Template:Pico e-Paper 3.7 Spec

Overview

3.7inch EPD (Electronic Paper Display) Module For Raspberry Pi Pico, 480×280 Pixels, Black / White, SPI Interface

Features

• No backlight keeps displaying last content for a long time even when power down
• Ultra-low power consumption, basically power is only required for refreshing
• SPI interface requires minimal IO pins

Specification

• Operating voltage: 3.3V
• Interface:SPI
• Outline dimensions: 56.9mm × 95.3mm
• Display size: 47.32mm x 81.12mm
• Dot pitch: 0.169mm x 0.169mm
• Resolution: 480×280 pixels
• Display color: Black, white
• Greyscale: 4
• full refresh time: 3s
• Partial refresh time: 0.3s
• Refresh power: 26.4mW(typ.)
• Standby current: <0.01uA(almost 0)
• Viewing Angle: >170°

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 a 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 an idle state. When CPOL = 0, the level is Low. However, CPOL has little effect on transmission.
2. CPHA determines whether data is collected at the first clock edge or at the second clock edge of the 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. Here, SPI0 is in used, and data is transferred by bits, MSB first.

Working protocoal

Pixel & Byte

We define the pixels in a monochrome picture, 0 is black and 1 is white.
White：□，Bit 1
Black：■：Bit 0

• The dot in the figure is called a pixel. As we know, 1 and 0 are used to define the color, therefore we can use one bit to define the color of one pixel, and 1 byte = 8pixels
  
• For example, If we set first 8 pixels to black and the last 8 pixels to white, we show it by codes, they will be 16 bit as below:
  

For computer, the data is saved in MSB format:

So we can use two bytes for 16 pixels.

• In addition to bicolor displaying, the 3.7inch e-Paper also supports four grayscale.

To display grey pixels, we need to define data for gray

• Black: 00b
• Dark Grey: 10b
• Light Grey: 01b
• White: 11b

Pixel	1		2		3		4		5		6		7		8
Bit	7	6	5	4	3	2	1	0	7	6	5	4	3	2	1	0
Data	0	0	0	0	1	0	1	0	0	1	0	1	1	1	1	1
Color	Black		Black		Dark Grey		Dark Grey		Light Grey		Light Grey		White		White
Byte	0x0A								0x5F

The display divides a four-grayscale picture into two pictures. The pixels in the same position of pictures are combined into one pixel.

Register	White	Light Grey	Dark Grey	Black
0x24	0x01	0x00	0x01	0x00
0x26	0x01	0x01	0x00	0x00

With the tables above, you can define the data which can be used to display grayscale pixels in the 3.7inch e-Paper

Pixel	1		2		3		4		5		6		7		8
Bit	7	6	5	4	3	2	1	0	7	6	5	4	3	2	1	0
Data	0	0	0	0	1	0	1	0	0	1	0	1	1	1	1	1
Color	Black		Black		Dark Grey		Dark Grey		Light Grey		Light Grey		White		White
Byte	0x0A								0x5F
Bit in 0x24 register	0		0		1		1		0		0		1		1
Bit in 0x26 register	0		0		0		0		1		1		1		1
Data sent to 0x24 register	0x33
Data sent to 0x26 register	0x0F