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MPASM Tutorial - Liquid Crystal Display (LCD) Print E-mail
( 2 Votes )
Written by P Joyce   
Tuesday, 21 April 2009 11:06

Although single wire serial data communication with your PC is relatively hassle free (especially if coupled with a PICKit 2), sometimes a 'stand alone' solution is required. This is where the Liquid Crystal Display (LCD) comes in very handy.

 

There are a number of reasons why I prefer to use LCD's over segment/matrix displays, a lot of them are straight forward; such as, far less wires, no need to continually 'refresh' display data and much more displayed information in less area. As mentioned, there is no need to continually refresh the screen information as most generic "Character LCD's" have an on-board controller (HD44780) that looks after all of that. You simply send a character to a row/column position, the on-board controller will keep it visible in that location for as long as there is power or you choose to change that information.

 

Consider the wiring diagram (note that the power supply has not been included for simplicity);

 

16F628A_LCD_WIRING_DIAGRAM

The 4.7K potentiometer can be any value upto 10K. You need to adjust it up/down to set the contrast of the LCD. Full contrast will lead to black blocks across the screen, while no contrast will make the screen look as if nothing is on it at all. I find setting up the LCD without the data pins (just power and ground) is a good way to set the contrast - as the on-board controller will fill the first row with rectangles - allowing you to set the contrast correctly (without the chance of trying to set it with a clear screen!)

 

Notice the data pins are connected to RB4:RB7. If you use a different port, then simply wire up the LCD in a similar fashion, and declare the appropriate PORT.Pins for the new connections. (code below).

 

And of course, the code. I'm not going to break down the code overall much, it probably better if you simply copy & paste it into a MPLAB project and simulate it. Step through the code and read the comments - there's quite a few handy little techniques throughout such as defining the PORT.PIN at the start of the program so you only have to change that information for the rest to follow suit);

	LIST	p=16F628A			;tell assembler what chip we are using
	include "P16F628A.inc"			;include the defaults for the chip
	ERRORLEVEL	0,	-302		;suppress bank selection messages
	__config 0x3D18				;sets the configuration settings (osc type etc.)     
 
	LCD_PORT	Equ	PORTB
	LCD_TRIS	Equ	TRISB
	LCD_RS		Equ	0x02		;LCD handshake lines
	LCD_E		Equ	0x03
 
	CBLOCK	0x20
		count				; Counter used when switch pressed has stopped
		count1				; 160us Counter variable
		counta				; variables for delay timers
		countb				; variables for delay timers
		LCDTemp				; 4 bit for LCD
	ENDC					;
 
	org	0x000				; 
	goto	Init			;
 
HEX_Table  	
	addwf   PCL, f
	retlw   0x30
	retlw	0x31
	retlw	0x32
	retlw	0x33
	retlw	0x34
	retlw	0x35
	retlw	0x36
	retlw	0x37
	retlw	0x38
	retlw	0x39
	retlw	0x41
	retlw	0x42
	retlw	0x43
	retlw	0x44
	retlw	0x45
	retlw	0x46
 
Text		
	addwf	PCL, f
	retlw	'H'
	retlw	'e'
	retlw	'l'
	retlw	'l'
	retlw	'o'
	retlw	' '
	retlw	'W'
	retlw	'o'
	retlw	'r'
	retlw	'l'
	retlw	'd'
	retlw	'!'
	retlw	0x00
 
Text2	
	addwf   PCL, f
	retlw   'L'
	retlw   'C'
	retlw   'D'
	retlw   ' '
	retlw   'i'
	retlw   's'
	retlw   ' '
	retlw	'O'
	retlw	'N'
	retlw	'L'
	retlw	'I'
	retlw	'N'
	retlw	'E'
	retlw   0x00
 
; Initialize the PIC and the LCD
Init						;
	movlw	0x07				; Turn comparators off and
	movwf	CMCON				; enable pins for I/O functions
	bsf 	STATUS,	RP0			; select bank 1	
	clrf	PORTA				; Initialize PORTA by setting output data latches
	movlw	b'00000000'			; PortA Outputs
	movwf	TRISA				; All portA pins are inputs
	movlw	b'00000000'			; PortB Outputs
	movwf	TRISB				; Change PortB I/O
	bcf 	STATUS,	RP0			; select bank 0
 
	call	LCDInit				; Initialize the LCD Display	
 
; Main program...
Message	
	movf	count, w			; put counter value in W
	call	Text				; get a character from the text table
	xorlw	0x00				; is it a zero?
	btfsc	STATUS, Z
	goto	NextMessage			; display next message if finished
	call	LCD_Char
	incf	count, f
	goto	Message
 
NextMessage	
	call	LCD_L2				;move to 2nd row, first column
	clrf	count				;set counter register to zero
 
Message2	
	movf	count, w			;put counter value in W
	call	Text2				;get a character from the text table
	xorlw	0x00				;is it a zero?
	btfsc	STATUS, Z
	goto	EndMessage
	call	LCD_Char
	incf	count, f
	goto	Message2
 
EndMessage	
 
; Infinate loop		
Stop
	goto	Stop				;endless loop
 
; LCD routines and subs
LCDInit						;  4 Bit Initialization...
		call	Del05			;  Wait 15 msecs
		call	Del05			;
		call	Del05			;
		movlw	0x030			; Send the Reset Instruction
		movwf	LCD_PORT		;
		call	Pulse_e			; Pulse LCD_E
		call	Del05			; Delay 5ms
		call	Pulse_e			; Pulse LCD_E
		call	D160us			; Delay of 160us
		call	Pulse_e			; Pulse LCD_E
		call	D160us			; Delay of 160us
		movlw	0x020			; Send the Data Length Specification
		movwf	LCD_PORT		;
		call	Pulse_e			; Pulse LCD_E
		call	D160us			; Delay of 160us
		movlw	0x028			; Set Interface Length
		call	LCDIns			;
		movlw	0x010			; Turn Off Display
		call	LCDIns			; 
		movlw	0x001			; Clear Display RAM
		call	LCDIns			;
		movlw	0x006			; Set Cursor Movement
		call	LCDIns			;
		movlw	0x00C			; Turn on Display/Cursor
		call	LCDIns			;
		call	LCD_Clr			; Clear the LCD
		return				;
 
LCDIns						; Send the Instruction to the LCD
		movwf	LCDTemp			; Save the Value
		andlw	0xF0			; Most Significant Nibble first
		movwf	LCD_PORT		;
		bcf	LCD_PORT, LCD_RS	;
		call	Pulse_e			;
		swapf	LCDTemp, w		; Least Significant Nibble Second
		andlw	0xF0			;
		movwf	LCD_PORT		;
		bcf	LCD_PORT, LCD_RS	;
		call	Pulse_e			;
		call	Del01			; wait 1 ms
		movf	LCDTemp, w		;
		andlw	0xFC			; Have to Delay 5 msecs?
		btfsc	STATUS, Z		;
		call	Del01			; 1ms
		return				;
 
LCD_CharD
		addlw	0x30			; add 0x30 to convert to ASCII
LCD_Char					; Send the Character to the LCD
		movwf	LCDTemp			; Save the Value
		andlw	0xF0			; Most Significant Nibble first
		movwf	LCD_PORT		;
		bsf	LCD_PORT, LCD_RS	;	
		call	Pulse_e			;
		swapf	LCDTemp, w		; Least Significant Nibble Second
		andlw	0xF0			;
		movwf	LCD_PORT		;
		bsf	LCD_PORT, LCD_RS	;
		call	Pulse_e			;
		call	Del05			;
		nop				;
		return				;
 
 
LCD_L2:	movlw	0xc0				; move to 2nd row, first column
		call	LCDIns			;
		retlw	0x00			;
 
LCD_Clr	movlw	0x01				; Clear display
		call	LCDIns			;
		retlw	0x00			;
 
Pulse_e						;
		bsf	LCD_PORT, LCD_E		; LCD Enable pulse to write data from PORTB
		nop				; into LCD module.
		bcf	LCD_PORT, LCD_E		; 
		nop				;
		retlw 0x00			;
 
 
; Delay routines...
D160us	
		clrf count1			;	
		bsf	count1, 5		;  Delay 160 usecs
		bsf	count1, 4		;
		decfsz	count1, f		;
		goto $ - 1			;
		return				;
 
Del255	movlw	0xff				; delay 255 mS
		goto	d0			;
Del200	movlw	d'200'				; delay 200mS
		goto	d0			;
Del100	movlw	d'100'				; delay 100mS
		goto	d0			;
Del50	movlw	d'50'				; delay 50mS
		goto	d0			;
Del20	movlw	d'20'				; delay 20mS
		goto	d0			;
Del05	movlw	0x05				; delay 5.000 ms (4 MHz clock)
		goto	d0			;
Del01	movlw	0x01				; delay 1.000 ms (4 MHz clock)
d0		movwf	count1			;
d1		movlw	0xC7			; delay 1mS
		movwf	counta			;
		movlw	0x01			;
		movwf	countb			;
Del_0	decfsz	counta,f			;
		goto	$+2			;
		decfsz	countb,f		;
		goto	Del_0			;
		decfsz	count1,f		;
		goto	d1			;
		retlw	0x00			;
 
		end

 

As there is no real "setup" guide for the 16F628(A) on the digital-diy.com site - I might make one up as I think the little $2 chip is worth the time.

Comments (4)
  • 2009-04-22 03:17:23 | Harold

    The article looks like its being pushed off the page, probably from that large image?

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  • 2009-04-22 04:48:02 | digital-diy

    It was due to additional spaces on some code comments, I corrected it for him :wink:

    +5 for a great tutorial. Clear approach and lots of commenting

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  • 2009-09-30 10:56:19 | Question  - Why the 5ms delay?

    In the LCDIns routine there is logic that ands the LCDTemp with 0xFC and if its zero adds a delay of 1ms.

    Why is this used? I have a DMC20261 LCD and can not find anything in the data sheet about adding an extra delay for certain instructions. Just wondering.

    Thanks.

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  • 2009-10-01 19:53:37 | Random Reader  - Answer?

    Given that 0xFC = 11111100, it looks like it calls the 5ms delay should any command byte be in the following format: 000000xx (Where xx could equal either 1 or 0)

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