7.5inch e-Paper HAT

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7.5inch e-Paper HAT
640x384, 7.5inch E-Ink display HAT for Raspberry Pi
7.5inch-e-paper-hat-4.jpg

640x384, 7.5inch E-Ink display HAT for Raspberry Pi, SPI interface
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Category: OLEDs / LCDs, LCD
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Brand: Waveshare
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Onboard Interfaces
SPI
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Introduction

640x384, 7.5inch E-Ink display HAT for Raspberry Pi, SPI interface

More

Interfaces

VCC 3.3V
GND GND
DIN SPI MOSI
CLK SPI SCK
CS SPI chip select (Low active)
DC Data/Command control pin (High for data, and low for command)
RST External reset pin (Low for reset)
BUSY Busy state output pin (Low for busy)

Working principle

Introduction

This product is an E-paper device adopting the image display technology of Microencapsulated Electrophoretic Display, MED. The initial approach is to create tiny spheres, in which the charged color pigments are suspending in the transparent oil and would move depending on the electronic charge. The E-paper screen display patterns by reflecting the ambient light, so it has no background light requirement. Under sunshine, the E-paper screen still has high visibility with a wide viewing angle of 180 degree. It is the ideal choice for E-reading.

Communication protocol

SPI timing

Note: Different from the traditional SPI protocol, the data line from the slave to the master is hidden since the device only has display requirement.

  • CS is slave chip select, when CS is low, the chip is enabled.
  • DC is data/command control pin, when DC = 0, write command, when DC = 1, write data.
  • SCLK is the SPI communication clock.
  • SDIN is the data line from the master to the slave in SPI communication.

SPI communication has data transfer timing, which is combined by CPHA and CPOL.

  1. CPOL determines the level of the serial synchronous clock at idle state. When CPOL = 0, the level is Low. However, CPOL has little effect to the transmission.
  2. CPHA determines whether data is collected at the first clock edge or at the second clock edge of serial synchronous clock; when CPHL = 0, data is collected at the first clock edge.
  • There are 4 SPI communication modes. SPI0 is commonly used, in which CPHL = 0, CPOL = 0.

As you can see from the figure above, data transmission starts at the first falling edge of SCLK, and 8 bits of data are transferred in one clock cycle. In here, SPI0 is in used, and data is transferred by bits, MSB first.

How to use

Working with Raspberry Pi

Installing libraries required

If you want to connect your E-paper screen to Raspberry Pi, you should install some necessary libraries, or else the Demo (click to download) below may work improperly. For more information about how to install the Raspberry Pi libraries, please visit the website: Libraries Installation for RPi.

You can find the detailed presentation about the installations of libraries wiringPi, bcm2835 and python.

Hardware connection

Here is the connection between Raspberry Pi 3B and E-paper.

e-Paper Raspberry Pi 3B
3.3V 3.3V
GND GND
DIN MOSI
CLK SCLK
CS CE0
DC 25 (BCM)
RST 17 (BCM)
BUSY 24 (BCM)

Expected result

1) After the corresponding libraries installed, you can copy the relative programs into your Raspberry Pi, and then enter the corresponding file.

  • BCM2835: Execute the command: make, to compile the code and generate a file epd. Execute the command: sudo ./epd, the program will run.
  • WringPi: Execute the command: make, to compile the code and generate a file epd. Execute the command: sudo ./epd, the program will run.
  • Python: Execute the command: sudo python main.py

2) Image will be displayed on the screen.

  • Note:The refresh rate of this module is slow (about 6s), and it will flicker for several times during refreshing. Please be patient.

Working with Arduino

Hardware connection

e-Paper Arduino UNO
3.3V 3V3
GND GND
DIN D11
CLK D13
CS D10
DC D9
RST D8
BUSY D7

Expected result

  1. Copy the libraries file of Arduino demo code to the libraries folder which is under the installation directory of Arduino IDE, it usually is C:\users\username\documents\arduino\libraries. You can also designate the location on Arduino IDE--> File -->Preferences -->Sketchbook location
  2. Upload the project.
  3. E-paper will display the image.

Note:

  • The refresh rate of this module is slow (about 6s), and it will flicker for several times during refreshing. Please be patient.

Working with the STM32 development board

  • Here we use STM32F103ZE development board.
  • The demo code is based on HAL, so you can port the demo code to other STM board with STM32CubeMX.
  • The demo code can be compiled successfully in Keil v5

Hardware connection

Connecting to the STM32F103ZE:

e-Paper STM32F103ZE
3.3V 3V3
GND GND
DIN PA7 (MOSI)
CLK PA5 (SCK)
CS PA4
DC PA2
RST PA1
BUSY PA3

Expected result

  1. Open the project (epd-demo.uvprojx), which is under the MDK-ARM directory.
  2. Click Build to compile the project.
  3. Click Download to load the code to your board.
  • Reset, the screen will display image.
    Note: The refresh rate of this module is slow (about 6s), and it will flicker for several times during refreshing. Please be patient.

Code analysis

Here, we will analyze the driving code and take the demos for Raspberry Pi based on WiringPi library as examples.

Hardware interface function

The functions of drive code like DigitalWrite, DigitalRead, SendCommand, SenData and DelayMs call the interface functions which are provided by hardware device (epdif.h, epdif.c, epdif.cpp) to respectively implements the functions that Control IO Level, Read IO Level, Send SPI Command, Send SPI Data and Delay For Millisecond. If you want to port the demo code, you need to implement all the interfaces of epdif (e-paper display interface) according to the corresponding hardware device.

Note that Raspberry Pi uses hardware chip select while transmitting SPI data. So we needn’t set the CS pin to LOW before transmitting data, and the code will set it automatically while transmitting. However, for Arduino and STM32, etc. you need to explicitly set the CS pin to LOW with codes to start the SPI transmission of module.

Send Commands and Data (SendCommand and SendData)

SendCommand and SendData are used to send commands and data to module respectively. What the difference between them is that, D/C pin is set to LOW for sending commands and HIGH for sending data. If the D/C pin is LOW, the data transmitted from SPI interface to module will be recognized as commands and executed. If the D/C pin is HIGH, the data will be recognized as normal data. Generally, normal data will follow the command, works as parameter or image data.

Reset (Reset)

Module will reset if RST pin is LOW. It is used to restart the module after powered on or awakened. After restarting, you need to initialize module with initialization function (Init) for working properly.

Initialization (Init)

After powering on the module, initialization function (Init) will configure the parameters of module. It can also wake up module from sleep mode. Process of initialization: reset --> power setting --> panel setting --> booster soft start --> power on --> PLL control --> temperature calibration --> VCOM and data interval setting --> TCON setting --> TCON resolution --> VCM DC setting.

Display a Frame (DisplayFrame)

DisplayFrame is used to send a frame to the module, and the screen will refresh and display it.

Process: Send command data start transmission 1 --> Send data of an image to display --> Refresh the screen.

Note:

  • About the image data: 4 bits = 1 pixel, doesn’t support Gray scale (can only display black and white). 0000 (binary) stands for a black pixel, otherwise 0011 (binary) stands for a white pixel.

For example:

0x00: 2 pixels ■■
0x03: 2 pixels ■□
0x30: 2 pixels □■
0x33: 2 pixels □□
but 0x66: the state of pixels is uncertain
  • This module doesn’t support partial refresh.
  • The refresh rate of this module is slow (about 6s), and it will flicker for several times during refreshing. Please be patient.

Sleep mode (Sleep)

Sleep can make the module go into sleep mode to reduce the consumption.

Process: power off --> deep sleep

If you want to wake up the module from sleep mode, you need to give a LOW pulse to RST pin. Then maybe you need to reconfigure the parameter of power (According to the batches, some of them need to reconfigure, some needn’t). So if you want to wake up module, you had better use the Init function instead of Reset. Reset function and relative commands will be executed while executing the Init function.

How to display an image

There are two ways to display pictures. One is display directly and other is indirectly.

Display directly: Read the data of pictures with library functions, and decode. Then convert it to arrays and send to module. About how to implement it, you can refer to the python examples of Raspberry Pi. (The C demo doesn’t display pictures directly)

Display indirectly: Converting pictures to relative arrays on PC and save as c file. Then you can use the c file on your project. This chapter we will talk about how to convert a picture to array.

  1. Open a picture with drawing tool comes with Windows system, create a new image, and set the pixel to 640x384.
  2. Because this module can only display two gray level (Only black and white), we need to convert picture to monochrome bitmap before converting it to array. That is, File --> BMP picture --> Monochrome Bitmap.
    There is a monochrome bitmap on examples pack for demonstration (raspberrypi/python/monocolor.bmp).
  3. Use Image2Lcd.exe software to generate corresponding array for picture (.c file). Open picture with this software, set the parameters:
    • Output data type: C language array
    • Scanning mode: vertical scanning
    • Output gray: single color (gray level of two)
    • Maximum width and height: 640 and 384
    • Include the data of Image Header: Don’t check
    • Inverse color: Check (Check: the white on image will be inversed to 1, and black is inversed to 0)
  4. Click “Save”, to generate .c file. Copy the corresponding array into your project, and you can display picture by calling this array.
  • Note: the module decodes the image data as: 4 bits = 1 pixel, but due to Gray scale unsupported, a monochrome bitmap array (1 bit = 1 pixel) is enough. Once the program finds a bit set in a bitmap table, it will send 0x3; but if a bit reset, it will send 0x0. For example, if the program decodes a byte 0xAA in a bitmap table, it will send 0x30303030 in fact.

Resources

Documentation

Demo code

Datasheets

FAQ

Question:
1. Why the e-Paper cant work with Arduino?
Answer:

The I/O level of Arduino is 5V, and the e-Paper should be driven with 3V3. If your Arduino cant drive the e-Paper successfully, please try to convert the level to 3.3V

You can also try to connect the Vcc pin to the 5V of Arduino to see whether the e-Paper works, but we recommend you not to use 5V for a long time.


Question:
2. Why does the color of e-Paper look a little black or grey?
Answer:

You can try to change the value of Vcom on demo codes.



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