Alphanumeric
Liquid-Crystal Display
Copyright University of Colorado, 2005
ASEN 4519/5519
Lecture #10
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Announcements
• Lab 3 due at 22:00
– Zip lab2 directory
– Submit via WebCT
Copyright University of Colorado, 2005
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Lecture #10
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Liquid Crystal Display
• Purpose
– Display information
– 8 bit binary numbers are mapped to 256 different characters
– Mapping is done using ASCII table
• American Standard Code for Information Interchange
• Qwikflash board LCD
– LCD is an Optrex 8x2 character display
– LCD controller is a Hitachi HD44780 LCD controller chip
• This is the defacto standard
– Controller and display are mounted on a single board
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ASCII
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Extended ASCII
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Notes on ASCII
• Characters 0x00 to 0x1F are control characters
• The numbers go from 0x30 to 0x39
• Upper case letters run from
– 0x41 (A) to 0x5A (Z)
• Lower case letters run from
– 0x61(a) to 0x7A (z)
• To convert from lower case to upper case letters add
0x20
• To convert from decimal numbers to their ASCII
equivalent add 0x30
• Characters below 128 are standard and above 128
are not standard characters
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Liquid Crystal Display
• Hitachi HD44780 LCD
–
–
–
–
–
–
Can interface in either 8 bit (byte) or 4 bit (nibble) mode
The Qwikflash board uses the 4 bit (nibble) mode
Qwikflash uses 2 control lines (RS & E)
Data is clocked in on falling edge of E
Read/Write (R/W) is grounded disabling reads
RS selects LCD mode
• RS=0, indicates following data are control statements
• RS=1, indicates following data are characters
– Interface has specific timing requirements for data transfer
– User can send both ASCII codes and LCD control codes
– LCD required specific initialization sequence
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4 bit nibble interface
PIC 18F452
HD44780
Not
used
DB0
DB1
DB2
DB3
RD4
RD5
RD6
RD7
DB4
DB5
DB6
DB7
RE0
RE1
RS
E
RS & E are control lines
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Optrex pin assignment
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Optrex write timing requirements I
R/W line is grounded
on Qwikflash board
only write enabled
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Optrex write timing requirements II
Author indicates
this is 40µsec
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Liquid Crystal Display Initialization
1. Wait 0.1 sec after PIC comes out of reset
2. Drive RS low and leave it there because all of the following
bytes are configuration bytes
3. Raise E, send 0x3, drop E, wait
4. Raise E, send 0x3, drop E, wait
5. Raise E, send 0x3, drop E, wait
; Sets 4-bit interface
6. Raise E, send 0x2, drop E, wait
7. Raise E, send 0x2, drop E, wait
; Sets 2 line display
8. Raise E, send 0x8, drop E, wait
9. Raise E, send 0x0, drop E, wait
; Clears the display
10. Raise E, send 0x1, drop E, wait
11. Raise E, send 0x0, drop E, wait
; Turns cursor off and
12. Raise E, send 0xC, drop E, wait
; display on
13. Raise E, send 0x0, drop E, wait
; Increments cursor
14. Raise E, send 0x6, drop E, wait
; automatically
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Point Macro and LCD string
LCDstr db 0x33,0x32,0x28,0x01,0x0c,0x06,0x00 ;Initialization string for LCD
POINT
macro stringname
MOVLF high stringname, TBLPTRH
MOVLF low stringname, TBLPTRL
endm
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Lecture #10
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Liquid Crystal Display Initialization
InitLCD
MOVLF 10,COUNT
REPEAT_
rcall LoopTime
decf COUNT,F
UNTIL_ .Z.
bcf PORTE,0
POINT LCDstr
tblrd*
REPEAT_
bsf PORTE,1
movff TABLAT,PORTD
bcf PORTE,1
rcall LoopTime
bsf PORTE,1
swapf TABLAT,W
movwf PORTD
bcf PORTE,1
rcall LoopTime
tblrd+*
movf TABLAT,F
UNTIL_ .Z.
return
Copyright University of Colorado, 2005
;Wait 0.1 second
;Call LoopTime 10 times
;RS=0 for command
;Set up table pointer to initialization string
;Get first byte from string into TABLAT
;Drive E high
;Send upper nibble
;Drive E low so LCD will process input
;Wait ten milliseconds
;Drive E high
;Swap nibbles
;Send lower nibble
;Drive E low so LCD will process input
;Wait ten milliseconds
;Increment pointer and get next byte
;Is it zero?
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LCD positioning codes
0x80
0x81
0x82
0x83
0x84
0x85
0x86
0x87
0xC0
0xC1
0xC2
0xC3
0xC4
0xC5
0xC6
0xC7
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Displaying a static string
First store a static string in memory to put kHz right justified on the top row of the
LCD
kHz db
“\x85kHz\x00”
\xhh indicates interpret two characters after x as hex values otherwise values are
converted to hex using the ASCII mapping
kHz is interpreted as 856B487A00 in hexadecimal. This is the value stored in
memory.
To display a static string we do the following
POINT
rcall
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kHz
DISPLAYC
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DisplayC subroutine
DisplayC
bcf PORTE,0
tblrd*
movf TABLAT,F
IF_ .Z.
tblrd+*
ENDIF_
REPEAT_
bsf PORTE,1
movff TABLAT,PORTD
bcf PORTE,1
bsf PORTE,1
swapf TABLAT,W
movwf PORTD
bcf PORTE,1
rcall T40
bsf PORTE,0
tblrd+*
movf TABLAT,F
UNTIL_ .Z.
return
Copyright University of Colorado, 2005
;Drive RS pin low for cursor-positioning code
;Get byte from string into TABLAT
;Check for leading zero byte
;If zero, get next byte
;Drive E pin high
;Send upper nibble
;Drive E pin low so LCD will accept nibble
;Drive E pin high again
;Swap nibbles
;Write lower nibble
;Drive E pin low so LCD will process byte
;Wait 40 usec
;Drive RS pin high for displayable characters
;Increment pointer, then get next byte
;Is it zero?
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T40 subroutine
;;;;;;; T40 subroutine ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;
; Pause for 40 microseconds or 40/0.4 = 100 clock cycles.
; Assumes 10/4 = 2.5 MHz internal clock rate.
T40
movlw 100/3
movwf COUNT
REPEAT_
decf COUNT,F
UNTIL_ .Z.
return
Copyright University of Colorado, 2005
;Each REPEAT loop takes 3 cycles
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Homework
MONDAY 03-OCT-05 (LECTURE)
• Read chapter 7 (Alphanumeric LCD)
• Lab #3 due at 22:00 (submit via WebCT)
WEDNESDAY 05-OCT-05 (LECTURE)
• Read and start lab #4
• Look at Hitachi 44780 LCD datasheet (website)
FRIDAY 07-OCT-05 (LAB)
• Lab #4 - LCD
Copyright University of Colorado, 2005
ASEN 4519/5519
Lecture #10
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