piclist 2000\05\22\142156a >
Thread: AN556 and long look-up tables
picon face BY : rleggittTakeThisOuT@KILLspamconcentric.net

Hi David, something like this should work for arbitrary table of any
length located anywhere in the program space. -- Rich

               movlw high string       ; point to a string
               movwf look_hi           ; in reality, a macro...
               movlw low string
               movwf look_lo
               call process            ; go process it

; subroutine to process string at look_hi/look_lo
process         call lookup             ; get a byte (this is the magic)

               ; here, do something with byte in W
               ; also, return if end of string
               ; otherwise...

               goto process            ; do it again

; Jump to address in look_hi/look_lo, which presumably is an RETLW.
; Note pointer post increment.
; Equivalent to: W=*look_ptr++
lookup          movf look_hi,w          ; set PCLATH
               movwf PCLATH
               movf look_lo,w          ; and get PCL
               incf look_lo,f          ; but post inc
               incf look_hi,f
               movwf PCL               ; ok, now jump

On Sun, 21 May 2000, David Thompson wrote:

> Hi guys,
> Firstly, don't scream.. I HAVE read the datasheets!. My table reading
> subroutine is based on the final example in AN556 "Implementing a Table
> Read" and can be used across page boundaries and be located anywhere in
> memory... BUT!... will not work if the table itself is longer than 256
> bytes. This is because the table element offset is passed in w (8 bits) and
> then used in the calculation for the program counter. Trying to access
> elements that start beyond an offset of 256 bytes won't work.

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See also: www.piclist.com/techref/mems.htm?key=table
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Subject (change) AN556 and long look-up tables

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