Glider

Open-source Eink monitor with an emphasis on low latency.

Note: This repo only contains the hardware design, the gateware running on the FPGA is my open-source Caster EPDC design. This README also contains information about the Caster as well.

This is a long document, containing not just information about this project, but also pretty much everything I know about Eink. Given it's a bit hard to gather information about Eink online, I think this is the right thing to do. Use the following table of contents to navigate around.

Eink is a registered trademark and brand of E Ink Corporation. All the contents provided in this repo are based on publicly available information online and original research. They are not endorsed by Eink in any way and they may contain errors and/ or inaccuracies.

If you're interested in attaining a board, checkout the product page for the Modos Paper Monitor on Crowd Supply.

If you are interested in Eink or any other display technologies, I have a Discord server for that. Feel free to join: https://discord.gg/rtT7euSHQS . (This Discord server is also not endorsed by Eink or any other company. It's not a customer support server.) For Modos product support or other inqueries about Modos products, use https://discord.gg/6ktE6VxSyh.

Table of Contents

• Overview
  • Features
  • Hardware
  • Components
• Eink Screens
  • Basic Theory of Operation
  • Advantages and Disadvantages
  • The Role of Eink Controller
  • Screen Panel Types
  • Using Screen with Integrated Controller
  • Using Screen without Integrated Controller
  • Understanding Waveform
  • Greyscale Display
  • Color Display
  • Dithering
  • Eink Screen Generations
• Caster/ Glider Design
  • Low Latency Drive
  • Hybrid Greyscale Mode
  • Limitations
  • Hardware Design Decisions
  • Gateware Architecture
  • Firmware Functions
  • Resources Utilization
• Building
  • PCB
  • FPGA Bitstream
  • MCU Firmware
  • Flashing Board
  • Compatible Screens
• References
• License
• Appendix
  • Using Screens without Datasheet
  • Screen List

Overview

Features

• Complete solution for low-latency/ high-refresh-rate EPD monitor
• Supports electrophoretic display panels with parallel I/F (Eink(R), OED and DES)
• Supports both monochrome and color-filter-array (such as Kaleido(TM)) based color screen
• Extremely low processing delay of <20 us
• Supports binary, 4-level grayscale, and 16-level grayscale output modes
• Latency-optimized binary and 4-level grayscale driving modes
• Hybrid automatic binary and 16-level grayscale driving mode
• Host software runtime controllable regional update and mode switching
• Hardware bayer dithering, blue-noise dithering, and error-diffusion dithering with no additional latency
• Controller takes standard HVsync signal input
• Board-level design supports USB-C (USB Type-C DisplayPort Alt Mode) and DVI input

Hardware

• Xilinx(R) Spartan-6 LX16 FPGA running Caster
• DDR3-800 framebuffer memory
• Type-C DisplayPort Alt-Mode video input with onboard PTN3460 DP-LVDS bridge or
• DVI (via microHDMI connector) video input with onboard ADV7611 decoder
• Epaper power supply with up to 1A peak current on +/-15V rail supporting large panels
• VCOM kick-back voltage measurement support
• On-board STM32H750 microcontroller for USB communication and firmware upgrade
• Up to 133MP/s processing rate with error-diffusion dithering enabled, >200MP/s when disabled

The board is designed with KiCad. You may need the latest stable version of KiCad to open the source file.

Components

This repo hosts the PCB design and firmware source code. The RTL code is in a separate repo: https://gitlab.com/zephray/Caster/.

Eink Screens

Eink is the brand of a family of paper-like electrophoretic displays. The underlying technology is invented in the MIT Media Lab between 1995 and 1997 by Barrett Comiskey, J.D. Albert, and Joseph Jacobson. They later founded the E Ink Corporation to commercialize this technology.

Nowadays they are commonly used on e-readers and electronic shelf labels. You’ve probably seen them on Kindle, in stores, or maybe in some train stations as well.

| eReader/ Tablets | Electronic Shelf Label | Digital Signage | |-|-|-| | | | |

(Source: https://www.eink.com/application, image copyright Eink corporation)

This section gives an overview of the electrophoretic displays, including the screen panels available and underlying technology. Note this project doesn't and can't support all electrophoretic screens. This documentation also solely focuses on using existing off-the-shelf screen panels rather than the physics or manufacturing process of one.

Basic Theory of Operation

In the simplest form, you have charged particles with different colors, dispersed in some oil in some transparent container. By applying electric fields the particles can be moved up or down to produce either black or white, or a mixture of that.

(Source: https://www.eink.com/tech/detail/How_it_works , copyright Eink Corporation)

There are multiple technologies based on this basic concept, namely Eink’s micro-capsule display, SiPix (now acquired by Eink)’s micro-cup display, and WFT’s DES display. They differ in specific ways of confining the particles in containers, but otherwise very similar.

The pixels on the screen are typically arranged as a 2D array, driven with TFTs. The pixels are scanned/ driven periodically at a fixed refresh rate, typically ranging from 50Hz to 120Hz. Applying positive voltage on the pixel will typically drive the particles toward the white state while applying negative voltage will drive the particles towards the black state. This is similar to active matrix TN/IPS LCDs, which also use 2D TFT arrays and electrical fields for changing state. However, unlike LCDs, EPDs maintain their state after the electrical field is removed. So unlike LCDs which require continuous refreshing, the EPDs only need to be refreshed till the pixels are fully driven.

In terms of driving the screen panel, depending on the pixel value (1 or 0), each pixel would be driven either with a positive voltage or a negative voltage. A global counter can be used to count the frames elapsed and stop driving the pixels after a predefined period of time (for example, 100ms). Two framebuffers are typically used for determining if the pixel has changed color or not. If not, then the pixel does not need to be driven.

Advantages and Disadvantages

In terms of display quality, EPDs are no match for modern IPS LCDs. The following is a comparison table of key parameters. The specific number would vary depending on the screen used but should be within the same ballpark.

| | Monochrome EPD | CFA-based Color EPD | Transmissive TFT IPS LCD | Reflective TFT TN LCD | |-|-|-|-|-| | Contrast Ratio | ~17:1 | ~14:1 | ~1000:1 | ~14:1 | | Colors | 16 (Greyscale) | 4096 | 16M | 256 | | Color Gamut | N/A | ~1.5% sRGB | ~99.9% sRGB | N/A | | Reflectivity | ~45% | ~25% | N/A | ~15% | | Response Time | ~150ms | ~150ms | ~10ms | ~15ms |

It has a few advantages. It reflects lights instead of emitting lights, so it generally consumes less power and can be used outdoors, etc. It’s also bistable, which means that it retains the image after the power has been removed. Personally, the biggest differentiating factor for me (author of this README) is that it looks like paper.

The image above shows a comparison between reflective TFT LCD (SHARP memory LCD in this case) and Eink. The LCD has a mirror-like texture which changes reflectivity drastically in different angles, while the Eink is more paper-like.

| ZBD LCD | Ch LCD | STN LCD | |-|-|-| | | | | | Bistable, reflective, high contrast, no greyscale, ~10s refresh | Bistable, reflective, lower contrast, up to 32 level greyscale, ~5s refresh | Volatile, reflective, lower contrast, up to 32 level greyscale, ~100ms response |

There are many other reflective or bistable display technologies. They are all interesting displays on their own, but none of them feels like paper (yet).

Overall, there is no single perfect display technology. Each has its own unique strength. Pick the right one for your project.

The Role of Eink Controller

The Eink controller is in some ways similar to the display controller (DC/ CRTC) + timing controller (TCON) in a typical LCD-based system. It takes the raw image data and converts it to signals required to drive the screen.

To understand the actual work of an eink controller, start from the basic concept. The color of