I would suggest you visit the following page. It has truck loads of info
related to LCD's as well as source for the various aspects of LCD control
including the busy flag.

http://www.iaehv.nl/users/pouweha/lcd2.htm

___________________________________________
Wesley Moore
RMIT - BEng/BApp.Sc. 1st Year

wmoore@cs.rmit.edu.au
http://wmoore.tsx.org/

On Mon, 25 Oct 1999, John Considine wrote:

> I am having a problem when I try to check the busy flag coming from the
> LCD display (BUSY_FLAG? subroutine shown below).  It is always busy but
> if I remove the busy flag check it runs fine, as long as I use a slow
> clock (therefore the display must not be busy).   Any suggestions why
> this bit would be remaining high.   the power supply is a 7805 (5+)
> being supplied by a 1 amp 12v power supply (120V wall plug in).  I have
> a .1uf capacitor from +5 to ground and a .57uf cap from 12V to ground
> (for decoupling so I think).  All of this is on a breadboard along with
> the PIC 16C84 and Noritake LCD display 4X20 display with selectable 4 or
> 8 bit transfer.
> Thanks.
>
> ;October 25, 1999
> ;
>
>
>  LIST P=16C84, F=INHX8M, R=DEC
>  include <p16c84.inc>
>    __CONFIG _CP_OFF & _WDT_OFF & _RC_OSC
>
> DISP_ON   EQU  b'00001101'
> CLR_DISP  EQU  b'00000001'
> ENTRY_INC  EQU  b'00000110'
> MSD   EQU  H'0C'
> LSD   EQU  H'0D'
> TEMP   EQU  H'0E'
> TEMP1   EQU  H'0F'
> CHAR   EQU  H'10'
> RESET_V   EQU  0
> LCD_DELAY_SET  EQU  H'FF'
> DD_RAM_ADD  EQU  H'80'
> E   EQU  4  ; LCD Enable control line
> RW   EQU  3  ; LCD Read/Write control line
> RS   EQU  2
> ;interupt routine varibles
> INT_W  EQU  H'11'
> INT_S  EQU  H'12'
>
>    org 0
> RESET   goto    START
>    org 4     ; RESET vector location
>
> Int
>  movwf INT_W
>  movf STATUS, 0
>  movwf  INT_S
>
> START
> ;pic 16c84 setup for initial state
>  movlw b'00000000'
>  movwf STATUS   ;Clear Status
>  clrf INTCON   ;Initialize INTCON Disable all interupts
> ; bsf INTCON, 3  ;Makes interupt if RB<7:4> change state
>  bsf STATUS,RP0
>  movlw b'11000111'
>  movwf OPTION_REG  ;Set Option Register
>  bcf STATUS,RP0
>
>  clrf PORTA
>  clrf PORTB
>  bsf STATUS,RP0
>  CLRF TRISA
>  CLRF TRISB
>  bcf STATUS,RP0
>
> ;display initialize
>
>  movlw H'20'   ;four bit transfer =h'20' 8 = h'38'
>  movwf PORTB
>  bsf PORTA,E
>  bcf PORTA,E
>
>  movlw DISP_ON
>
>  nop
>  nop
>  nop
>  nop
>  nop
>  movwf PORTB
>  call CMD_SEND
>
> ; movlw CLR_DISP
> ; call CMD_SEND
>
> ;send characters
>  movlw 'F'
>  call CHAR_REC
>  movlw 'I'
>  call CHAR_REC
>  movlw 'N'
>  call CHAR_REC
>  movlw 'A'
>  call CHAR_REC
>  movlw 'L'
>  call CHAR_REC
>
>  goto $
>
>
>
> CMD_SEND
>   movwf   CHAR            ; Command to be sent is in W
>        call    BUSY_FLAG?      ; Wait for LCD to be ready
>         movf    CHAR, 0
>        movwf   PORTB         ; Send data to LCD
>         bcf     PORTA, RW  ; Set LCD in read mode
>         bcf     PORTA, RS ; Set LCD in command mode
>         bsf     PORTA, E      ; toggle E for LCD
>         bcf     PORTA, E
>     ;send lower nibble
>  swapf CHAR, 0
>  movwf   PORTB
>  bsf PORTA, E
>  bcf PORTA, E
>         RETURN
>
> CHAR_REC
>   movwf   CHAR            ; Command to be sent is in W
>        call    BUSY_FLAG?      ; Wait for LCD to be ready
>         movf    CHAR, 0
>        movwf   PORTB         ; Send data to LCD
>         bcf     PORTA, RW  ; Set LCD in read mode
>         bsf     PORTA, RS ; Set LCD in command mode
>         bsf     PORTA, E      ; toggle E for LCD
>         bcf     PORTA, E
>     ;low nibble transfer
>  swapf CHAR,0
>  movwf PORTB
>  bsf PORTA, E
>  bcf PORTA, E
>         RETURN
> ;
> BUSY_FLAG?
>
>   bsf     STATUS,RP0       ; Select Register page 1
>         movlw   0xF0             ; Set port_B for input
>         movwf   TRISB
>         bcf     STATUS, RP0      ; Select Register page 0
>         bcf     PORTA, RS     ; Set LCD for command mode
>         bsf     PORTA, RW    ; Setup to read busy flag
>         bsf     PORTA, E      ; Set E high
>         bcf     PORTA, E      ; Set E low
>         movf    PORTB, 0      ; Read busy flag, DDram address
>  andlw 0xF0
>  movwf TEMP
>  bsf PORTA, E
>  bcf PORTA, E
>  swapf PORTB, 0
>  andlw 0x0F
>  iorwf TEMP
>  btfsc TEMP,7
> ; goto BUSY_FLAG?
>  nop
>  bsf STATUS, RP0
>  movlw 0x00
>  movwf TRISB
>  bcf STATUS, RP0
>  RETURN
>
> ; movwf
> ;       movwf   TEMP
> ;        btfsc   TEMP, 7
> ;           ; Check busy flag, high=busy
> ;        goto    BUSY_FLAG?
> ;        bcf     PORTA, RW
> ;        bsf     STATUS, RP0      ; Select Register page 1
> ;        movlw   H'00'
> ;        movwf   TRISB      ; Set port_B for output
> ;        bcf     STATUS, RP0      ; Select Register page 0
> ;        RETURN
>
>  end
>