I think ePaper displays are really cool and they are one of the reasons why I chose this project for my residency. Instead of using a backlight like LCDs, they reflect ambient light from their surface like traditional paper and ink and have an ultra-wide viewing angle. Since displays can consume nearly half the power of any computing device, this amounts to a drastic reduction in power consumption. ePaper displays are also bistable so the images will be retained even when there is no power source. LCD screens need to be refreshed at least 30Hz even when the image is static. Bistability also reduces power consumption.
The screen I am using for this project is an active matrix display and composed of a plastic substrate with an array of thin film transistors (TFT) attached to each pixel. These are directly connected to the row and column drivers. The row driver consists of a shift register, level shifter, an enable system and output buffer.
The driver operates with a gate pulse that controls the line of pixels to be programmed by turning the TFT switch on or off. A level shifter raises the voltages to the desired logical output and the output buffer reduced the impedance of the row driver so that these lines can be driven by the output signals. A storage capacitor maintains the state of the pixel until the next refresh cycle.
ePaper displays are composed of microcapsules that contain black and white ink particles suspended in a clear fluid. These are the same pigments that are used in printing, though in ePaper displays, they are electrically charged and laminated in a single layer onto film. When a negative electrical field is applied to the bottom of the film the negatively charged black particles are repelled and migrate to the top. When a positive field is applied the white particles float to the top.
The threshold voltages for eInk screens are quite high, commonly +15V / -15V for the source and +20v / -22V for the gate though other displays may use different ranges. The timing for this needs to be highly accurate and is controlled by software. Waveforms are used for this process and since they are specific to the display model, they should be provided by the manufacturer. Since temperature affects the update time, an internal temperature sensor is needed and this data is passed over I2C.
For more information about the technical details of ePaper displays, Petteri Aimonen’s site is an excellent resource.