lcd

Hitachi HD44780 LCD driver

A Node.js Hitachi HD44780 LCD driver for Linux boards such as the BeagleBone or Raspberry Pi. Heavily inspired by the Arduino LiquidCrystal library.

Most LCDs compatible with the HD44780 have a sixteen pin interface. This Node.js module uses six of these interface pins for controlling such displays. Register select (RS), enable (E), and four data bus pins (D4-D7). The read/write (R/W) pin is assumed to be tied low to permanently select write mode.

$ [sudo] npm install lcd

lcd requires Node.js v0.8.0 or higher.

BeagleBone Prerequisites

There are no prerequisites for using lcd on the BeagleBone or BeagleBone Black, when Debian is being used.

Before installing lcd on stock Ångström on the BeagleBone or BeagleBone Black, three Python modules need to be installed; python-compiler, python-misc, and python-multiprocessing. They can be installed with the following commands:

$ opkg update
$ opkg install python-compiler
$ opkg install python-misc
$ opkg install python-multiprocessing

As of lcd v0.2.0 the print method is asynchronous. In previous versions it was synchronous.

The following nine line program can be used to make a UTC digital clock.

var Lcd = require('../lcd'),
  lcd = new Lcd({rs: 27, e: 65, data: [23, 26, 46, 47], cols: 8, rows: 1});
 
lcd.on('ready', function () {
  setInterval(function () {
    lcd.setCursor(0, 0);
    lcd.print(new Date().toISOString().substring(11, 19));
  }, 1000);
});

Here it is up and running on a BeagleBone Black wired up to an 8x1 display:

After requiring the lcd module, the above program creates an Lcd object. The constructor function is passed all the necessary information.

The six LCD interface pins used to control the display need to be wired up to six GPIOs on the BeagleBone Black. GPIOs on Linux are identified by unsigned integers. The relevant information for all six GPIOs used here is shown in the following table:

BBB Expansion HeaderGPIO No.LCD FunctionLCD Pin No.
P8_1323Data Bus Bit 411
P8_1426Data Bus Bit 512
P8_1547Data Bus Bit 714
P8_1646Data Bus Bit 613
P8_1727Register Select4
P8_1865Enable6

The constructor function is also told how many columns and rows the display has, eight and one respectively in this case.

It takes several milliseconds to initialize an LCD. The constructor starts the initialization process, but it doesn't wait for it to complete. Instead, a 'ready' event is emitted after the LCD has been completely initialized and is ready for usage.

The 'ready' handler leverages setInterval to execute a function that updates the time displayed on the LCD once a second.

Adding the following few lines will turn the digital clock into a good citizen that cleans up after itself.

// If ctrl+c is hit, free resources and exit. 
process.on('SIGINT', function () {
  lcd.close();
  process.exit();
});

Lcd(config)

Returns a new Lcd object which inherits from EventEmitter. A 'ready' event will be emitted when the display is ready for usage.

The config object has these possibilities:

  • cols LCD column count. Defaults to 16.
  • rows LCD row count. Defaults to 1.
  • largeFont Use 5x10 dot font. Defaults to false for 5x8 dot font.
  • rs Register select GPIO number.
  • e Enable GPIO number.
  • data Array of four GPIO numbers for data bus bits D4 through D7.

print(val, [cb])

Converts val to string and writes it to the display asynchronously.

If the optional completion callback is omitted, a 'printed' event is emitted after the operation has completed. The string representation of val is passed to the 'printed' event handler as the first argument. If an error occurs, an 'error' event will be emitted and an error object will be passed to the 'error' event handler as the first argument.

If the optional completion callback is specified, it gets two arguments (err, str), where err is reserved for an error object and str is the string representation of val. If the optional completion callback is specified, no 'printed' or 'error' event will be emitted.

The example print-twice-20x4.js demonstrates how to print two strings in succession using events.

clear([cb])

Clears display and returns cursor to the home position asynchronously.

If the optional completion callback is omitted, a 'clear' event is emitted after the operation has completed. If an error occurs, an 'error' event will be emitted and an error object will be passed to the 'error' event handler as the first argument.

If the optional completion callback is specified, it gets one argument (err), where err is reserved for an error object. If the optional completion callback is specified, no 'clear' or 'error' event will be emitted.

home([cb])

Returns cursor to home position asynchronously. Also returns display being shifted to the original position.

If the optional completion callback is omitted, a 'home' event is emitted after the operation has completed. If an error occurs, an 'error' event will be emitted and an error object will be passed to the 'error' event handler as the first argument.

If the optional completion callback is specified, it gets one argument (err), where err is reserved for an error object. If the optional completion callback is specified, no 'home' or 'error' event will be emitted.

setCursor(col, row) Moves the cursor to the specified col and row. Numbering for col and row starts at zero.

cursor() Turn cursor on.

noCursor() Turn cursor off.

blink() Turn cursor blink on.

noBlink() Turn cursor blink off.

scrollDisplayLeft() Shift display to the left. Cursor follows the display shift.

scrollDisplayRight() Shift display to the right. Cursor follows the display shift.

leftToRight() Sets cursor move direction to left to right.

rightToLeft() Sets cursor move direction to right to left.

autoscroll() Automatically shift display when data is written to display.

noAutoscroll() Turn automatic shifting off.

close() Frees (unexports) all GPIOs used by the Lcd.

"Hello, World!" is five characters too long for an 8x1 display, but by moving the cursor to the ninth column, turning autoscroll on, and displaying a new character every 300 milliseconds the text can be scrolled onto the display character by character. Note that an 8x1 display actually has eighty columns but only eight of them are visible.

var Lcd = require('../lcd'),
  lcd = new Lcd({rs: 27, e: 65, data: [23, 26, 46, 47], cols: 8, rows: 1});
 
function print(strpos) {
  pos = pos || 0;
 
  if (pos === str.length) {
    pos = 0;
  }
 
  lcd.print(str[pos]);
 
  setTimeout(function () {
    print(str, pos + 1);
  }, 300);
}
 
lcd.on('ready', function () {
  lcd.setCursor(8, 0);
  lcd.autoscroll();
  print('Hello, World! ** ');
});

NHD-0108FZ-FL-YBW-33V3

NHD-0420DZ-FL-YBW-33V3