LCD Programming - Sending command and Data

►Sending Commands to LCD

To send commands we simply need to select the command register. Everything is same as we have done in the initialization routine. But we will summarize the common steps and put them in a single subroutine. Following are the steps:
• Move data to LCD port
• select command register
• select write operation
• send enable signal
• wait for LCD to process the command

Keeping these steps in mind we can write LCD command routine as.

CODE:
;Ports used are same as the previous example
;Routine to send command to LCD

LCD_command:
mov LCD_data,A ;Move the command to LCD port
clr LCD_rs ;Selected command register
clr LCD_rw ;We are writing in instruction register
setb LCD_en ;Enable H->L
clr LCD_en
acall LCD_busy ;Wait for LCD to process the command
ret ;Return from busy routine

; Usage of the above routine
; A will carry the command for LCD
; e.g. we want to send clear LCD command
;
; mov a,#01H ;01H is command for clearing LCD
; acall LCD_command ;Send the command


The equivalent C code Keil C compiler. Similar code can be written for SDCC.
CODE:
void LCD_command(unsigned char var)
{
LCD_data = var; //Function set: 2 Line, 8-bit, 5x7 dots
LCD_rs = 0; //Selected command register
LCD_rw = 0; //We are writing in instruction register
LCD_en = 1; //Enable H->L
LCD_en = 0;
LCD_busy(); //Wait for LCD to process the command
}
// Using the above function is really simple
// var will carry the command for LCD
// e.g.
//
// LCD_command(0x01);



Setting cursor position on LCD
To set the cursor position on LCD, we need to send the DDRAM address...
CODE:
Bit7 6 5 4 3 2 1 0
1 AD6 AD5 AD4 AD3 AD2 AD1 AD0


The seventh bit is always 1, and bit 0 to 7 are DDRAM address (See the introduction section of LCD). so if you want to put the cursor on first position the address will be '0000000B' in binary and 7th bit is 1. so address will be 0x80, so for DDRAM all address starts from 0x80.

For 2 line and 16 character LCD. The adress from 0x80 to 0x8F are visible on first line and 0xC0 to 0xCF is visible on second line, rest of the DDRAM area is still available but is not visible on the LCD, if you want to check this thing, then simply put a long sting greater than 16 character and shift the entire display, you will see all the missing character coming from the back.. this way you can make scrolling line on LCD (see more on shifting display in commands section).

Below is an example for setting cursor position on LCD.
CODE:
;We are placing the cursor on the 4th position
;so the DDRAM address will be 0x03
;and the command will be 0x80+0x03 = 0x83
mov a,#83H ;load the command
acall LCD_command ;send command to LCD

CODE:
// to do the same thing is C
// as we done before
LCD_command(0x83);



►Sending Data to LCD

To send data we simply need to select the data register. Everything is same as the command routine. Following are the steps:
• Move data to LCD port
• select data register
• select write operation
• send enable signal
• wait for LCD to process the data

Keeping these steps in mind we can write LCD command routine as.

CODE:
;Ports used are same as the previous example
;Routine to send data (single character) to LCD

LCD_senddata:
mov LCD_data,A ;Move the command to LCD port
setb LCD_rs ;Selected data register
clr LCD_rw ;We are writing
setb LCD_en ;Enable H->L
clr LCD_en
acall LCD_busy ;Wait for LCD to process the data
ret ;Return from busy routine

; Usage of the above routine
; A will carry the character to display on LCD
; e.g. we want to print A on LCD
;
; mov a,#'A' ;Ascii value of 'A' will be loaded in accumulator
; acall LCD_senddata ;Send data


The equivalent C code Keil C compiler. Similar code can be written for SDCC.
CODE:
void LCD_senddata(unsigned char var)
{
LCD_data = var; //Function set: 2 Line, 8-bit, 5x7 dots
LCD_rs = 1; //Selected data register
LCD_rw = 0; //We are writing
LCD_en = 1; //Enable H->L
LCD_en = 0;
LCD_busy(); //Wait for LCD to process the command
}
// Using the above function is really simple
// we will pass the character to display as argument to function
// e.g.
//
// LCD_senddata('A');


Now you have seen that its really easy to send command and data to LCD. Now what if we have a string to send to LCD? how we are going to do that?

Is simple, we will store the LCD string in the ROM of controller and call the string character by character. A simple exmple is shown below.

CODE:
;Sending string to LCD Example

LCD_sendstring:
clr a ;clear Accumulator for any previous data
movc a,@a+dptr ;load the first character in accumulator
jz exit ;go to exit if zero
acall lcd_senddata ;send first char
inc dptr ;increment data pointer
sjmp LCD_sendstring ;jump back to send the next character
exit:
ret ;End of routine

; Usage of the above routine
; DPTR(data pointer) will carry the address
; of string to send to LCD.
; e.g. we want to print "LCD Tutorial" on LCD then
;
; mov dptr,#my_string ;my_string is the label where the string is stored
; acall LCD_sendstring ;Send string
;
; To store a string..
; my_string:
; DB "LCD Tutorial", 00H
; 00H indicate that string is finished.


The equivalent C code Keil C compiler. Similar code can be written for SDCC.
CODE:
void LCD_sendstring(unsigned char *var)
{
while(*var) //till string ends
LCD_senddata(*var++); //send characters one by one
}
// Using the above function is really simple
// we will pass the string directly to the function
// e.g.
//
// LCD_sendstring("LCD Tutorial");




Now we are ready with sending data and sending command to LCD. Now the last and final section which is creating custom characters or patterns to display on LCD. Please proceed to the next section to read more.