the measured internal rise at the OLED is 15°C, the local display temperature is 60°C. Check startup, reset, brightness, color, touch, adhesive, and communication at the required low and high limits. OLED luminance can change with panel temperature.^[9]

Area	Test
Temperature	Cold start, hot start, reset, thermal cycling, local OLED temperature
Humidity	Condensation, fogging, FPC/connector corrosion, adhesive edge condition
Touch	Bare finger, actual glove, dry surface, wet surface, center and edge points
Cleaning	Real chemical, concentration, wiping material, contact time, total cycles
EMC/ESD	Pixel corruption, lost communication, brightness-register change, ghost touch, lock-up, recovery

Cleaning-cycle targets can be calculated from actual use. One cleaning per day equals 365 cycles/year; three per day equals 1,095 cycles/year; ten per day equals 3,650 cycles/year. Over five years, those become 1,825, 5,475, and 18,250 cycles. Use the product's real cleaning frequency instead of a generic wipe count.

IEC 62341-5-2 covers mechanical endurance test methods for OLED panels and modules.^[10] IEC 60068-2-78 covers steady-state damp-heat testing.^[11] IEC 61000-4-2 covers ESD immunity testing,^[12] and IEC 61000-4-3 covers radiated RF immunity testing.^[13] Use the levels required by the standards that apply to the finished product.

Controller, Documentation, and Supply Control

Before Approval	Required Information
Controller	Exact IC part number and revision/configuration where relevant
Firmware	Initialization sequence, pixel format, orientation, sleep/wake, brightness control, tearing signal if used
Electrical	Pinout, power rails, reset timing, interface timing, recommended circuit
Mechanical	Controlled drawing with revision number
Touch	Touch-controller IC, firmware/configuration, cover-stack limits
Change control	PCN rules for controller, touch IC, FPC, adhesive, cover material, PCB parts, and production site
EOL	Notice period, last-time-buy process, replacement plan
Traceability	Lot code, date code, production site, revision identification

Do not approve “compatible controller” or “equivalent IC” wording without a change-control agreement. A controller substitution can change initialization, memory addressing, timing, sleep behavior, or firmware even when the module outline is unchanged.

Production-Sample Validation

Measure on Every Sample	Record
Optics	Brightness setting, measured luminance, visible color/uniformity, test condition
Mechanical	Outline, active-area position, FPC position, connector fit, assembly clearance
Electrical	Startup, reset, normal current, bright-screen current, interface clock
UI	Full-screen transfer time, largest partial update time, tearing or visible artifacts
Touch	Center/edge response, glove result, wet result, false-touch recovery
Environment	Local display temperature and pass/fail result after required tests
Traceability	Sample serial/lot/date code and module revision

Use production-representative samples and more than one lot when available. Run the final checks with the real cover glass, adhesive/bonding, enclosure, connector, processor, firmware, power supply, touch stack, and brightness settings.

Finally

Reject a round OLED early if its numbers do not fit the system. A 454 × 454 RGB565 screen needs about 403 KiB per frame and 98.9 Mbit/s of raw data at 30 fps, versus 112.5 KiB and 27.6 Mbit/s for 240 × 240. A 100 × 40 partial update cuts 240 × 240 pixel traffic by 93.1%. For lifetime, 16 h/day over 10 years is 58,400 hours and 24/7 use is 87,600 hours. Approve the module only after mechanical fit, measured transfer time, RAM, finished-stack readability, local temperature, touch behavior, controller identity, and supply-change rules all meet the product limits.