LCD Interface on I/O ports

April 1, 2011 — Leave a comment

Most projects you create with the 8051 microcontroller require some form of  display.  The most common way to accomplish this is with the LCD  (Liquid Crystal Display).  LCDs have become a cheap and easy way to get text display for an embedded system Common displays are set up as 16 to 20 characters by 1 to 4 lines.

UNDERSTANDING LCD

 

Pinout


8 data pins D7:D0

Bi-directional data/command pins.
Alphanumeric characters are sent in ASCII format.

RS:  Register Select

RS = 0 -> Command Register is selected
RS = 1 -> Data Register is selected

R/W: Read or Write

0 -> Write,  1 -> Read

E: Enable (Latch data)

Used to latch the data present on the data pins.
A high-to-low edge is needed to latch the data.

VEE : contrast control



NOTE: When writing to the display, data is transferred only on the high to low transition of this signal. However, when reading from the display, data will become available shortly after the low to high transition and remain available until the signal falls low again.

Registers

The HD44780 has two 8-bit registers, an instruction register (IR) and a data register (DR). The IR stores instruction codes. The DR temporarily stores data to be written into DDRAM or CGRAM and temporarily stores data to be read from DDRAM or CGRAM. Data written into the DR is automatically written into DDRAM or CGRAM by an internal operation. . These two registers can be selected by the register selector (RS) signal. See the table below:

Register Selection
RS R/W Operation
0 0 IR write as an internal operation (display clear, etc.)
0 1 Read busy flag (DB7) and address counter (DB0 to DB6)
1 0 DR write as an internal operation (DR to DDRAM or CGRAM)
1 1 DR read as an internal operation (DDRAM or CGRAM to DR)

Busy Flag (BF)

When the busy flag is 1, the LCD  is in the internal operation mode, and the next instruction will not be accepted. When RS = 0 and R/W = 1 (see the table above), the busy flag is output to DB7 (MSB of LCD data bus). The next instruction must be written after ensuring that the busy flag is 0.

LCD Commands

The LCD’s internal controller accept several commands and modify the display accordingly. These commands would be things like:
– Clear screen
– Return home
– Shift display right/left

Instruction Decimal HEX
Function set (8-bit interface, 2 lines, 5*7 Pixels)
56
38
Function set (8-bit interface, 1 line, 5*7 Pixels)
48
30
Function set (4-bit interface, 2 lines, 5*7 Pixels)
40
28
Function set (4-bit interface, 1 line, 5*7 Pixels)
32
20
Entry mode set
See Below
See Below
Scroll display one character right (all lines)
28
1E
Scroll display one character left (all lines)
24
18
Home (move cursor to top/left character position)
2
2
Move cursor one character left
16
10
Move cursor one character right
20
14
Turn on visible underline cursor
14
0E
Turn on visible blinking-block cursor
15
0F
Make cursor invisible
12
0C
Blank the display (without clearing)
8
08
Restore the display (with cursor hidden)
12
0C
Clear Screen
1
01
Set cursor position (DDRAM address)
128 + addr
80+ addr
Set pointer in character-generator RAM (CG RAM address)
64 + addr
40+ addr

INTERFACING  LCD TO 8051

The 44780 standard requires 3 control lines as well as either 4 or 8 I/O lines for the data bus. The user may select whether the LCD is to operate with a 4-bit data bus or an 8-bit data bus. If a 4-bit data bus is used, the LCD will require a total of 7 data lines. If an 8-bit data bus is used, the LCD will require a total of 11 data lines. The three control lines are  EN, RS, and RW. Note that the EN line must be raised/lowered before/after each instruction sent to the LCD regardless of whether that instruction is read or write, text or instruction. In short, you must always manipulate EN when communicating with the LCD. EN is the LCD’s way of knowing that you are talking to it. If you don’t raise/lower EN, the LCD doesn’t know you’re talking to it on the other lines.




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Jagan

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