How e‐Paper works and how to work with it · bitbank2/bb_epaper Wiki · GitHub

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How e‐Paper works and how to work with it

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Larry Bank edited this page Aug 22, 2026<br>&middot;<br>36 revisions

What's in a name?

e-Paper is the agreed upon name to describe various types of electrophoretic displays (EPDs) - displays composed of arrays of colored particles suspended in clear oil which are manipulated with electric fields. The company famous for commercializing this technology is Eink. They have trademarked the name for their company and the EPDs they sell. Other companies sell similar products, but must use the term e-Paper instead of Eink.

As a display type, e-Paper is unique. It presents some serious challenges (slow updates, reflective only), and some amazing benefits (high contrast, wide viewing angles and no energy needed to retain the image). Because of it's unique properties, the software (your software) which drives it also needs to be different from more common display software. There are two main types of interfaces to EPDs: serial and parallel. The serial type have a built-in, low power microcontroller with its own memory. These are controlled by sending single byte commands and image pixel data. The commands trigger the on-glass MCU to redraw the display with the contents of the internal memory. The parallel panels (usually found in e-book readers) have a simpler digital state machine inside them and only contain memory for a single row of the display. They require the external MPU to control all aspects of updating the display.

What's inside the glass?

EPDs are composed of arrays of tiny vertical cells of clear oil with small colored grains holding a static charge (one color is positively charged, the other negatively) suspended in them. The top of each cell (viewing side) has a transparent electrode and the bottom has a semi-transparent electrode. These 2 electrodes are connected to a strong positive and negative voltage source (15 to 28 Volts) controlled by a pair of transistors. When a voltage is applied to either side, the pigment grains with the opposite polarity move towards that side and the grains with the same polarity are repelled and move towards the other side. Changing the contents of the display requires 'pushing' the charged particles one way or the other to get the desired color. It takes a finite amount of time for the particles to move through the oil and they move slower in colder temperatures. You've probably seen e-Paper displays update with a pulsating or flashing between black and white. This is needed to ensure the known state of each pixel before pushing it towards the desired color. The continuous movement also makes use of inertia to get the particles moved fully into their final positions. Care must also be taken to not push the particles more towards one color versus the other or a static charge can accumulate and cause the display to misbehave. The industry term for this is the "balance of charge". Unbalanced pushes can cause the particles to move to the wrong color after power is removed; this can look like a 'graying' of the display after an update. Pushing the pixels many times between fully black and white can clear this charge if needed. In other words, a charge imbalance is usually not a fatal situation for the display.

Black/White vs Multicolor

The first versions of e-Paper were only able to display black and white. Each cell had black and white colored particles which would change places (top/bottom) to change the visible color. Later models introduced red particles. The red particles were lighter and had a lower charge so that updates occurred in two steps: first, the heavier black and white particles were moved, followed by the lighter red particles. These were soon followed by 4-color panels which paired black/white and red/yellow. Newer panels added a third pair of colors to allow 6 at a time (Spectra6). The addition of more color pairs increased the time to update the display since each pair must be controlled independently of the others. Update times for the 6-color panels are usually 25-35 seconds. Another multi-color idea was created called Kaleido. This makes use of a precisely placed color filter array (CFA) similar to those used in LCD displays. The position of the black/white pixels under the color filter allows for a wider range of colors. Use of a CFA allows for fast updates since the underlying panel is only black/white. One downside to the CFA panels is...