Introduction:LCD

Liquid Crystal Display(LCDs) provide a cost effective way to put a text output unit for a microcontroller. As we have seen in the previous tutorial, LEDs or 7 Segments do no have the flexibility to display informative messages. This display has 2 lines and can display 16 characters on each line. Nonetheless, when it is interfaced with the micrcontroller, we can scroll the messages with software to display information which is more than 16 characters in length.

LCD Internal Controller

Fig 1: LCD Block diagram

The LCD is a simple device to use but the internal details are complex. Most of the 16x2 LCDs use a Hitachi HD44780 or a compatible controller. Yes, a micrcontroller is present inside a Liquid crystal display as shown in figure 1.

The Display Controller takes commands and data from a external microcontroller and drivers the LCD panel(LCDP). It takes a ASCII value as input and generate a patter for the dot matrix. E.g., to display letter 'A', it takes its value 0X42(hex) or 66(dec) decodes it into a dot matrix of 5x7 as shown in figure 2.

Basic Commands

The LCD controller uses RS and RW lines along with E to operate the LCD.

• Resister Select (RS): Determines weather a command(RS = 0) is sent (to set up the display) or actual data(RS=1) is sent.
• Read/Write RW=0; writes to the LCD. RW=1;Reads from the LCD.

The commonly used instructions are shown in the instruction set below. Observe the Bit names: I/D, S, D, C etc at the bottom of instruction set to decode the instructions completely.

• Clear Display
• Cursor Home
• Set Entry Mode
• Display on/off control
• Cursor/display shift
• Function Set
• Read Busy Flag
• Data Read
• Data Write
  

Objective

fig LCD display

There is lot of stuff that can be done with the LCDs, to start with we will simple display a couple of strings on the 2 lines of the LCD as shown in the image.

Schematic Discription

• Data Lines: In this mode, all of the 8 datalines DB0 to DB7 are connected from the micrcontroller to a LCD module as shown the schematic.
• Control Lines:' The RS, RW and E are control lines, as discussed earlier.
• Power & contrast:Apart from that the LCD should be powered with 5V between PIN 2(VCC) and PIN 1(gnd). PIN 3 is the contrast pin and is output of center terminal of potentiometer(voltage divider) which varies voltage between 0 to 5v to vary the contrast.
• Back-light: The PIN 15 and 16 are used as backlight. The led backlight can be powered through a simple current limiting resistor as we do with normal leds.

Code

As with all the interfaces to simplify thing we have separated code into two files, main.c and lcd_8_bit.c. You may go through the tools setup tutorial on configuring the code.

The main.c is very simple it includes the standard library files. Then it uses several functions from the lcd_8_bit.c file to set up and display messages. As you can see, it makes things very simple when the libraries are well written. We will discuss the implementation while discussing the lcd_8_bit.c file.

#define databus	P2   //	LCD databus connected to PORT2
sbit rs= P0^0;		 // Register select pin connected to P0.0
sbit rw= P0^1;		 // Read Write pin connected to P0.1
sbit en= P0^2;		 // Enable pin connected to P0.2

Let us look at three important functons in lcd_8_bit.c
1. void LCD_CmdWrite( char cmd)
Commands are required to sent to lcd in order to set it up or initialize. The timing diagrams for command write are shown in the figure,

• step1: Send the I/P command to LCD.
• step2: Select the Control Register by making RS low.
• step3: Select Write operation making RW low.
• step4: Send a High-to-Low pulse on Enable PIN with some delay_us.

void LCD_CmdWrite( char cmd)
{
   databus=cmd;     // Send the command to LCD
     rs=0;          // Select the Command Register by pulling RS LOW
     rw=0;          // Select the Write Operation  by pulling RW LOW
     en=1;          // Send a High-to-Low Pusle at Enable Pin
     delay_us(10);
     en=0;
   delay_ms(1);
}

2. void LCD_DataWrite( char dat) This function sends a character to be displayed on LCD in the following steps.

• step1: Send the character to LCD.
• step2: Select the Data Register by making RS high.
• step3: Select Write operation making RW low.
• step4: Send a High-to-Low pulse on Enable PIN with some delay_us.

The timings are similar as above only change is that RS is made high.

void LCD_DataWrite( char dat)
{
   databus=dat;	   // Send the data to LCD
     rs=1;	   // Select the Data Register by pulling RS HIGH
     rw=0;         // Select the Write Operation by pulling RW LOW
     en=1;	   // Send a High-to-Low Pusle at Enable Pin
     delay_us(10);
     en=0;
   delay_ms(1);
}

3. void LCD_Init()
Looking at the instruction set of the LCD controller, we can initialize the LCD with following steps

• Set the display mode as 2 lines, 5 x 7 matrix
• Turn On the dislay, and cursor.
• Clear the LCD
• Get the cursor to first line first position.

The code is listed below.

void LCD_Init()
{
   delay_us(5000);
   LCD_CmdWrite(0x38);   // LCD 2lines, 5*7 matrix
   LCD_CmdWrite(0x0E);	// Display ON cursor ON  Blinking off
   LCD_CmdWrite(0x01);	// Clear the LCD
   LCD_CmdWrite(0x80);	// Cursor to First line First Position
}

The remaining code is listed below. You could observe that the basic functions to read data and write write command are used extensively and entire library is built upon them. There are also other functions that are used to display number etc in the lcd library file. The entire listing can be found here.

Schematic

There are following differences in 4 bit mode.

• Only data lines D4 to D7 are used as shown in the schematic below.
• In code, we need to send the command to select 4 bit mode as shown in the instruction set above.

The main program remains exactly as in 8 bit mode, we simply include the lcd_4_bit.c to work in 4 bit mode. The important and required code is listed below, you find other useful functions and entire library here