Lookup

A classic application for Lookup is to convert the numbers 0 through 9 (or 0 through F in hexadecimal) into patterns for seven-segment displays. Throughout this book, you will see other applications, such as the table Pinz that some routines use to change a number in the range of 0 through 7 into a byte with a 1 in the corresponding position for use as a bit mask.

Lookup tables are an inherent PIC capability made possible by the retw instruction, which stands for "return with literal value in w." A table consists of a way to select from a list of retws. The instruction jmp pc+w takes care of this. Since the program counter always points to the next instruction, a 0 in w selects the first retw from the list; a 1 the second; a 2 the third, and so on.

The Parallax assembler allows great flexibility in formatting data for tables. You can type one retw n per line, or use the shorthand retw n1,n2,n3...For tables of text, you can type retw 'Text data in retw table'. The assembler will turn this into a series of retws with the ASCII codes for each character of the text. For an example of such a table, see the program listing for Lookdown.

A couple of cautions about using tables. First, your program must ensure that the number in w never exceeds the maximum entry in the table. If it does, program execution will jump completely over the table. Second, tables must be in the first 256 words of a 512-word program-memory page because of the way the PIC's computed-jump mechanism works.

Demonstrating Lookup.

;
; ***************************************************************************
; ***  Bubble Software Parallax to PIC Source Converter. Copyright 1999.  ***
; ***  http://www.picnpoke.com                 email: sales@picnpoke.com  ***
; ***************************************************************************
;
; LOOKUP index (in w), table
; Finds the table entry corresponding to index (in w) and 
; returns it in w. This demonstration uses a table to map the 
; numbers 0 through F hex to patterns on a seven-segment LED
; display. It assumes that the display is connected with segment
; a to rb.0, b to rb.1... and g to rb.6 through current-limiting 
; resistors of 220 ohms minimum. When the program runs, it counts
; upward 0, 1, 2, 3...through F, then begins the count again at 0. 

; Device data and reset vector
        P = pic16c55
        #include <16c55.inc>   ; processor assembler definitions
        _CONFIG _xt_osc & _wdt_off & _protect_off
 reset start

 org     8
count   Res d'1' ; Counter variable for demo.
temp    Res d'1' ; Variables for delays
temp2   Res d'1' ; used in demo subroutines. 

 org 0

Lookup       ADDWF pcl                  ; Jump to entry spec'd by w.
             RETLW d'63'                ; 0, 1, 2, 3
             RETLW d'6'
             RETLW d'91'
             RETLW d'79'
             RETLW d'102'               ; 4, 5, 6
             RETLW d'109'
             RETLW d'125'
             RETLW d'7'                 ; 7, 8, 9
             RETLW d'127'
             RETLW d'103'
             RETLW d'119'               ; A, B, C
             RETLW d'124'
             RETLW d'57'
             RETLW d'94'                ; D, E, F
             RETLW d'121'
             RETLW d'113'

start        MOVLW d'0'                 ; All outputs for LEDs
             TRIS 6h
             CLRF 6h                    ; All LEDs off to begin.
             CLRF count                 ; Start count at 0. 
start_loop   MOVF count,w               ; Put number to look up into w.
             ANDLW b'00001111'          ; Strip off high bits. 
             CALL Lookup                ; Call the table. 
             MOVWF 6h                   ; Write pattern in w to LEDs. 
start_wait   DECFSZ temp                ; Short delay. 
             GOTO start_wait
             DECFSZ temp2               
             GOTO start_wait
             INCF count                 ; count = count + 1
             GOTO start_loop            ; Do it again.

             
             
             end

; LOOKUP index (in w), table
; Finds the table entry corresponding to index (in w) and 
; returns it in w. This demonstration uses a table to map the 
; numbers 0 through F hex to patterns on a seven-segment LED
; display. It assumes that the display is connected with segment
; a to rb.0, b to rb.1... and g to rb.6 through current-limiting 
; resistors of 220 ohms minimum. When the program runs, it counts
; upward 0, 1, 2, 3...through F, then begins the count again at 0. 

        org     8
count   ds      1       ; Counter variable for demo.
temp    ds      1       ; Variables for delays
temp2   ds      1       ; used in demo subroutines. 

; Device data and reset vector
        device  pic16c55,xt_osc,wdt_off,protect_off
        reset   start
        org     0

Lookup  jmp     pc+w            ; Jump to entry spec'd by w.
        retw    63,6,91,79      ; 0, 1, 2, 3
        retw    102,109,125     ; 4, 5, 6
        retw    7,127,103       ; 7, 8, 9
        retw    119,124,57      ; A, B, C
        retw    94,121,113      ; D, E, F

start   mov     !rb, #0         ; All outputs for LEDs
        clr     rb              ; All LEDs off to begin.
        clr     count           ; Start count at 0. 
:loop   mov     w,count         ; Put number to look up into w.
        AND     w,#00001111b    ; Strip off high bits. 
        call    Lookup          ; Call the table. 
        mov     rb,w            ; Write pattern in w to LEDs. 
:wait   djnz    temp,:wait      ; Short delay. 
        djnz    temp2,:wait
        inc     count           ; count = count + 1
        jmp     :loop           ; Do it again.

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