how to select row and column in LCD display

Question

I want to display letter on specific row and column in 16x2 LCD display with 8051 MCU. For Example:

Display "R" at 2nd column in first row
Display "W" at 3rd column in second row

I use these routines for the LCD:

 #include<reg51.h>

/* Data pins connected to port P1 of 8051 */
#define  Data_Port_Pins                            (P1)
sbit    Register_Select_Pin   = P2^0;           /* Register Pin of LCD connected to Pin 0 of Port P2 */
sbit    Read_Write_Pin        = P2^1;           /* Read/Write Pin of LCD connected to Pin 1 of Port P2 */
sbit    Enable_Pin            = P2^2;           /* EN pin connected to pin 2 of port P2 */
/* Function for creating delay in milliseconds */
void Delay(unsigned int wait)
   {
      volatile unsigned i, j;
      for(i = 0; i < wait; i++)
         for(j = 0; j < 1200; j++);
   }

/* Function to send command instruction to LCD */
void LCD_Command (unsigned char command)
{
    Data_Port_Pins = command;
    Register_Select_Pin =0;
    Read_Write_Pin=0;
    Enable_Pin =1;
    Delay (2);
    Enable_Pin =0;
}
/* Function to send display data to LCD */
void LCD_Data (unsigned char Data)
{
    Data_Port_Pins = Data;
    Register_Select_Pin=1;
    Read_Write_Pin=0;
    Enable_Pin =1;
    Delay(2);
    Enable_Pin =0;
}
/* Function to prepare the LCD  and get it ready */
void LCD_Initialization()
{
    LCD_Command (0x38);
    LCD_Command (0x0e);
    LCD_Command (0x01);
    LCD_Command (0x81);
}

And this is my attempt:

Does it make any sense?

   void LCD_Position( char row, char column)
   {
    unsigned char cmd = 0x80 ;   /* Start address */

   if( row != 0 )               /*If second row selected ...*/
         {
       cmd += 0x40 ;            /*add start address of second row */
     }
         cmd += row & 0x0f ; 
         LCD_Command (cmd);

   }

Answer 1

Refer to the data sheet for the LCD device in question. For the common 1602 type module (which the initialisation sequence shown suggests is what you are using) you set the position for the next data write using the Set DDRAM address instruction.

In 2-line display mode the 1st line starts at address 0x00, and the the 2nd line starts at 0x40.

void LCD_Position( int row, int pos)
{
    LCD_Command( 0x80 |                     // Set DDRAM Address
                 (row == 0) ? 0x00 : 0x40 | // Row selector
                 (pos & 0x0f) ) ;           // Position in row
}

Given (from the data sheet):

The code sets DB7 to 1 (0x80)indicating the Set DDRAM Addresss instruction. The other bits are address bits, but there are more locations in the display RAM than the width of the display, so only 0x00 to 0x0f and 0x40 to 0x4f refer to visible display locations. So if the second row is selected, 0x40 is masked in ((row == 0) ? 0x00 : 0x40), then the character position is masked in ((pos & 0x0f)). Although I have used bit-wise manipulation, the expression could equally be performed arithmetically:

0x80 + (row == 0) ? 0x00 : 0x40 + (pos & 0x0f)

In both cases the & 0x0f ensures the command is not modified and that the character is placed on the display even if the position if out-of-range.

Less succinctly, but perhaps easier to follow:

// Set DDRAM Address command - start of row 0
unsigned char cmd = 0x80 ;

// If second row selected ...
if( row != 0 )  
{
    // ... add start address of second row
    cmd += 0x40 ; 
}

// Add row offset.  Masked to protect other 
// bits from change if row is out of range.
cmd += row & 0x0f ;  

// Write command
LCD_Command( cmd ) ;