Sending Data To LCD

►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.

Sending Commands to LCD

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);

Checking Busy Flag

►Reading the busy Flag

As discussed in the previous section, there must be some delay which is needed to be there for LCD to successfully process the command or data. So this delay can be made either with a delay loop of specified time more than that of LCD process time or we can read the busy flag, which is recomended. The reason to use busy flag is that delay produced is almost for the exact amount of time for which LCD need to process the time. So is best suited for every application.


Steps to read busy flag

when we send the command, the BF or D7th bit of the LCD becomes 1 and as soon as the command is processed the BF = 0. Following are the steps to be kept in mind while reading the Busy flag.

• Select command register
• Select read operation
• Send enable signal
• Read the flag

So following the above steps we can write the code in assembly as below...

CODE:

;Ports used are same as the previous example

LCD_busy:
         setb   LCD_D7        ;Make D7th bit of LCD data port as i/p
         setb   LCD_en        ;Make port pin as o/p
         clr    LCD_rs        ;Select command register
         setb   LCD_rw        ;we are reading
check:
         clr    LCD_en        ;Enable H->L
         setb   LCD_en
         jb     LCD_D7,check  ;read busy flag again and again till it becomes 0
         ret                  ;Return from busy routine

The equivalent C code Keil C compiler. Similar code can be written for SDCC.

CODE:

void LCD_busy()
{
     LCD_D7   = 1;           //Make D7th bit of LCD as i/p
     LCD_en   = 1;           //Make port pin as o/p
     LCD_rs   = 0;           //Selected command register
     LCD_rw   = 1;           //We are reading
     while(LCD_D7){          //read busy flag again and again till it becomes 0
           LCD_en   = 0;     //Enable H->L
           LCD_en   = 1;
     }
}

The above routine will provide the necessary delay for the instructions to complete. If you dont want to read the busy flag you can simply use a delay routine to provide the a specific ammount of delay. A simple delay routine for the LCD is given below.

CODE:

LCD_busy:
         mov  r7,#50H
back:
         mov  r6,#FFH
         djnz r6,$
         djnz r7,back
         ret                  ;Return from busy routine

CODE:

void LCD_busy()
{
         unsigned char i,j;
         for(i=0;i<50;i++)        //A simple for loop for delay
            for(j=0;j<255;j++);
}

Now we are ready with the initialization routine and the busy routine for LCD. In the next section we will see how to send data and command to the LCD.

Initialization of LCD

►LCD Initialization

Before using the LCD for display purpose, LCD has to be initialized either by the internal reset circuit or sending set of commands to initialize the LCD. It is the user who has to decide whether an LCD has to be initialized by instructions or by internal reset circuit. we will dicuss both ways of initialization one by one.


Initialization by internal Reset Circuit

An internal reset circuit automatically initializes the HD44780U when the power is turned on. The following instructions are executed during the initialization. The busy flag (BF) is kept in the busy state until the initialization ends (BF = 1). The busy state lasts for 10 ms after VCC rises to 4.5 V.

• Display clear
• Function set:
  DL = 1; 8-bit interface data
  N = 0; 1-line display
  F = 0; 5 x 8 dot character font
• Display on/off control:
  D = 0; Display off
  C = 0; Cursor off
  B = 0; Blinking off
• Entry mode set:
  I/D = 1; Increment by 1
  S = 0; No shift

Note: If the electrical characteristics conditions listed under the table Power Supply Conditions Using Internal Reset Circuit are not met, the internal reset circuit will not operate normally and will fail to initialize the HD44780U. For such a case, initial-ization must be performed by the MCU as explained in the section, Initializing by Instruction.

As mentioned in the Note, there are certain condtions that has to be met, if user want to use initialization by internal reset circuit. These conditions are shown in the Table 5 below.


Table 5: Power Supply condition for Internal Reset circuit
Figure 7 shows the test condition which are to be met for internal reset circuit to be active.

Figure 7: Internal Power Supply reset

Now the problem with the internal reset circuit is, it is highly dependent on power supply, to meet this critical power supply conditions is not hard but are difficult to achive when you are making a simple application. So usually the second menthod i.e. Initialization by instruction is used and is recommended most of the time.



Initialization by instructions

Initializing LCD with instructions is really simple. Given below is a flowchart that describles the step to follow, to initialize the LCD.

Figure 8: Flow chart for LCD initialization
As you can see from the flow chart, the LCD is initialized in the following sequence...
1) Send command 0x30 - Using 8-bit interface
2) Delay 20ms
3) Send command 0x30 - 8-bit interface
4) Delay 20ms
5) Send command 0x30 - 8-bit interface
6) Delay 20ms
7) Send Function set - see Table 4 for more information
8) Display Clear command
9) Set entry mode command - explained below

The first 3 commands are usually not required but are recomended when you are using 4-bit interface. So you can program the LCD starting from step 7 when working with 8-bit interface. Function set command depends on what kind of LCD you are using and what kind of interface you are using (see Table 4 in LCD Command section).

LCD Entry mode
From Table 3 in command section, you can see that the two bits decide the entry mode for LCD, these bits are:
a) I/D - Increment/Decrement bit
b) S - Display shift.
With these two bits we get four combinations of entry mode which are 0x04,0x05,0x06,0x07 (see table 3 in LCD Command section). So we get different results with these different entry modes. Normally entry mode 0x06 is used which is No shift and auto incremement. I recommend you to try all the possible entry modes and see the results, I am sure you will be surprised.


Programming example for LCD Initialization

CODE:

LCD_data equ P2    ;LCD Data port
LCD_D7   equ P2.7  ;LCD D7/Busy Flag
LCD_rs   equ P1.0  ;LCD Register Select
LCD_rw   equ P1.1  ;LCD Read/Write
LCD_en   equ P1.2  ;LCD Enable

LCD_init:
         mov   LCD_data,#38H  ;Function set: 2 Line, 8-bit, 5x7 dots
         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
         mov   LCD_data,#0FH  ;Display on, Curson blinking command
         clr   LCD_rs         ;Selected instruction 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
         mov   LCD_data,#01H  ;Clear LCD
         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
         mov   LCD_data,#06H  ;Entry mode, auto increment with no shift
         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 routine

Now we can do the same thing in C, I am giving example using Keil C. Similar code can be written for SDCC.

CODE:

#include <AT89X51.H>.
#define LCD_data P2
#define LCD_D7   P2_7
#define LCD_rs   P1_0
#define LCD_rw   P1_1
#define LCD_en   P1_2

void LCD_init()
{
     LCD_data = 0x38;     //Function set: 2 Line, 8-bit, 5x7 dots
     LCD_rs   = 0;        //Selected command register
     LCD_rw   = 0;        //We are writing in data register
     LCD_en   = 1;        //Enable H->L
     LCD_en   = 0;
     LCD_busy();          //Wait for LCD to process the command
     LCD_data = 0x0F;     //Display on, Curson blinking command
     LCD_rs   = 0;        //Selected command register
     LCD_rw   = 0;        //We are writing in data register
     LCD_en   = 1;        //Enable H->L
     LCD_en   = 0;
     LCD_busy();          //Wait for LCD to process the command
     LCD_data = 0x01;     //Clear LCD
     LCD_rs   = 0;        //Selected command register
     LCD_rw   = 0;        //We are writing in data register
     LCD_en   = 1;        //Enable H->L
     LCD_en   = 0;
     LCD_busy();          //Wait for LCD to process the command
     LCD_data = 0x06;     //Entry mode, auto increment with no shift
     LCD_rs   = 0;        //Selected command register
     LCD_rw   = 0;        //We are writing in data register
     LCD_en   = 1;        //Enable H->L
     LCD_busy();
}

With the help of the above code, you are able to initialize the LCD. Now there is a function/subroutine coming in the code i.e. LCD_busy, which is used to put delay for LCD so that there should not be any command or data sent to the LCD untill it finish executing the command. More on this delay routine is explained in the next section.

LCD COMMANDS

►Commands and Instruction set

Only the instruction register (IR) and the data register (DR) of the LCD can be controlled by the MCU. Before starting the internal operation of the LCD, control information is temporarily stored into these registers to allow interfacing with various MCUs, which operate at different speeds, or various peripheral control devices. The internal operation of the LCD is determined by signals sent from the MCU. These signals, which include register selection signal (RS), read/write signal (R/W), and the data bus (DB0 to DB7), make up the LCD instructions (Table 3). There are four categories of instructions that:

• Designate LCD functions, such as display format, data length, etc.
• Set internal RAM addresses
• Perform data transfer with internal RAM
• Perform miscellaneous functions

Table 3: Command and Instruction set for LCD type HD44780

Although looking at the table you can make your own commands and test them. Below is a breif list of useful commands which are used frequently while working on the LCD.

No.	Instruction	Hex	Decimal
1	Function Set: 8-bit, 1 Line, 5x7 Dots	0x30	48
2	Function Set: 8-bit, 2 Line, 5x7 Dots	0x38	56
3	Function Set: 4-bit, 1 Line, 5x7 Dots	0x20	32
4	Function Set: 4-bit, 2 Line, 5x7 Dots	0x28	40
5	Entry Mode	0x06	6
6	Display off Cursor off (clearing display without clearing DDRAM content)	0x08	8
7	Display on Cursor on	0x0E	14
8	Display on Cursor off	0x0C	12
9	Display on Cursor blinking	0x0F	15
10	Shift entire display left	0x18	24
12	Shift entire display right	0x1C	30
13	Move cursor left by one character	0x10	16
14	Move cursor right by one character	0x14	20
15	Clear Display (also clear DDRAM content)	0x01	1
16	Set DDRAM address or coursor position on display	0x80+add*	128+add*
17	Set CGRAM address or set pointer to CGRAM location	0x40+add**	64+add**

Table 4: Frequently used commands and instructions for LCD
* DDRAM address given in LCD basics section see Figure 2,3,4
** CGRAM address from 0x00 to 0x3F, 0x00 to 0x07 for char1 and so on..

The table above will help you while writing programs for LCD. But after you are done testing with the table 4, i recommend you to use table 3 to get more grip on working with LCD and trying your own commands. In the next section of the tutorial we will see the initialization with some of the coding examples in C as well as assembly.

LCD Tutorial Index

Introduction to LCD Sending command & Data to LCD Basics of LCD Creating custom characters LCD Commands LCD in 4-bit Mode - Introduction Initializing the LCD LCD in 4-bit Mode - Programming Checking Busy Flag of LCD

Basics Of LCD

►DDRAM - Display Data RAM

Display data RAM (DDRAM) stores display data represented in 8-bit character codes. Its extended capacity is 80 X 8 bits, or 80 characters. The area in display data RAM (DDRAM) that is not used for display can be used as general data RAM. So whatever you send on the DDRAM is actually displayed on the LCD. For LCDs like 1x16, only 16 characters are visible, so whatever you write after 16 chars is written in DDRAM but is not visible to the user.

Figures below will show you the DDRAM addresses of 1 Line, 2 Line and 4 Line LCDs.



Figure 2: DDRAM Address for 1 Line LCD


Figure 3: DDRAM Address for 2 Line LCD


Figure 4: DDRAM Address for 4 Line LCD

►CGROM - Character Generator ROM

Now you might be thinking that when you send an ascii value to DDRAM, how the character is displayed on LCD? so the answer is CGROM. The character generator ROM generates 5 x 8 dot or 5 x 10 dot character patterns from 8-bit character codes (see Figure 5 and Figure 6 for more details). It can generate 208 5 x 8 dot character patterns and 32 5 x 10 dot character patterns. Userdefined character patterns are also available by mask-programmed ROM.

Figure 5: LCD characters code map for 5x8 dots


Figure 6: LCD characters code map for 5x10 dots
As you can see in both the code maps, the character code from 0x00 to 0x07 is occupied by the CGRAM characters or the user defined characters. If user want to display the fourth custom character then the code to display it is 0x03 i.e. when user send 0x03 code to the LCD DDRAM then the fourth user created charater or patteren will be displayed on the LCD.


►CGRAM - Character Generator RAM

As clear from the name, CGRAM area is used to create custom characters in LCD. In the character generator RAM, the user can rewrite character patterns by program. For 5 x 8 dots, eight character patterns can be written, and for 5 x 10 dots, four character patterns can be written. Later in this tutorial i will explain how to use CGRAM area to make custom character and also making animations to give nice effects to your application.


►BF - Busy Flag

Busy Flag is an status indicator flag for LCD. When we send a command or data to the LCD for processing, this flag is set (i.e BF =1) and as soon as the instruction is executed successfully this flag is cleared (BF = 0). This is helpful in producing and exact ammount of delay. for the LCD processing.

To read Busy Flag, the condition RS = 0 and R/W = 1 must be met and The MSB of the LCD data bus (D7) act as busy flag. When BF = 1 means LCD is busy and will not accept next command or data and BF = 0 means LCD is ready for the next command or data to process.


►Instruction Register (IR) and Data Register (DR)

There are two 8-bit registers in HD44780 controller Instruction and Data register. Instruction register corresponds to the register where you send commands to LCD e.g LCD shift command, LCD clear, LCD address etc. and Data register is used for storing data which is to be displayed on LCD. when send the enable signal of the LCD is asserted, the data on the pins is latched in to the data register and data is then moved automatically to the DDRAM and hence is displayed on the LCD.
Data Register is not only used for sending data to DDRAM but also for CGRAM, the address where you want to send the data, is decided by the instruction you send to LCD. We will discuss more on LCD instuction set further in this tutorial.

Introduction To LCD

►Introduction

The most commonly used Character based LCDs are based on Hitachi's HD44780 controller or other which are compatible with HD44580. In this tutorial, we will discuss about character based LCDs, their interfacing with various microcontrollers, various interfaces (8-bit/4-bit), programming, special stuff and tricks you can do with these simple looking LCDs which can give a new look to your application.

For Specs and technical information HD44780 controller Click Here

►Pin Description

The most commonly used LCDs found in the market today are 1 Line, 2 Line or 4 Line LCDs which have only 1 controller and support at most of 80 charachers, whereas LCDs supporting more than 80 characters make use of 2 HD44780 controllers.

Most LCDs with 1 controller has 14 Pins and LCDs with 2 controller has 16 Pins (two pins are extra in both for back-light LED connections). Pin description is shown in the table below.



Figure 1: Character LCD type HD44780 Pin diagram

Pin No.	Name	Description
Pin no. 1	VSS	Power supply (GND)
Pin no. 2	VCC	Power supply (+5V)
Pin no. 3	VEE	Contrast adjust
Pin no. 4	RS	0 = Instruction input 1 = Data input
Pin no. 5	R/W	0 = Write to LCD module 1 = Read from LCD module
Pin no. 6	EN	Enable signal
Pin no. 7	D0	Data bus line 0 (LSB)
Pin no. 8	D1	Data bus line 1
Pin no. 9	D2	Data bus line 2
Pin no. 10	D3	Data bus line 3
Pin no. 11	D4	Data bus line 4
Pin no. 12	D5	Data bus line 5
Pin no. 13	D6	Data bus line 6
Pin no. 14	D7	Data bus line 7 (MSB)

Table 1: Character LCD pins with 1 Controller

Pin No.	Name	Description
Pin no. 1	D7	Data bus line 7 (MSB)
Pin no. 2	D6	Data bus line 6
Pin no. 3	D5	Data bus line 5
Pin no. 4	D4	Data bus line 4
Pin no. 5	D3	Data bus line 3
Pin no. 6	D2	Data bus line 2
Pin no. 7	D1	Data bus line 1
Pin no. 8	D0	Data bus line 0 (LSB)
Pin no. 9	EN1	Enable signal for row 0 and 1 (1stcontroller)
Pin no. 10	R/W	0 = Write to LCD module 1 = Read from LCD module
Pin no. 11	RS	0 = Instruction input 1 = Data input
Pin no. 12	VEE	Contrast adjust
Pin no. 13	VSS	Power supply (GND)
Pin no. 14	VCC	Power supply (+5V)
Pin no. 15	EN2	Enable signal for row 2 and 3 (2ndcontroller)
Pin no. 16	NC	Not Connected

Table 2: Character LCD pins with 2 Controller

Usually these days you will find single controller LCD modules are used more in the market. So in the tutorial we will discuss more about the single controller LCD, the operation and everything else is same for the double controller too. Lets take a look at the basic information which is there in every LCD.