Lec10 LCD Interface

8/10/2019 Lec10 LCD Interface

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Interfacing LCD

(Liquid Crystal Display)

Interface a 2-line x 20

character LCD module withthe built-in HD44780controller to I/O ports of the8051 microcontroller


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Converting to ASCII

The LCD can represent characters in ASCII

For example number 08H must be converted to 38H


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Interfacing LCD

Hardware

20 x 2-line LCD displays (twolines with 20 characters perline)

LCD has a display Data RAM(registers) that stores data in8-bit character code.

Each register in Data RAMhasits own address thatcorresponds to its position on

the line. The address rangefor Line 1 is

00 to 13H and Line 2 is 40H to53H.

0x38

0x39

0x00

2x20

0x013

0x38

0x014

0x040

0x053

8

8


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Interfacing LCD

Driver HD77480 Three control signals:

RS Register Select (RA3)

R/W Read/Write (RA2)

E Enable (RA1)

Three power connections Power, ground, and the variable register to control

the brightness


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Interfacing LCD

Driver (HD77480) has two 8-bit internal registers Instruction Register (IR) to write instructions to set up

LCD

Data Register (DR) to write data (ASCII characters)

IR REGISTER

DR REGISTER


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Interfacing LCD

LCD Operation When the MPU writes an instruction to IR or data to DR,

the controller: Sets the data line DB7 high as a flag indicating that the

controller is busy completing the operation

Sets the data line DB7 low after the completion of theoperation

The MPU should always check whether DB7 is low beforesending an instruction or a data byte

After the power up, DB7 cannot be checked for the first

two initialization instructions.


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Interfacing LCD

Writing to or reading from LCD

The MPU: Asserts RSlow to select IR

Reads from LCD by asserting the R/W signal high

Asserts the E signal high and then low (toggles) to latch a data

byte or an instruction

Asserts RShigh to select DR

Writes into LCD by asserting the R/W signal low

Asserts the E signal high and then low (toggles) to latch a databyte or an instruction


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HD44780 Bus Timing

Read timing diagram

Write timing diagram


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Interfacing LCD (Write)

Software To write into the LCD, the program should:

Send the initial instructions (commands) before it cancheck DB7 to set up the LCD in the 4-bit or the 8-bitmode.

Check DB7 and continue to check until it goes low.

Write instructions to IR to set up the LCD parameters

such as the number of display lines and cursorstatus.

Write data to display a message.


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Subroutine to check Busy FLAG of LCD.

CHKBSY:CLR RS ;RS=0SETB RW ;R/W=1(READ)

SETB EN ;E=1MOV C,P1.7CLR EN ;E=0JC CHKBSY

RET


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Subroutine to Write LCD Instruction

INSWR:ACALL CHKBUSYCLR RS ;RS=0CLR RW ;R/W=0SETB EN ;E=1MOV P1,ACLR EN ;E=0RET


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Subroutine to Write LCD Character

DTWR:ACALL CHKBUSYSETB RS ;RS=1CLR RW ;R/W=1SETB ENMOV P1,ACLR ENRET

Lec10 LCD Interface

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