Writing To The E-ink Display With Micropython

I’d gotten as far as running a few lines of code on the Pico, so the next thing was to work out if I could display anything at all on the e-ink display. Since I’d already messed up by buying the wrong HAT type, I still had a small doubt about whether I would get the two to work together at all.

Waveshare has its own Github respository so I headed there to track down the correct Python file. I knew I needed one of the files that started with “Pico_ePaper-2.13-“, but which one?

There was a little stick on the display telling me I had a “v3”, but working out whether I had an A, B, C or D model was a mixture of guesswork and deduction. The file for both B and C appeared to support multiple colours, which is not what I had, so it wasn’t those. I had nothing else to go on, so I tried both A and D. Neither seemed to work properly, although I did get the screen to flicker a bit so something was happening.

I can’t even remember how I finally got the thing working, but it appears to be through a hybrid of files that started as Pico_ePaper-2.13-B_V4.py with all the code that referred to the red image data removed. The whole process was made more difficult by the fact that I seemed to have messed up connecting two of the pins, having not spotted that there was a pin labelled “GND” right between “GP21” and “GP22” on the Pico board.

This was my “driver” module:


from machine import Pin, SPI
import framebuf
import utime

EPD_WIDTH       = 122
EPD_HEIGHT      = 250

RST_PIN         = 12
DC_PIN          = 8
CS_PIN          = 9
BUSY_PIN        = 13

class EPD:
    def __init__(self):
        self.reset_pin = Pin(RST_PIN, Pin.OUT)
        
        self.busy_pin = Pin(BUSY_PIN, Pin.IN, Pin.PULL_UP)
        self.cs_pin = Pin(CS_PIN, Pin.OUT)
        if EPD_WIDTH % 8 == 0:
            self.width = EPD_WIDTH
        else :
            self.width = (EPD_WIDTH // 8) * 8 + 8
        self.height = EPD_HEIGHT
        
        self.spi = SPI(1)
        self.spi.init(baudrate=4000_000)
        self.dc_pin = Pin(DC_PIN, Pin.OUT)
        
        
        self.buffer_black = bytearray(self.height * self.width // 8)
        self.init()

    def digital_write(self, pin, value):
        pin.value(value)

    def digital_read(self, pin):
        return pin.value()

    def delay_ms(self, delaytime):
        utime.sleep(delaytime / 1000.0)

    def spi_writebyte(self, data):
        self.spi.write(bytearray(data))

    def module_exit(self):
        self.digital_write(self.reset_pin, 0)

    # Hardware reset
    def reset(self):
        self.digital_write(self.reset_pin, 1)
        self.delay_ms(50)
        self.digital_write(self.reset_pin, 0)
        self.delay_ms(2)
        self.digital_write(self.reset_pin, 1)
        self.delay_ms(50)


    def send_command(self, command):
        self.digital_write(self.dc_pin, 0)
        self.digital_write(self.cs_pin, 0)
        self.spi_writebyte([command])
        self.digital_write(self.cs_pin, 1)

    def send_data(self, data):
        self.digital_write(self.dc_pin, 1)
        self.digital_write(self.cs_pin, 0)
        self.spi_writebyte([data])
        self.digital_write(self.cs_pin, 1)
        
    def ReadBusy(self):
        print('busy')
        while(self.digital_read(self.busy_pin) == 1): 
            self.delay_ms(10) 
        print('busy release')
        self.delay_ms(20)
        
    def TurnOnDisplay(self):
        self.send_command(0x20)  # Activate Display Update Sequence
        self.ReadBusy()
        
    def TurnOnDisplayPart(self):
        self.send_command(0x20)        
        self.ReadBusy()

    def SetWindows(self, Xstart, Ystart, Xend, Yend):
        self.send_command(0x44) # SET_RAM_X_ADDRESS_START_END_POSITION
        self.send_data((Xstart>>3) & 0xFF)
        self.send_data((Xend>>3) & 0xFF)

        self.send_command(0x45) # SET_RAM_Y_ADDRESS_START_END_POSITION
        self.send_data(Ystart & 0xFF)
        self.send_data((Ystart >> 8) & 0xFF)
        self.send_data(Yend & 0xFF)
        self.send_data((Yend >> 8) & 0xFF)
        
    def SetCursor(self, Xstart, Ystart):
        self.send_command(0x4E) # SET_RAM_X_ADDRESS_COUNTER
        self.send_data(Xstart & 0xFF)

        self.send_command(0x4F) # SET_RAM_Y_ADDRESS_COUNTER
        self.send_data(Ystart & 0xFF)
        self.send_data((Ystart >> 8) & 0xFF)
    

    def init(self):
        print('init')
        self.reset()
        
        self.ReadBusy()   
        self.send_command(0x12)  #SWRESET
        self.ReadBusy()   

        self.send_command(0x01) #Driver output control      
        self.send_data(0xf9)
        self.send_data(0x00)
        self.send_data(0x00)

        self.send_command(0x11) #data entry mode       
        self.send_data(0x03)

        self.SetWindows(0, 0, self.width-1, self.height-1)
        self.SetCursor(0, 0)

        self.send_command(0x3C) #BorderWavefrom
        self.send_data(0x05)

        self.send_command(0x18) #Read built-in temperature sensor
        self.send_data(0x80)

        self.send_command(0x21) #  Display update control
        self.send_data(0x80)
        self.send_data(0x80)

        self.ReadBusy()
        
        return 0       
        
    def display(self):
        self.send_command(0x24)
        for j in range(0, self.height):
            for i in range(0, int(self.width / 8)):
                self.send_data(self.buffer_black[i + j * int(self.width / 8)])   
        self.TurnOnDisplay()

    
    def Clear(self, colorblack):
        self.send_command(0x24)
        for j in range(0, self.height):
            for i in range(0, int(self.width / 8)):
                self.send_data(colorblack)
        self.TurnOnDisplay()

    def sleep(self):
        self.send_command(0x10) 
        self.send_data(0x01)
        
        self.delay_ms(2000)
        self.module_exit()
        

I haven’t yet managed to track down what all the send_command statements actually do, but I feel sure there must be some documentation for them somewhere.

Anyway, finally, I had the equivalent of “hello world” on the display, without really knowing how. Also, I had discovered that the epaper display naturally preferred to display in portrait mode, which wouldn’t work for the layout I had in mind for my weather display.

    epd = EPD()
    epd.Clear(0xff)
    epd.imageblack = framebuf.FrameBuffer(epd.buffer_black, epd.width, epd.height, framebuf.MONO_HLSB)
    
    epd.imageblack.fill(0xff)
    #epd.imagered.fill(0xff)
    epd.imageblack.text("Waveshare", 0, 10, 0x00)
    epd.imageblack.text("ePaper-2.13-B", 0, 25, 0x00)
    epd.imageblack.text("RPi Pico", 0, 40, 0x00)
    #epd.imagered.text("Hello World", 0, 55, 0x00)
    epd.display()
    epd.delay_ms(2000)
    
    epd.imageblack.vline(10, 90, 40, 0x00)
    epd.imageblack.vline(90, 90, 40, 0x00)
    epd.imageblack.hline(10, 90, 80, 0x00)
    epd.imageblack.hline(10, 130, 80, 0x00)
    epd.imageblack.line(10, 90, 90, 130, 0x00)
    epd.imageblack.line(90, 90, 10, 130, 0x00)
    epd.display()
    epd.delay_ms(2000)

It felt like I’d conquered most of the unknowns anyway, so now onto showing some actual weather.