875 E-ink display driver :: Quicker, easier and cheaper to make your own chip!

How it works

This is an example hardware driver for an e-ink display. Adafruit makes a nice series of small e-ink displays, but they are designed for an Arduino and driven by software. This project shows how to build a display driver in verilog. To keep memory overhead to a minimum, it operates like a VGA screen saver, displaying simple patterns that can be computed in real time as the pixel positions are counted and transmitted to the driver.

The driver instantiates an SPI master which communicates with the SSD1680 chipset on the e-ink display. Whenever a bit from the input PMOD is set to "1", and initialization sequence is send to the display, followed by a transmission of the display image, followed by a deep sleep power-down. Once in deep sleep mode, the displayed image will remain indefinitely, even if the display is disconnected from the development board.

How to test

The input/output PMOD is used to connect to the e-ink display pins. Since the e-ink display is not PMOD-compatible, it is necessary to install a header onto the e-ink display and create a bundle of jumper wires to connect to the PMOD as follows:

pin signal direction PMOD pin

ECS: uio[0] output 1 MOSI: uio[1] output 2 MISO: uio[2] input 3 SCK: uio[3] output 4 SRCS: uio[4] output 7 RST: uio[5] output 8 BUSY: uio[6] input 9 D/C: uio[7] output 10 GND: 11 or 5 VIN: 12 or 6

To test the eight example patterns, raise one of the input pins to value "1". This can be done with a set of external buttons on the input PMOD, or the input PMOD value can be set from software.

ui[5] is a special case in which the contents of the display board's SRAM are copied directly to the e-ink display. This uses an unusual method in which the SRAM is set to a sequential read mode and then is left enabled while the e-ink display is initialized. Commands being sent to the display are ignored by the SRAM, which outputs one bit on every clock cycle. The SRAM contents are then copied into the display starting at offset address 30 (which is the number of SPI bytes clocked while initializing the display). The SRAM is volatile and so unprogrammed at power-up. It can be programmed using the "pass-through" mode, in which the SRAM's SPI can be bit-banged from the ui[] port using software. Enable "pass-through" mode by setting ui[7:4] to 0xf, then bit-bang using ui[0] for clock and ui[1] for data (if the SRAM is given a READ command, then output from the SRAM can be read from uo[0]). First put the SRAM into sequential mode with command 0x01 0x40. End pass-through mode with ui = 0x00, then re-enter pass-through mode with ui = 0xf0. Continue with the command 0x02 0x00 0x1e and then write 3904 bytes of image data (32 bytes x 122 lines). End pass-through mode again with ui = 0x00, then display the image data with ui = 0x20.

External hardware

Every e-ink display has a very specific driver, and making a general-purpose driver is prohibitive for Tiny Tapeout. The project is designed to drive the Adafruit 2.13" e-ink display, Product ID: 4197, URL https://www.adafruit.com/product/4197 (as of this writing, cost is $22.50).

#	Input	Output	Bidirectional
0	All white \| Bitbang SCK	SRAM MISO (out)	ECS (out)
1	All black \| Bitbang MOSI		MOSI (out)
2	Vertical stripes		MISO (in, unused)
3	Horizontal stripes		SCK (out)
4	Small checkerboard		SRCS (out, =1)
5	User SRAM contents		RSTB (out)
6	Large checkerboard		BUSY (in)
7	Low-res smiley face		D/C (out)

875 E-ink display driver