Bài giảng Vi xử lý - Chương 5: Thiết kế hệ vi xử lý (Phần 2) - Hồ Trung Mỹ
Chương 5
Thiết kế hệ vi xử lý
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5.6 Giao tiếp bộ hiển thị (Display)
5.6.2 Giao tiếp với LCD
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LCD controller
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LCD Operation
LCD is gaining popular and replacing LEDs
(7-segment …), due to
1. declining price
2. the ability to display numbers, characters,
and graphics
3. relieving the CPU task by incorporating a
refreshing controller
4. ease of programming for characters and
graphics (OLED is the coming display)
4
LCD Pin Descriptions
14-pin LCD module is discussed here, table 12-1 lists pin’s function,
Fig 12-1 shows the pin positions for various LCDs
– Vcc, Vss provide +5V and ground
– Vee is used for contrast controlling
– RS (register select) is used to select the instruction command code
register (RS = 0) or data register (RS = 1)
– LCD command codes is listed at table 12-2
– R/W (read/write) allows user to write to (R/W = 0) or read from
(R/W = 1) information
– E (enable) latch information at data pins; when data is supplied to
data pins, a high-to-low pulse must be applied to this pin
– D0-D7 are the 8-bit data pins; send information to LCD (R/W = 0)
and read contents of LCD internal registers (R/W = 1)
– to display letters and numbers, ASCII codes are sent while RS = 1
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6
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Pin diagrams
– RS = 0, the command code register is selected, we can send instruction
to LCD to perform clear, shift, blink …
– when RS = 0, and R/W = 1, D7 is busy flag, when D7 = 0, LCD is ready
to receive new information; it is recommended to check the busy flag
before writing any data to the LCD
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LCD Interfacing
• Liquid Crystal Displays (LCDs) have become a cheap and
easy way to display text for an embedded system
– Various configurations (1 line by 20 characters upto 8
lines by 80 characters).
• LCD needs a driving circuit to work.
• Driving circuit and LCD are often integrated into a single
chip Hitachi LM015 can display one line of 16 characters
• The display has one register into which commands are sent
and one register into which data to be displayed are sent
• Two registers are differentiated by the RS input
• Data lines (DB7-DB0) are used to transfer both commands
(clearing, cursor positioning, etc) and data (character to be
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displayed)
Alphanumeric LCD Interfacing
Microcontroller
• Pinout
E
communications
bus
– 8 data pins D7:D0
R/W
RS
– RS: Data or Command
Register Select
– R/W: Read or Write
– E: Enable (Latch data)
• RS – Register Select
DB7–DB0
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LCD
controller
LCD Module
– RS = 0 → Command Register
– RS = 1 → Data Register
• R/W = 0 → Write, R/W = 1 → Read
• E – Enable
– Used to latch the data present on the data pins.
• D0 – D7
– Bi-directional data/command pins.
– Alphanumeric characters are sent in ASCII format.
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LCD Commands
• The LCD’s internal controller can accept several
commands and modify the display accordingly.
These commands would be things like:
– Clear screen
– Return home
– Decrement/Increment cursor
• After writing to the LCD, it takes some time for it
to complete its internal operations. During this
time, it will not accept any new commands or
data.
– We need to insert time delay between any two
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commands or data sent to LCD
Interfacing LCD with 8051
8051
LM015
P3.4
P3.5
RW
E
P3.3
RS
P1.7-P1.0
D7-D0
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Interfacing LCD with 8051
Command and Data Write Routines
DATA: MOV P1, A ; A is ascii data
SETB P3.3 ; RS=1 data
CLR P3.4 ; RW=0 for write
SETB P3.5 ; H->L pulse on E
CLR P3.5
In main program:
. . .
MOV A, COMMAND
CALL CMD
CALL DELAY
MOV A, ANOTHER_CMD
CALL CMD
RET
CALL DELAY
CMD: MOV P1, A ; A has the cmd word
CLR P3.3 ; RS=0 for cmd
CLR P3.4 ; RW=0 for write
SETB P3.5 ; H->L pulse on E
CLR P3.5
MOV A, #’A’
CALL DATA
CALL DELAY
MOV A, #’B’
CALL DATA
RET
CALL DELAY ….
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16
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LCD
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LCD Timing
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