Interfacing LCD with LPC 2148

Let us start with basics of LCD. China made JHD 162A 16*2 LCD is widely used and easily available in market. It has total 16 pins which can be divided into three categories.

1.   Power Control pins: 1,2,3,15,16

2.   Control Pins: 4,5,6

3.   Data Pins: 7 to 14

Functions of pins are as follows.

LCD needs to be configured properly. To configure LCD, we must know the command structure which is as given below.

This table must be understood properly for programming.

Well to reduce your efforts, generally required commands are short listed here.

Once you are aware of command structure, let’s move ahead.

LCD can operate in two modes.

1.   Command Mode: In this mode, RS pin is at logic 0 and the data on pin 7-14 represents the command which specifies the actions to be performed like clear LCD, go to home position, blink cursor etc.

2.   Data Mode: In this mode, RS pin is at logic 1 and data op pin 7-14 represents the data to be displayed on LCD.

While operating in data mode, we must keep in mind that the pattern to be displayed on dot matrix is already stored in internal memory of LCD. It is given below.

[Courstey: www.ocfreaks.com]

So to display any character on LCD, we must provide ASCII value of that on data pins and LCD is smart enough to do the rest. To convert

any number into ASCII, we must add 48 decimal value to it. So ASCII of 0 is 48, of 1 is 49 etc.

How many pins LCD needs ??

As provided earlier, LCD has 8 data pins and 3 control pins. So it needs 11 pins  of controller for operation. But by smart thinking, we can reduce it to six. Let’s see how we can do that.

RW pin is for specifying read or write operation. Well if LCD is output device so where we have to read it ? Let’s understand this properly.

LCD is slow compared to Microc-controllers and it needs time to perform the operations. Assume that controller is asking LCD to print data continuously. When LCD will receive command for that, it will start displaying on LCD but if controller will send another data and if LCD has not finished the task of displaying first data, the later data will be lost and will not be displayed on it.

To avoid this, LCD has one method known as busy flag. In this, MSB of data pins ie. Pin 14 is used as busy flag. Once LCD will start performing and actions, it will raise pin 14 high to indicate that it is in busy state. Once the operation is completed, it will bring it down. Controller must check the status of pin 14 before sending further data or command to avoid any loss of data.

Here it is necessary to read pin 14 and RW has to be made high to read. We can save 1 controller pin here. Although LCD is slow, it is able to finish executaion of command within few machine cycles. So after giving any command or data, if we provide delay of few microseconds, LCD will finish its task within that much time. So we don’t have to read status of pin14 and we can permenately ground it and save controller pin.

Another beautiful option available is to use LCD in 4bit mode. Here the data will be sent in two nibbles instead of a complete byte at a time. In this, only higher nibble of data pins has been connected with controller and pinf 7-10 of LCD are in NC state.

In this way, we need only 6 pins (4 bit data bus, RS and E) are required for LCD interafcing.

Once you are clear with basics of LCD, command structure and basic commands, let’s move ahead with hardware connections.

As mentioned earlier, LPC 2148 operates on 3.3V while LCD requires 5V for operation as well as 5V signals on data and control pins. So we need level shifter in between LCD and LPC 2148.

Some of the buffers / level shifters than can be used are : SN74AHC244N , SN74AC241N , CD74AC244E , CD4050B , etc.. out of which CD74AC244E is readily available. The below circuit explains how we can interface LCD with LPC 2148.

Project Source Code

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Let us start with programming now.
Before reading further, you must understand what is MODULAR PROGRAMMING.
 
Functions for LCD are as follows.
Function Name
Description
void Delay(unsigned long b);
Provide delay of few microseconds
void write_command(int cmd);
Give command to LCD
void write_data(int dat);
Display data on LCD
void init_lcd(void);
Initialize LCD
void printlcd(char *CPtr);
Display string on LCD
 
We have declared thess functions in LCD4BIT.C files and defined in LCD4BIT.H file using moddular programming approach.
 
Once above files are created, we can use code to display data on LCD.
1.   Program for LCD.h file
 
#include 
 
void Delay(unsigned long b)
{
  while (--b!=0); 
}
 
void write_command(int cmd)
 {
  IO1CLR  |= 0x00f00000;                    // Clear Data pins
  IO1CLR  |= 0x00040000;                            // RW = 0         
  IO1CLR  |= 0X00020000;                            // RS= 0,
  IO1SET  |= 0x00f00000 & cmd;           //Set Data pins
  IO1SET  |= 0X00080000;                            // Enable = 1                                
  Delay(30000);                                   // Provide Delay
  IO1CLR  |= 0x00080000;                    // Set Enable=0
  }
 
void write_data(int dat)
{
  IO1CLR  |= 0x00f00000;                    // Clear Data pins4-D7 
  IO1CLR  |= 0x00040000;                   // RW= 0                  
  IO1SET  |= 0X00020000;                    //RS= 1
  IO1SET  |= 0x00f00000 & dat;            // Set Data pins
  IO1SET  |= 0X00080000;                    // Enable = 1
  Delay(30000);                                   //Provide Delay
  IO1CLR  |= 0x00080000;                    //Set Enable=0
  }
 
void lcd_data(char dat)
{
  write_data(dat << 16);
  write_data(dat << 20);
}
 
void lcd_command(char cmd)
{
  write_command(cmd << 16);
  write_command(cmd << 20);
}
 
void printlcd(char *CPtr)
{
  while(*CPtr != '�')
 {
  lcd_data(*CPtr);
  CPtr++;
  Delay(20000);
 }
}
 
void init_lcd(void)
 {                                                                              
  IO1DIR    |= 0x00FE0000;   
  Delay(200000) ;
  Delay(100000);
  write_command(0x30 << 16);
  Delay(100000);
  write_command(0x20 << 16);
 
  lcd_command(0x01);                          /* clear display */
  lcd_command(0x06);                          /* auto address inc */
  lcd_command(0x0c);                          /* cursor off */
  lcd_command(0x80);                          /* first location */
  }
 

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Project Source Code

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//Program for main application
 
#include 
#include "lcd4bit.h"
 
int main(void)
 {                                                                              
  init_lcd();
  printlcd("Welcome To");
  lcd_command(0xC0);
  printlcd("Engineers Garage ");
 
  while(1);
 
  }


 
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Circuit Diagrams

4.-Circuit-Diagram_0