The e-ink screen
Why the screen never glows, why it sometimes flashes black and white, and what those faint shadows are.
The ZerryBit's screen is e-ink (e-paper), the same technology as an e-reader. The image is made of physical ink particles that are moved into place electrically and then stay put with zero power. That single property shapes everything about how the device behaves:
- The screen is always on, yet uses power only in the moment it changes.
- It emits no light. Like paper, you read it by the light of the room: perfectly readable in sunlight, invisible in the dark.
- The image survives even with the battery empty.
The panel is 7.5 inches, 800 × 480 pixels, pure black and white (1 bit per pixel), driven by a UC8179 controller over SPI. Shades of grey in widgets are created with fine dot patterns, which suits e-ink well.
Three kinds of refresh
The firmware updates the screen in three ways, and you can see the difference:
| What it looks like | When it's used | |
|---|---|---|
| Partial refresh | The changed area updates quietly, no flashing | Most updates: widget changes, menu navigation |
| Fast full refresh | The whole screen flashes black and white once | At the end of button-press wakes, keeps the image crisp |
| Full refresh | A slower, more thorough flashing cycle | About once a day, when the device is turned off, and after a factory reset |
Ghosting, and the self-cleaning refresh
If a partial refresh only nudges the ink that changed, faint traces of the old image can build up over time. You may notice light shadows of previous content. This is called ghosting, and it is normal for e-ink.
The firmware manages it for you. Manual wake-ups end with a fast full refresh, and the daily full refresh runs the panel's deep-clean waveform, which resets the ink completely and keeps the panel healthy. If you ever see shadows building up, press the Zerry button; the refresh at the end of the wake clears them.
The occasional black-and-white flashing is not a glitch. It is the panel doing its hygiene, and suppressing it entirely would slowly degrade the image quality.
Temperature behaviour
Ink particles move through a fluid, and that fluid thickens in the cold. Below roughly 5 °C refreshes get slower and lower-contrast, and e-ink is generally not usable well below freezing. Room temperature is where the panel performs best, worth knowing before mounting one in an unheated porch or garage.