HD44780

Introduction

The HD44780 is sufficiently different from the other supported LCD displays to warrant a dedicated page to describe its features (and limitations).

It is a very popular LCD display that is widely available, cheap and available in several form factors with 16 character x 2 line and 20 character by 4 line displays the most common. As with most LCD displays, it comes with a backlight which luma.lcd can control using a GPIO pin. PWM modulation is supported if varying the intensity of the backlight is desired.

They are normally connected to SBCs using a parallel 6800-series interface implemented bitbang-style using the SBCs GPIO pins. Alternatively, they are also sold pre-connected to a few different I2C backpacks including the PCF8574, MCP23008 and MCP23017 I2C expanders.

Capabilities

The HD44780 is mainly a character-based device and can be purchased with different font tables installed in its ROM. The two most popular are the A00 ROM (English/Japanese) and the A02 ROM (English/European). See HD44780 for details on which characters are included in each.

In addition to the built-in characters, HD44780s can have up to eight custom written into their RAM. This feature is used by luma.lcd to provide a limited graphics capability.

Character-mode Usage

When using luma.lcd.device.hd44780 the most common way of controlling the display is to use the text property which operates similarly to the luma.core.virtual.sevensegment wrapper class. To use it, assign the text property a string containing the values that you want displayed:

from luma.core.interface.parallel import bitbang_6800
from luma.lcd.device import hd44780

interface = bitbang_6800(RS=7, E=8, PINS=[25, 24, 23, 27])
device = hd44780(interface, width=16, height=2)

device.text = 'Hello World'

The newline character can be used to display characters on the next line of the display:

device.text = 'Hello\nWorld'

It is the only control character that works however. Other control characters such as carriage return (ascii 13) and line feed (ascii 10) will be silently ignored.

Graphics-mode Usage

It is also possible to display graphical content using the display method which works similarly to other luma.lcd and luma.oled displays. However there is a significant limitation. There can only be 8 characters worth of ‘custom’ content displayed at any point in time. If you exceed this limit, the undefined character (a value set during initialization of the hd44780) will be displayed instead. While 8 custom characters may seem limited, with some creativity it can be very useful.

To understand how to leverage this capability requires some explanation of how the screen of the display is organized and how luma.lcd manages the custom character space.

Each character position on an HD44780 display is made up of a 5 pixel by 8 pixel grid. When a character is requested to be displayed, the HD44780 looks up the character from its font table and copies the pixels from the font table ROM to the address in RAM that corresponds to the current character position. If there is not a character within the font table that matches what needs to be displayed, a custom character can be created within a small space in the display’s RAM that is reserved for this purpose. There are several restrictions that have to be kept in mind though:

• The custom character must be a 5x8 image

• It can only be displayed in alignment with the other characters on the display

• There can be a maximum of 8 special characters on the screen at any given time

• If the driver has run out of custom characters for a screen, the remaining cells with non-standard content will display the undefined character instead.

However there are a few features that can be leveraged to extend beyond these restrictions:

• The HD44780 class will automatically create the appropriate custom characters

• The content of a special character can be used multiple times on the screen

• The content of all of the special characters can be changed every time the screen is redrawn

Here is a small example of how this can be leveraged:

from luma.core.interface.parallel import bitbang_6800
from luma.lcd.device import hd44780
from luma.core.render import canvas
from PIL import Image, ImageDraw

interface = bitbang_6800(RS=7, E=8, PINS=[25, 24, 23, 27])
device = hd44780(interface, width=16, height=2)

def progress_bar(width, height, percentage):
    img = Image.new('1', (width, height))
    drw = ImageDraw.Draw(img)
    drw.rectangle((0, 0, width - 1, height - 1), fill='black', outline='white')
    drw.rectangle((0, 0, width * percentage, height - 1), fill='white', outline='white')
    return img

progress = 0.25
fnt = device.get_font('A00')
with canvas(device) as draw:
    draw.text((5, 0), f'Installing {progress*100:.0f}%', font=fnt, fill='white')
    draw.bitmap((5, 8), progress_bar(70, 8, progress), fill='white')

There are a few of things that deserve highlighting in this code.

• The get_font method of the hd44780 class is used to retrieve the internal font used by the device. This enables us to place exact replicas of the characters within the font tables on the canvas. When these characters are displayed, because they are already normal characters, they do not consume any of the customer character RAM. The hd44780 class contains both the A00 and A02 font tables. You should request the table that matches what is installed in your display.

• The progress bar is drawn using normal PIL.ImageDraw primitives in this case a couple of calls to the rectangle method and a call to the text method.

• The size of the progress bar was carefully chosen. It is 70 pixels wide by 8 pixels high. This will fill 14 characters worth of space. This is because each cell is 5 pixels wide (70/5=14) and it is 8 pixels high (8/8=1). Normally 14 cells worth of graphical content would be a problem. However, the progress bar only requires four custom characters regardless of what position the progress value is set to. To see why that is you need to look at what each cell looks like within the progress bar at each state the progress bar.

  The different conditions of the progress bar can be expressed in 5 states:

  CUSTOM CHARACTERS

  First	Middle	End
  Left Empty	Middle Empty	Right Empty
  Filling	Middle Empty	Right Empty
  Filled	Filling, Middle Empty	Right Empty
  Filled	Filled, Filling, Middle Empty	Right Empty
  Filled	Filled	Right Filling
  Filled	Filled	Filled

  So the maximum number of custom characters is as little as one, but never exceeds four.

• The progress bar was carefully placed to align with the character cell boundaries. If we had placed the progress bar image one pixel to the left if would have consumed two additional custom characters as the beginning and end of the progress bar would be spread across character cell boundaries. In this particular case, it would still have displayed correctly because we would be under the 8 character limit.

Tip

When displaying text, to avoid using custom character space you should:

• Use the internal font installed in your device which can be retrieved using get_font

• Make sure to align the placement of the text to the 5x8 cell structure of the display

  Good:

  draw.text( (5,0), 'Good', fill='white' font=device.get_font('A00'))
  

  Bad:

  draw.text( (6,1), 'Bad', fill='white' font=device.get_font('A00'))
  

Embedded Font Tables

The luma.lcd.device.hd44780 class leverages luma.core.bitmap_font to include two fonts that replicate the two font tables that are commonly available for the hd44780 device. These are:

Font Table	Font Name
A00	English Japanese
A02	English European

You can retrieve either of these fonts using the get_font method:

fnt = device.get_font('A02')

You can also combine fonts together in order to display characters not included within your device’s character table.

As an example, the ‘Black Right-Pointing Triable’ symbol Unicode U+25b6 is not contained in the A00 character table but is frequently used as a ‘Play’ symbol for multi-media systems. It is however included in the A02 font table. We can pull the symbol from A02 and add it to the current embedded font to enable us to use it.

fnt = device.get_font('A02')
device.font.combine(fnt, '\u25b6')
device.text = '\u25b6 Play'

This feature leverages the custom character capability so it has the same 8 character limitation. If you exceed 8 characters within a screen, the undefined character will be used for all additional characters that are not contained within the devices font table.

The table for all display characters can be found in the luma.lcd.const.hd44780.FONTDATA attribute.

See the documentation for luma.core.bitmap_font for more information on how to use the bitmap_font module.