LCD Programming Example using Assembly Language

Four-bit interface using the busy flag.

If you really want to use the busy flag in your application then this is the version to use. This version allows you to use any available I/O pin for any of the seven signals.

The source code is here.
A similar program written in 'C' is here.
A similar program written for the Arduino IDE is here.
The list of all LCD programming examples is here.

; *************************************************************************** ; LCD-AVR-4f.asm - Use an HD44780U based LCD with an Atmel ATmega processor ; ; Copyright (C) 2013 Donald Weiman (weimandn@alfredstate.edu) ; ; This program is free software: you can redistribute it and/or modify ; it under the terms of the GNU General Public License as published by ; the Free Software Foundation, either version 3 of the License, or ; (at your option) any later version. ; ; This program is distributed in the hope that it will be useful, ; but WITHOUT ANY WARRANTY; without even the implied warranty of ; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the ; GNU General Public License for more details. ; ; You should have received a copy of the GNU General Public License ; along with this program. If not, see <http://www.gnu.org/licenses/>. ; ; *************************************************************************** ; File: LCD-AVR-4f.asm ; Date: September 9, 2013 ; ; Target: ATmega328 ; Assembler: Atmel AvrAssembler2 (AVR Studio 6) ; Author: Donald Weiman ; ; Summary: 4-bit data interface, busy flag implemented. ; Any LCD pin can be connected to any available I/O port. ; Includes a simple write string routine. ; ; ****************************** Program Notes ****************************** ; ; This program uses a 4-bit data interface and it uses the busy ; flag to determine when the LCD controller is ready. The LCD ; RW line (pin 5) must therefore be connected to the uP. ; ; The use of the busy flag does not mean that all of the software ; time delays have been eliminated. There are still several ; required in the LCD initialization routine where the busy flag ; cannot yet be used. These delays are have been implemented at ; least twice as long as called for in the data sheet to ; accommodate some of the out of spec displays that may show up. ; There are also some other software time delays required to ; implement timing requirements such as setup and hold times for ; the various control signals. ; ; *************************************************************************** ; ; The four data lines as well as the three control lines may be ; implemented on any available pin of any port. These are the ; connections used for this program: ; ; ----------- ---------- ; | ATmega328 | | LCD | ; | | | | ; | PD7|---------------->|D7 | ; | PD6|---------------->|D6 | ; | PD5|---------------->|D5 | ; | PD4|---------------->|D4 | ; | | |D3 | ; | | |D2 | ; | | |D1 | ; | | |D0 | ; | | | | ; | PB1|---------------->|E | ; | PB2|---------------->|RW | ; | PB0|---------------->|RS | ; ----------- ---------- ; ; *************************************************************************** /* ; Studio 6 handles this by default ;.nolist ;.include "m328def.inc" ;.list */ .equ fclk = 16000000 ; system clock frequency (for delays) ; register usage .def temp = R16 ; temporary storage ; LCD interface (should agree with the diagram above) .equ lcd_D7_port = PORTD ; lcd D7 connection .equ lcd_D7_bit = PORTD7 .equ lcd_D7_ddr = DDRD .equ lcd_D7_pin = PIND ; busy flag .equ lcd_D6_port = PORTD ; lcd D6 connection .equ lcd_D6_bit = PORTD6 .equ lcd_D6_ddr = DDRD .equ lcd_D5_port = PORTD ; lcd D5 connection .equ lcd_D5_bit = PORTD5 .equ lcd_D5_ddr = DDRD .equ lcd_D4_port = PORTD ; lcd D4 connection .equ lcd_D4_bit = PORTD4 .equ lcd_D4_ddr = DDRD .equ lcd_E_port = PORTB ; lcd Enable pin .equ lcd_E_bit = PORTB1 .equ lcd_E_ddr = DDRB .equ lcd_RS_port = PORTB ; lcd Register Select pin .equ lcd_RS_bit = PORTB0 .equ lcd_RS_ddr = DDRB .equ lcd_RW_port = PORTB ; lcd Read Write pin .equ lcd_RW_bit = PORTB2 .equ lcd_RW_ddr = DDRB ; LCD module information .equ lcd_LineOne = 0x00 ; start of line 1 .equ lcd_LineTwo = 0x40 ; start of line 2 ;.equ lcd_LineThree = 0x14 ; start of line 3 (20x4) ;.equ lcd_lineFour = 0x54 ; start of line 4 (20x4) ;.equ lcd_LineThree = 0x10 ; start of line 3 (16x4) ;.equ lcd_lineFour = 0x50 ; start of line 4 (16x4) ; LCD instructions .equ lcd_Clear = 0b00000001 ; replace all characters with ASCII 'space' .equ lcd_Home = 0b00000010 ; return cursor to first position on first line .equ lcd_EntryMode = 0b00000110 ; shift cursor from left to right on read/write .equ lcd_DisplayOff = 0b00001000 ; turn display off .equ lcd_DisplayOn = 0b00001100 ; display on, cursor off, don't blink character .equ lcd_FunctionReset = 0b00110000 ; reset the LCD .equ lcd_FunctionSet4bit = 0b00101000 ; 4-bit data, 2-line display, 5 x 7 font .equ lcd_SetCursor = 0b10000000 ; set cursor position ; ****************************** Reset Vector ******************************* .org 0x0000 jmp start ; jump over Interrupt Vectors, Program ID etc. ;******************************* Program ID ********************************* .org INT_VECTORS_SIZE program_author: .db "Donald Weiman",0 program_version: .db "LCD-AVR-4f (asm)",0,0 program_date: .db "Sep 9, 2013",0 ; ****************************** Main Program Code ************************** start: ; initialize the stack pointer to the highest RAM address ldi temp,low(RAMEND) out SPL,temp ldi temp,high(RAMEND) out SPH,temp ; configure the microprocessor pins for the data lines sbi lcd_D7_ddr, lcd_D7_bit ; 4 data lines - output sbi lcd_D6_ddr, lcd_D6_bit sbi lcd_D5_ddr, lcd_D5_bit sbi lcd_D4_ddr, lcd_D4_bit ; configure the microprocessor pins for the control lines sbi lcd_E_ddr, lcd_E_bit ; E line - output sbi lcd_RS_ddr, lcd_RS_bit ; RS line - output sbi lcd_RW_ddr, lcd_RW_bit ; RW line - output ; initialize the LCD controller as determined by the equates (LCD instructions) call lcd_init_4f ; initialize the LCD display for a 4-bit interface ; display the first line of information ldi ZH, high(program_author) ; point to the information that is to be displayed ldi ZL, low(program_author) ldi temp, lcd_LineOne ; point to where the information should be displayed call lcd_write_string_4f ; display the second line of information ldi ZH, high(program_version) ; point to the information that is to be displayed ldi ZL, low(program_version) ldi temp, lcd_LineTwo ; point to where the information should be displayed call lcd_write_string_4f ; endless loop here: rjmp here ; ****************************** End of Main Program Code ******************* ; ============================== 4-bit LCD Subroutines ====================== ; Name: lcd_init_4f ; Purpose: initialize the LCD module for a 4-bit data interface ; Entry: equates (LCD instructions) set up for the desired operation ; Exit: no parameters ; Notes: uses the busy flag instead of time delays when possible lcd_init_4f: ; Power-up delay ldi temp, 100 ; initial 40 mSec delay call delayTx1mS ; IMPORTANT - At this point the LCD module is in the 8-bit mode and it is expecting to receive ; 8 bits of data, one bit on each of its 8 data lines, each time the 'E' line is pulsed. ; ; Since the LCD module is wired for the 4-bit mode, only the upper four data lines are connected to ; the microprocessor and the lower four data lines are typically left open. Therefore, when ; the 'E' line is pulsed, the LCD controller will read whatever data has been set up on the upper ; four data lines and the lower four data lines will be high (due to internal pull-up circuitry). ; ; Fortunately the 'FunctionReset' instruction does not care about what is on the lower four bits so ; this instruction can be sent on just the four available data lines and it will be interpreted ; properly by the LCD controller. The 'lcd_write_4' subroutine will accomplish this if the ; control lines have previously been configured properly. ; Set up the RS, E, and RW lines for the 'lcd_write_4' subroutine. cbi lcd_RS_port, lcd_RS_bit ; select the Instruction Register (RS low) cbi lcd_E_port, lcd_E_bit ; make sure E is initially low cbi lcd_RW_port, lcd_RW_bit ; write to LCD module (RW low) ; Reset the LCD controller. ldi temp, lcd_FunctionReset ; first part of reset sequence call lcd_write_4 ldi temp, 10 ; 4.1 mS delay (min) call delayTx1mS ldi temp, lcd_FunctionReset ; second part of reset sequence call lcd_write_4 ldi temp, 200 ; 100 uS delay (min) call delayTx1uS ldi temp, lcd_FunctionReset ; third part of reset sequence call lcd_write_4 ldi temp, 200 ; this delay is omitted in the data sheet call delayTx1uS ; Preliminary Function Set instruction - used only to set the 4-bit mode. ; The number of lines or the font cannot be set at this time since the controller is still in the ; 8-bit mode, but the data transfer mode can be changed since this parameter is determined by one ; of the upper four bits of the instruction. ldi temp, lcd_FunctionSet4bit ; set 4-bit mode call lcd_write_4 ; ldi temp, 80 ; 40 uS delay (min) ; call delayTx1uS ; --> from this point on the busy flag is available <-- call lcd_check_BF_4 ; make sure LCD controller is ready ; Function Set instruction ldi temp, lcd_FunctionSet4bit ; set mode, lines, and font call lcd_check_BF_4 ; make sure LCD controller is ready call lcd_write_instruction_4f ; The next three instructions are specified in the data sheet as part of the initialization routine, ; so it is a good idea (but probably not necessary) to do them just as specified and then redo them ; later if the application requires a different configuration. ; Display On/Off Control instruction ldi temp, lcd_DisplayOff ; turn display OFF call lcd_check_BF_4 ; make sure LCD controller is ready call lcd_write_instruction_4f ; Clear Display instruction ldi temp, lcd_Clear ; clear display RAM call lcd_check_BF_4 ; make sure LCD controller is ready call lcd_write_instruction_4f ; Entry Mode Set instruction ldi temp, lcd_EntryMode ; set desired shift characteristics call lcd_check_BF_4 ; make sure LCD controller is ready call lcd_write_instruction_4f ; This is the end of the LCD controller initialization as specified in the data sheet, but the display ; has been left in the OFF condition. This is a good time to turn the display back ON. ; Display On/Off Control instruction ldi temp, lcd_DisplayOn ; turn the display ON call lcd_check_BF_4 ; make sure LCD controller is ready call lcd_write_instruction_4f ret ; --------------------------------------------------------------------------- ; Name: lcd_write_string_4f ; Purpose: display a string of characters on the LCD ; Entry: ZH and ZL pointing to the start of the string ; (temp) contains the desired DDRAM address at which to start the display ; Exit: no parameters ; Notes: the string must end with a null (0) ; uses the busy flag instead of time delays when possible lcd_write_string_4f: ; preserve registers push ZH ; preserve pointer registers push ZL ; fix up the pointers for use with the 'lpm' instruction lsl ZL ; shift the pointer one bit left for the lpm instruction rol ZH ; set up the initial DDRAM address ori temp, lcd_SetCursor ; convert the plain address to an address instruction call lcd_check_BF_4 ; make sure LCD controller is ready call lcd_write_instruction_4f ; set up the first DDRAM address ; write the string of characters lcd_write_string_4f_01: lpm temp, Z+ ; get a character cpi temp, 0 ; check for end of string breq lcd_write_string_4f_02 ; done ; arrive here if this is a valid character call lcd_check_BF_4 ; make sure LCD controller is ready call lcd_write_character_4f ; display the character rjmp lcd_write_string_4f_01 ; not done, send another character ; arrive here when all characters in the message have been sent to the LCD module lcd_write_string_4f_02: pop ZL ; restore pointer registers pop ZH ret ; --------------------------------------------------------------------------- ; Name: lcd_write_character_4f ; Purpose: send a byte of information to the LCD data register ; Entry: (temp) contains the data byte ; Exit: no parameters ; Notes: configures RW (busy flag is implemented) lcd_write_character_4f: cbi lcd_RW_port, lcd_RW_bit ; write to LCD module (RW low) sbi lcd_RS_port, lcd_RS_bit ; select the Data Register (RS high) cbi lcd_E_port, lcd_E_bit ; make sure E is initially low call lcd_write_4 ; write the upper 4-bits of the data swap temp ; swap high and low nibbles call lcd_write_4 ; write the lower 4-bits of the data ret ; --------------------------------------------------------------------------- ; Name: lcd_write_instruction_4f ; Purpose: send a byte of information to the LCD instruction register ; Entry: (temp) contains the data byte ; Exit: no parameters ; Notes: configures RW (busy flag is implemented) lcd_write_instruction_4f: cbi lcd_RW_port, lcd_RW_bit ; write to LCD module (RW low) cbi lcd_RS_port, lcd_RS_bit ; select the Instruction Register (RS low) cbi lcd_E_port, lcd_E_bit ; make sure E is initially low call lcd_write_4 ; write the upper 4-bits of the instruction swap temp ; swap high and low nibbles call lcd_write_4 ; write the lower 4-bits of the instruction ret ; --------------------------------------------------------------------------- ; Name: lcd_write_4 ; Purpose: send a nibble (4-bits) of information to the LCD module ; Entry: (temp) contains a byte of data with the desired 4-bits in the upper nibble ; RS is configured for the desired LCD register ; E is low ; RW is low ; Exit: no parameters ; Notes: use either time delays or the busy flag lcd_write_4: ; set up D7 sbi lcd_D7_port, lcd_D7_bit ; assume that the data is '1' sbrs temp, 7 ; check the actual data value cbi lcd_D7_port, lcd_D7_bit ; arrive here only if the data was actually '0' ; set up D6 sbi lcd_D6_port, lcd_D6_bit ; repeat for each data bit sbrs temp, 6 cbi lcd_D6_port, lcd_D6_bit ; set up D5 sbi lcd_D5_port, lcd_D5_bit sbrs temp, 5 cbi lcd_D5_port, lcd_D5_bit ; set up D4 sbi lcd_D4_port, lcd_D4_bit sbrs temp, 4 cbi lcd_D4_port, lcd_D4_bit ; write the data ; 'Address set-up time' (40 nS) sbi lcd_E_port, lcd_E_bit ; Enable pin high call delay1uS ; implement 'Data set-up time' (80 nS) and 'Enable pulse width' (230 nS) cbi lcd_E_port, lcd_E_bit ; Enable pin low call delay1uS ; implement 'Data hold time' (10 nS) and 'Enable cycle time' (500 nS) ret ; --------------------------------------------------------------------------- ; Name: lcd_check_BF_4 ; Purpose: check busy flag, wait until LCD is ready ; Entry: no parameters ; Exit: no parameters ; Notes: main program will hang if LCD module is defective or missing ; data is read while 'E' is high ; both nibbles must be read even though desired information is only in the high nibble lcd_check_BF_4: push temp ; preserve register cbi lcd_D7_ddr, lcd_D7_bit ; set D7 data direction to input cbi lcd_RS_port, lcd_RS_bit ; select the Instruction Register (RS low) sbi lcd_RW_port, lcd_RW_bit ; read from LCD module (RW high) in temp, SREG ; save the status register push temp sec ; set up the carry flag to 'mirror' the busy flag status ; get high nibble lcd_check_BF_4bit_01: sbi lcd_E_port, lcd_E_bit ; Enable pin high call delay1uS ; implement 'Delay data time' (160 nS) and 'Enable pulse width' (230 nS) sbis lcd_D7_pin, lcd_D7_bit ; check busy flag while 'E' is high ; arrive here if busy flag is clear (LCD is ready for next instruction) clc ; clear our busy flag 'mirror' to indicate that we are done cbi lcd_E_port, lcd_E_bit ; Enable pin low call delay1uS ; implement 'Address hold time' (10 nS), 'Data hold time' (10 nS), and 'Enable cycle time' (500 nS ) ; get (and ignore) the lower nibble sbi lcd_E_port, lcd_E_bit ; Enable pin high call delay1uS ; implement 'Delay data time' (160 nS) and 'Enable pulse width' (230 nS) cbi lcd_E_port, lcd_E_bit ; Enable pin low call delay1uS ; implement 'Address hold time (10 nS), 'Data hold time' (10 nS), and 'Enable cycle time' (500 nS ) ; check our busy flag 'mirror' brcs lcd_check_BF_4bit_01 ; check again if busy flag was high ; arrive here if busy flag is clear - clean up and return pop temp ; restore status register out SREG, temp cbi lcd_RW_port, lcd_RW_bit ; write to LCD module (RW low) sbi lcd_D7_ddr, lcd_D7_bit ; reset D7 data direction to output pop temp ; restore register ret ; ============================== End of 4-bit LCD Subroutines =============== ; ============================== Time Delay Subroutines ===================== ; Name: delayYx1mS ; Purpose: provide a delay of (YH:YL) x 1 mS ; Entry: (YH:YL) = delay data ; Exit: no parameters ; Notes: the 16-bit register provides for a delay of up to 65.535 Seconds ; requires delay1mS delayYx1mS: call delay1mS ; delay for 1 mS sbiw YH:YL, 1 ; update the the delay counter brne delayYx1mS ; counter is not zero ; arrive here when delay counter is zero (total delay period is finished) ret ; --------------------------------------------------------------------------- ; Name: delayTx1mS ; Purpose: provide a delay of (temp) x 1 mS ; Entry: (temp) = delay data ; Exit: no parameters ; Notes: the 8-bit register provides for a delay of up to 255 mS ; requires delay1mS delayTx1mS: call delay1mS ; delay for 1 mS dec temp ; update the delay counter brne delayTx1mS ; counter is not zero ; arrive here when delay counter is zero (total delay period is finished) ret ; --------------------------------------------------------------------------- ; Name: delay1mS ; Purpose: provide a delay of 1 mS ; Entry: no parameters ; Exit: no parameters ; Notes: chews up fclk/1000 clock cycles (including the 'call') delay1mS: push YL ; [2] preserve registers push YH ; [2] ldi YL, low (((fclk/1000)-18)/4) ; [1] delay counter ldi YH, high(((fclk/1000)-18)/4) ; [1] delay1mS_01: sbiw YH:YL, 1 ; [2] update the the delay counter brne delay1mS_01 ; [2] delay counter is not zero ; arrive here when delay counter is zero pop YH ; [2] restore registers pop YL ; [2] ret ; [4] ; --------------------------------------------------------------------------- ; Name: delayTx1uS ; Purpose: provide a delay of (temp) x 1 uS with a 16 MHz clock frequency ; Entry: (temp) = delay data ; Exit: no parameters ; Notes: the 8-bit register provides for a delay of up to 255 uS ; requires delay1uS delayTx1uS: call delay1uS ; delay for 1 uS dec temp ; decrement the delay counter brne delayTx1uS ; counter is not zero ; arrive here when delay counter is zero (total delay period is finished) ret ; --------------------------------------------------------------------------- ; Name: delay1uS ; Purpose: provide a delay of 1 uS with a 16 MHz clock frequency ; Entry: no parameters ; Exit: no parameters ; Notes: add another push/pop for 20 MHz clock frequency delay1uS: push temp ; [2] these instructions do nothing except consume clock cycles pop temp ; [2] push temp ; [2] pop temp ; [2] ret ; [4] ; ============================== End of Time Delay Subroutines ==============