Graphic LCD

Necessary knowledge: [HW] User Interface Module, [LIB] Graphic LCD, [LIB] Delay, [PRT] Alphanumeric LCD

Graphical LCD liquid crystal display is a display which allows displaying pictures and text. Its construction is similar to the alphanumerical LCD, with a difference that on the graphic display all pixels are divided as one large matrix. If we are dealing with a monochrome LCD, then a pixel is one square segment. Color displays’ one 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.

The text formed of pixels of a graphic LCD.

Monochrome graphic displays have usually passive matrix, large color displays including computer screens have active matrix. All information concerning the color of the background and the pixels of the graphic LCDs are similar to alphanumerical LCDs. Similar to the alphanumerical displays, graphic displays are also equipped with separate controller, which takes care of receiving information through the communication interface and generates the electrical field for the segments. If for alphanumerical LCD is enough to send indexes of the signs in order to display text, then graphic displays are not capable of generating letters by themselves – all the pictures and text needs to be generated pixel by pixel by the user.

In the Home-Lab set is a 84×48 pixels monochrome graphic LCD. It is the same display as used in Nokia 3310 mobile phones. Philips PCD8544 controller is attached to the display which can be communicated through SPI-like serial interface. The background lighting of the display module is separately controlled. Communicating with the display is not very difficult, but due to the large amount of the functions it is not explained here. Home-Labs library has functions for using it.

The functions of the graphic LCD are similar to the alphanumeric LCD functions. First, the screen must be started with lcd_gfx_init function. After start-up it is advised to clean the screen, more precisely controllers memory with the lcd_gfx_clear function. There is a letter map in side of the library with full Latin alphabet, numbers and with most common signs written. The height of the letter is 7 and the width of the letter is 5 pixels. The gap between each letter is horizontally 6 and vertically 8 pixels, i.e. in total it fits 6 rows and 14 columns of letters. To display a letter or text, first its position must be determined by using function lcd_gfx_goto_char_xy . For displaying s letter is lcd_gfx_write_char function and for displaying text lcd_gfx_write_string function.

The following is an example of time counter. The program counts seconds (approximately), minutes and hours. For converting time to text 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("Aja loendur");
 
	// Endless loop.	
	while (true)
	{
		// Converting the seconds to the form of 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);
	}
}