A Note on Character LCD Displays

There are many display devices used by the hobbyists. LCD displays are one of the most sophisticated display devices used by them. Once you learn how to interface it, it will be the easiest and very reliable output device used by you! More, for micro controller based project, not every time any debugger can be used. So LCD displays can be used to test the outputs. Obviously, for last possibility, you need to know how to use this stuff pretty well.

Note 1:  We have sub divided this article for easy navigation as shown below:-

1.  Pin Configuration  2. Block Diagram 3. Control and Display Commands  4. LCD Interfacing  5. LCD Initialization

Note 2: If you are interested in reading about working of LCD – we have an excellent article for you – Working of Liquid Crystal Displays (LCD)

                Hitachi has set up a mile stone by its LCD controller IC. All the LCD displays use the same, or any one of the IC s based upon the architecture introduced by Hitachi.

Ok, one minute, all I’m talking about is the character LCD display and not Graphical LCD Display.

 Most of the LCD Displays available in the market are 16X2 (That means, the LCD displays are capable of displaying 2 lines each having 16 Characters a), 20X4 LCD Displays (4 lines, 20 characters). It has 14 pins. It uses 8lines for parallel data plus 3 control signals, 2 connections to power, one more for contrast adjustment and two connections for LED back light. Let us have a look to typical pin configurations:

Pin Configuration table for a 16X2 LCD character display:-

Pin Number	Symbol	Function
1	Vss	Ground Terminal
2	Vcc	Positive Supply
3	Vdd	Contrast adjustment
4	RS	Register Select; 0→Instruction Register, 1→Data Register
5	R/W	Read/write Signal; 1→Read, 0→ Write
6	E	Enable; Falling edge
7	DB0	Bi-directional data bus, data transfer is performed once, thru DB0 to DB7, in the case of interface data length is 8-bits; and twice, through DB4 to DB7 in the case of interface data length is 4-bits. Upper four bits first then lower four bits.
8	DB1
9	DB2
10	DB3
11	DB4
12	DB5
13	DB6
14	DB7
15	LED-(K)	Back light LED cathode terminal
16	LED+(A)	Back Light LED anode terminal

Data/Signals/Execution of LCD

Now that was all about the signals and the hardware. Let us come to data, signals and execution.

LCD accepts two types of signals, one is data, and another is control. These signals are recognized by the LCD module from status of the RS pin. Now data can be read also from the LCD display, by pulling the R/W pin high. As soon as the E pin is pulsed, LCD display reads data at the falling edge of the pulse and executes it, same for the case of transmission.

                LCD display takes a time of 39-43µS to place a character or execute a command. Except for clearing display and to seek cursor to home position it takes 1.53ms to 1.64ms. Any attempt to send any data before this interval may lead to failure to read data or execution of the current data in some devices. Some devices compensate the speed by storing the incoming data to some temporary registers.

                LCD displays have two RAMs, naming DDRAM and CGRAM. DDRAM registers in which position which character in the ASCII chart would be displayed. Each byte of DDRAM represents each unique position on the LCD display. The LCD controller reads the information from the DDRAM and displays it on the LCD screen. CGRAM allows user to define their custom characters. For that purpose, address space for first 16 ASCII characters are reserved for users. After CGRAM has been setup to display characters, user can easily display their custom characters on the LCD screen.

Images of LCD Display:-

Block Diagram of LCD Display:-

Control and display commands

Instruction

Instruction Code

Instruction Code Description

Execution time

RS

R/W

DB7

DB6

DB5

DB4

DB3

DB2

DB1

DB0

Read Data From RAM

1

1

D7

D6

D5

D4

D3

D2

D1

D0

Read data from internal RAM

1.53-1.64ms

Write data to RAM

1

0

D7

D6

D5

D4

D3

D2

D1

D0

Write data into internal RAM (DDRAM/CGRAM)

1.53-1.64ms

Busy flag & Address

0

1

BF

AC6

AC5

AC4

AC3

AC2

AC1

AC0

Busy flag (BF: 1→ LCD Busy) and contents of address counter in bits AC6-AC0.

39 µs

Set DDRAM Address

0

0

1

AC6

AC5

AC4

AC3

AC2

AC1

AC0

Set DDRAM address in address counter.

39 µs

Set CGRAM Address

0

0

0

1

AC5

AC4

AC3

AC2

AC1

AC0

Set CGRAM Address in address counter.

39 µs

Function Set

0

0

0

0

1

DL

N

F

X

X

Set interface data length (DL: 4bit/8bit), Numbers of display line (N: 1-line/2-line) display font type (F:0→ 5×8 dots, F:1→ 5×11 dots)

39 µs

Cursor or Display Shift

0

0

0

0

0

1

S/C

R/L

X

X

Set cursor moving and display shift control bit, and the direction without changing DDRAM data

39 µs

Display & Cursor On/Off

0

0

0

0

0

0

1

D

C

B

Set Display(D),Cursor(C) and cursor blink(b) on/off control

39 µs

Entry Mode Set

0

0

0

0

0

0

0

1

I/D

SH

Assign cursor moving direction and enable shift entire display.

0µs

Return Home

0

0

0

0

0

0

0

0

1

X

Set DDRAM Address to “00H” from AC and return cursor to its original position if shifted.

43µs

Clear Display

0

0

0

0

0

0

0

0

0

1

Write “20H” to DDRAM and set DDRAM Address to “00H” from AC

43µs

 

AC -Address Counter

Outline

Now the instruction can be divided mainly in four kinds

1)      Function set instructions

2)      Address set instructions

3)      Data transfer instructions with internal RAM

4)      Others

Details of the Instructions

1)      Read Data from RAM

RS	R/W	DB7	DB6	DB5	DB4	DB3	DB2	DB1	DB0
1	1	D7	D6	D5	D4	D3	D2	D1	D0

Read 8bit binary data from DDRAM/CGRAM

The selection of RAM is set by the previous address set instruction. If the address set instruction of RAM is not performed before this instruction, the data that is read first is invalid, because the direction of AC is not determined. If the RAM data is read several times without RAM address set instruction before read operation, the correct RAM data from the second, but the first data would be incorrect, as there is no time to transfer RAM data. In case of DDRAM read operation, cursor shift instruction plays the same role as DDRAM address set instruction; it also transfers RAM data to the output data registers.

After read operation, the data address counter is automatically increased or decreased by 1 according to the entry mode. After CGRAM read operation, display shift may not be executed properly.

*In case of RAM write operation, AC is increased or decreased by 1 like that of the read operation. In this time AC indicates the next address position, but the previous data can only by the read instruction.

2)      Write data to ram

RS	R/W	DB7	DB6	DB5	DB4	DB3	DB2	DB1	DB0
1	0	D7	D6	D5	D4	D3	D2	D1	D0

Write binary 8bit data to DDRAM/CGRAM. The selection of CGRAM or DRAM is set by the previous address set instruction; DDRAM address set, CGRAM address set. RAM set instruction can also determine the AC direction to RAM.

After write operation, the address is automatically increased or decreased by 1 according to the entry mode.

3)      Read Busy Flag and Address

RS	R/W	DB7	DB6	DB5	DB4	DB3	DB2	DB1	DB0
0	1	BF	AC6	AC5	AC4	AC3	AC2	AC1	AC0

By making this read out operation, it can be determined if the LCD is performing some internal operation or not. If Busy Flag (BF) is high, some internal operation is going inside the LCD at that particular moment. To perform further operation the data source (e.g. micro controller) must wait for the BF to go low. Here, the address counter value can also be read.

4)      Set DDRAM Address

RS	R/W	DB7	DB6	DB5	DB4	DB3	DB2	DB1	DB0
0	0	1	AC6	AC5	AC4	AC3	AC2	AC1	AC0

Set DDRAM address to AC, this instruction makes DDRAM data available from MPU. In 1-line display mode, DDRAM address rangers from “00H” to “4FH”. In 2-line display mode, DDRAM address in the first line ranges from “00H” to “27H”, and DDRAM address in the 2^nd line is from “40H” to “67H”.

5)      Set CGRAM address

RS	R/W	DB7	DB6	DB5	DB4	DB3	DB2	DB1	DB0
0	0	0	1	AC5	AC4	AC3	AC2	AC1	AC0

Set CGRAM address to AC. This instruction makes CGRAM data available from MPU.

6)      Function Set

RS	R/W	DB7	DB6	DB5	DB4	DB3	DB2	DB1	DB0
0	0	0	0	1	DL	N	F	X	X

DL: Interface data length control bit

DL=’1’ means 8bit mode of data transfer.

DL=’0’ means 4bit mode of data transfer

When 4 bit mode is activated, the data needs to be transferred in two parts, first higher 4bits, and then lower 4 bits.

N: display line number control bit

N=’1’ will allows to characters to display in 2-lines

N=’0’ will allows to characters to display in the first line only

F: display font control bit

F=’0’ will use 5×8 dots format display mode

F=’1’ will use 5×11 dots format display mode

7)      Cursor or display Shift

RS	R/W	DB7	DB6	DB5	DB4	DB3	DB2	DB1	DB0
0	0	0	0	0	1	S/C	R/L	X	X

Without writing or reading the display data, shifting right/left cursor position or display.

This instruction is made to correct or search or display data. During 2-line display mode, cursor moves to the 2^nd line after the 40^th digit of the 1^st line.

When displayed data is shifted repeatedly, each line shifts individually.

When display shift is performed, the contents of the address counter are not changed.

8)      Display On/Off Control

RS	R/W	DB7	DB6	DB5	DB4	DB3	DB2	DB1	DB0
0	0	0	0	0	0	1	D	C	B

This instruction controls Display, Cursor and cursor blink.

D: Display On/Off control bit

D=’1’ means entire display is turned on

D=’0’ means entire display is turned off. But Display data remains in DDRAM.

C: cursor On/Off control bit

C=’1’ turns on the cursor

C=’0’ turns off the cursor. But I/D register retains the data

B: Cursor blink On/Off control bit

B=’1’ makes cursor blink periodically.

B=’0’ stops the cursor to blink and cursor looks steady if the Cursor is turned on.

 

9)      Entry Mode Set

RS	R/W	DB7	DB6	DB5	DB4	DB3	DB2	DB1	DB0
0	0	0	0	0	0	0	1	I/D	SH

This instruction sets the moving direction of cursor and display.

When I/D= ’1’ cursor moves to the right and DDRAM address is increased by 1.

When I/D= ’0’ cursor moves to the left and DDRAM address is decreased by 1.

CGRAM operates in the same way in this setting.

10)   Return Home

RS	R/W	DB7	DB6	DB5	DB4	DB3	DB2	DB1	DB0
0	0	0	0	0	0	0	0	1	X

This instruction sets the address counter to ‘00H’, and returns the cursor to the first column of first line. And if display is shifted previously, this instruction shifts this too. The DDRAM contents don’t change in this instruction.

11)   Clear display

RS	R/W	DB7	DB6	DB5	DB4	DB3	DB2	DB1	DB0
0	0	0	0	0	0	0	0	0	1

Clear all the display data by writing “20H” (ASCII code of ‘space’ character) to all DDRAM address, AND set value DDRAM address counter (AC) to “00H”. It returns the cursor to the first column of first line and sets the entry mode to increment mode (I/D=’1’).

8-bit and 4-bit interfacing of LCD

Now the question is how to display data in the LCD or give command to it. There is two modes of data transfer are supported by LCD displays. One is 4bit mode, another is 8 bit mode. To transfer data In 8 bit mode, first put your data in the 8bit bus, then put command in the command bus and then pulse the enable signal.

                To send data in 4bit mode; first put upper 4bit in the 4 bit data bus connected to 4MSB pins of LCD display, then put control signals in the control bus, then pulse the E pin once. Next put the lower 4 bit in the data bus and pulse the E pin again. Here is a flowchart simply describing it.

LCD Display Interfacing – Flowchart:-

LCD Initialisation

We are pretty familiar how to send data. But before displaying characters on the LCD display, it must be configured first. To configure an LCD display, four command words must be sent to LCD in either 4 bit mode, or in 8 bit mode. The commands are:

1. Function set

2. Display On/Off control

3. Entry mode set

4. Display Clear

Here is a flow chart of the initialization sequence of LCD display.

Displaying Custom Characters

All these character display modules got the feature to create 8 user defined characters (ASCII Codes 0…7) in addition to the ROM fixed codes.

1.) The command “CG RAM Address Set” defines the ASCII code (Bit 3, 4, 5) and the dot line (Bit 0, 1, 2) of the new character. Example demonstrates creating ASCII code “00H”.

2.) Doing 8 times the write command “Data Write” defines line by line the new character. 8th byte stands for the cursor line.

3.) The new defined character can be used as a “normal” ASCII code (0…7); use with “DD RAM Address Set” and “Data Write”.

Now let us look up the character set that can be displayed using the LCD Displayed

	0x00	0x10	0x20	0x30	0x40	0x50	0x60	0x70	0x80 to 0xd0	0xE0	0xF0
0x00	CG1			0	@	P	`	p	CUSTOM REGIONAL CHARACHTERS	Α
0x01	CG2		!	1	A	Q	a	q		Ä
0x02	CG3		“	2	B	R	b	r		Β	θ
0x03	CG4		#	3	C	S	c	s		Ε
0x04	CG5		$	4	D	T	d	t		Μ	Ω
0x05	CG6		%	5	E	U	e	u		Σ	ϋ
0x06	CG7		&	6	F	V	f	v		Ρ	Σ
0x07	CG8			7	G	W	g	w		G	π
0x08	CG1		(	8	H	X	h	x		√
0x09	CG2		)	9	I	Y	i	y		¯¹	y
0x0A	CG3		*	:	J	Z	j	z		J
0x0B	CG4		+	;	K	[	k	{
0x0C	CG5		,	<	L	¥	l	|
0x0D	CG6		–	=	M	]	m	}
0x0E	CG7		.	>	N	^	n	→
0x0F	CG8		/	?	O	_	o	←			█

CG→ CGRAM User Defined Character

Custom Regional characters vary from manufacturer to manufacturer. The Greek letters may or may not appear. It too depends upon the design.