> ## Documentation Index
> Fetch the complete documentation index at: https://docs.zerokeyusb.com/llms.txt
> Use this file to discover all available pages before exploring further.

# Display: SSD1306 OLED

> 128×32 monochrome display wiring, power budget, and firmware usage.

ZeroKeyUSB uses a **0.91" SSD1306-based OLED module** to present menus, credentials, and status icons. The display is bright, low-power, and readable from multiple angles — ideal for a quick glance during logins.

***

## Electrical characteristics

| Parameter       | Value                       |
| --------------- | --------------------------- |
| Resolution      | 128 × 32 pixels             |
| Interface       | I²C (address `0x3C`)        |
| Supply voltage  | 3.3 V                       |
| Typical current | 10–12 mA at full brightness |
| Controller      | Solomon Systech SSD1306     |

The module connects directly to the SAMD21’s SERCOM3 I²C bus, shared with the external EEPROM. Pull-up resistors (4.7 kΩ) are located on the PCB, so the breakout resistors should be disabled when assembling.

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## Pin assignments

| OLED pin | Signal   | Notes                                          |
| -------- | -------- | ---------------------------------------------- |
| VCC      | 3V3      | Powered from the MCU regulator                 |
| GND      | GND      | Common ground                                  |
| SCL      | PA23     | Shared I²C clock                               |
| SDA      | PA22     | Shared I²C data                                |
| RES      | PA14     | Controlled by firmware during init             |
| DC       | Tied low | Command/data handled automatically in I²C mode |
| CS       | Tied low | Not used in I²C mode                           |

The firmware toggles the **RES** line during startup to ensure a clean boot sequence even if power is unstable.

***

## Frame buffer strategy

* The SSD1306 expects data in **pages of 8 vertical pixels**.
* Firmware maintains a 512-byte buffer in SRAM (`128 × 32 / 8`).
* Updates use **partial writes** to minimize I²C traffic when only a few characters change.
* A simple double-buffer diff tracks dirty regions so the screen refresh stays under 5 ms.

Animations such as smooth scrolling for long passwords rely on timer interrupts that shift the buffer between refreshes.

***

## Brightness control

* Default contrast value: `0x7F` (50%).
* Menu option allows dimming down to `0x20` for dark environments.
* After 60 seconds of inactivity the firmware sends `DISPLAY OFF` while keeping data in RAM.
* Any touch input or USB activity turns the screen back on instantly.

This approach balances legibility and OLED lifespan.

***

## Troubleshooting

| Symptom                        | Possible cause                   | Fix                                                  |
| ------------------------------ | -------------------------------- | ---------------------------------------------------- |
| No image, backlight off        | RES pin held low                 | Check solder joint or ensure the boot logo finished. |
| Display flashes or shows noise | I²C conflict with EEPROM         | Inspect pull-up resistors and cable length.          |
| Ghosting / burn-in             | Static content at max brightness | Lower contrast or enable auto-dim in settings.       |

If the OLED ever needs replacement, any SSD1306 I²C module with the same pin order can be swapped in without firmware changes.