Template: E-Paper Codes Description

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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)

Init has 3 effects: 1, Set the arguments at power up. 2, Awaken the module from deep sleep. 3, Set the mode to Full update or Partial update.

Process of initialization: reset -> driver output control -> booster soft start control -> write VCOM register -> set dummy line period -> set gate time -> data entry mode setting -> look-up table setting

Configuration of LUT table(SetLut)

Look-up table is used to set the update mode of the module. This table is provided by us but it may be different among different batches. If the table changed, we will update the demo code as soon as possible.

Set the frame memory (SetFrameMemory)

SetFrameMemory is used to write image data to the memory.

  • Process:
Set the area size (see the function SetMemoryArea) -> set the start point (see the function SetMemoryPointer) -> send the command Write RAM -> start image data transfer.
  • The module has two memory areas. Once DisplayFrame is invoked, the following action of SetFrameMemory will set the other memory area, e.g. to set all the two memory areas, the process is: SetFrameMemory -> DisplayFrame -> SetFrameMemory -> DisplayFrame, i.e. set and update twice.
  • The data from SPI interface is first saved into the memory and then updated if the module received the update command.
  • About the image to be sent: 1 byte = 8 pixels, doesn’t support Gray scale (Can only display black and white). A bit set stands for a white pixel, otherwise a bit reset stands for a black pixel.

For example:

0xC3:8 pixels □□■■■■□□
0x00:8 pixels ■■■■■■■■
0xFF:8 pixels □□□□□□□□
0x66:8 pixels ■□□■■□□■

Display a Frame (DisplayFrame)

DisplayFrame is used to display the data from the frame memory.

Note:

  • The module has two memory areas. Once DisplayFrame is invoked, the following function SetFrameMemory will set the other memory area, e.g. to set all the two memory areas, the process is: SetFrameMemory -> DisplayFrame -> SetFrameMemory -> DisplayFrame, i.e. set and update twice.
  • The data from SPI interface is first saved into the memory and then updated if the module received the update command.
  • The module will flicker during full update.
  • The module won't flicker during partial update, however, it may retain a "ghost image" of the last page.

Sleep mode (Sleep)

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

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.

Private function: Set the memory area (SetMemoryArea)

SetMemoryArea is used to specify the memory area, the arguments are the start/end points. Because 1 byte = 8 pixels of the image data to be sent, the x coordinates must be the multiple of 8, or else the last 3 bits will be ignored.

Private function: Set the memory pointer (SetMemoryPointer)

SetMemoryPointer is used to set the start point of the following image to be sent. Because 1 byte = 8 pixels of the image data to be sent, the x coordinates must be the multiple of 8, or else the last 3 bits will be ignored.

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 200x200.
  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: 200 and 200
    • Include the data of Image Header: Don’t check
    • Inverse color: Check (Check: the white on image will be converted to 1, and black is converted 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.

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