LCD screen

The graphical LCD element

The picture formed of pixels of a graphic LCD

A graphical LCD liquid crystal display is a display that allows displaying pictures and text. Its construction is similar to the alphanumeric LCD, with a difference that on the graphic display, all pixels are divided into a single large matrix. If we are dealing with a monochrome LCD, then a pixel is one square segment. In a color display, a pixel is formed of three subpixels. Each of the three subpixels lets only one colored light pass (red, green, or blue). Since the subpixels are positioned very close to each other, they seem like one pixel.

Monochrome graphic displays usually have a passive matrix, and large color displays, including computer screens, have an active matrix. All information concerning the color of the background and the pixels of the graphic LCDs is similar to that of alphanumeric LCDs. Similar to the alphanumeric displays, graphic displays are also equipped with a separate controller, which takes care of receiving information through the communication interface and generating the electrical field for the segments.

HomeLab User Interface is an 84×48-pixel monochrome graphic LCD. It is the same display as used in Nokia 3310 mobile phones. The Philips PCD8544 controller is attached to the display, which can be communicated through a SPI-like serial interface. The background lighting of the display module is separately controlled.

First, the graphical LCD screen must be started with lcd_gfx_init function. There is a letter map inside the library with the full Latin alphabet, numbers, and the most common signs written. To display a letter or text, first its position must be determined by using the function lcd_gfx_goto_char_xy. For displaying a letter is lcd_gfx_write_char function, and for displaying text lcd_gfx_write_string function.

The following is an example of a time counter. The program counts seconds (approximately), minutes, and hours. For converting time to text, the sprintf function is used.

// Example of using the graphic LCD of the HomeLab
// Time of day is displayed on LCD since the beginning of the program
#include <stdio.h>
#include <homelab/module/lcd_gfx.h>
#include <homelab/delay.h>
 
// Main program
int main(void)
{
	int seconds = 0;
	char text[16];
 
	// Set-up of the LCD
	lcd_gfx_init();
 
	// Cleaning the screen
	lcd_gfx_clear();
 
	// Switching on the background light
	lcd_gfx_backlight(true);	
 
	// Displaying the name of the program
	lcd_gfx_goto_char_xy(1, 1);
	lcd_gfx_write_string("Time counter");
 
	// Endless loop	
	while (true)
	{
		// Converting the seconds to the form of a clock
		// hh:mm:ss
		sprintf(text, "%02d:%02d:%02d",
			(seconds / 3600) % 24,
			(seconds / 60) % 60,
			 seconds % 60);
 
		// Displaying the clock text
		lcd_gfx_goto_char_xy(3, 3);
		lcd_gfx_write_string(text);
 
		// Adding one second
		seconds++;
 
		// Hardware delay for 1000 ms
		hw_delay_ms(1000);
	}
}

1. Combine with LED tasks (traffic light) and show on the LCD screen, which LED is currently active.