Question 13.4: You want to display text on an alphanumeric LCD display....

Use an HD44780-compatible LCD module and wire it to the GPIO connector.

You will find many low-cost LCD modules, such as the one shown connected to a Raspberry Pi in Figure 13-6. These have 16 pins, although fortunately, not all of them have to be connected to a GPIO pin.

These modules are available in a number of sizes and are specified by the number of columns and rows of letters they can display. So, for example, the module used in this recipe is described as 16×2, as it can display two rows each of 16 characters. Other common sizes are 8×1, 16×1, 16×2, 20×2, and 20×4.

To make this recipe, you will need:

• Breadboard and jumper wires (see “Prototyping Equipment” on page 380)

• 16×2 HD44780-compatible LCD module (see “Modules” on page 381)

• Row of 16 header pins (see “Miscellaneous” on page 382)

• 10kΩ trimpot (see “Resistors and Capacitors” on page 380)

Figure 13-7 shows the breadboard layout for connecting the display. The LCD modules are not normally supplied with header pins, so you will need to solder these in place.

The trimpot is used to control the contrast of the display. Note that if it looks like the project has not worked, try moving the knob of the trimpot over its entire range. It may well be that the contrast is so off that the display characters are not visible at all.

The Adafruit Raspberry Pi example code, available from GitHub, includes a library for controlling LCD displays that use the HD44780 driver. Before installing it, follow Recipe 8.3 to install the RPi.GPIO library.

The Adafruit library is not installed as a proper library, so to use it, you first need to download the folder structure. If you don’t have Git installed, install it now with this command (see Recipe 3.19):

Now, you can download the folder structure from GitHub:

Change directory into the Adafruit code using:

There is a test program here that displays the time and IP address of the Raspberry Pi. But first, if you are using a newer Raspberry Pi model B, revision 2, you will need to edit the file Adafruit_CharLCD.py:

Search for the line:

Replace the number 21 with 27 so that the line looks as follows, and then save and exit the file:

You can now run the example program with the command:

Discussion

These displays can operate either with a four- or eight-bit data bus and also require three control pins. The pins are connected as per Table 13-1.

The Adafruit library file Adafruit_CharLCD.py is responsible for setting the values on the data pins and then sending them to the display module. It provides the following functions, which you can use in your programs:

home()
Move to top left.

clear()
Clear all text off the display.

setCursor(column, row)
Set the cursor position from where text will be written.

cursor()
Turn on cursor display.

noCursor()
Turn off cursor display (default).

message(text)
Write the text at the current cursor position.

The following minimal example program shows how easy it is to display a simple message by using the library:

These modules are available in many sizes and are specified by the number of columns and rows of letters they can display. So, for example, the module used in this recipe is described as 16×2, as it can display two rows each of 16 characters. Other common sizes are 8×1, 16×1, 16×2, 20×2, and 20×4.

See Also

This recipe is based on the Adafruit tutorial at the Adafruit learning website.

Adafruit also sells a plug-in shield with an LCD display attached that is compatible with this recipe.