marcel wrote:

> Tanks for the reply's on this matter
> the thing is what i wand to do is more complicated
> I am reading 12 bit of data from an potentiometer when it is in the
> middle
> it should give something like 0x0800  "hex"
> when i turn left it decreases and when i turn right it increases
> The maximum increment should be from 0x80 to 0xEF  and the maximum
> decrement must not cross 0x00  from0x80 down to 0x00within one reading
> if so i have to increment or decrement
> with these maximums or less until i reach the last readout.
>
> I hope i am more clear on what i want to do
>
> I can get the data msb first in the registers datahi and datalo but
> the LTC1286 allows also to read LSB first
>
> Example   first readout  could be  x0800  the next  0x0880
> the differents is 80 positive is bigger than 0x7F so i add 0x7Fto
> previous readout
> and compare again with the last readout witch can have changed in this
> example
> i assume not so 0x880 sub 0x87F is 0x01 witch is less than 0x7F and
> i add 0x01 to the first readout and the process stops  until a new
> readout
> appears  to be differend .
>
> I don't understand what is the meaning msb stored at lower adress at the
>
> moment i am
> trying simulating this in MacPic
>
>   Storlox     equ   0x10
>   Storhix     equ   0x11
>   ;Storloy    equ   0x12
>   ;Storhiy    equ   0x13
>   TmpReg1     equ   0x14
>   TmpReg2     equ   0x15
>   datalo      equ   0x16
>   datahi      equ   0x17
>
>    movlw   0x30
>    movwf   datalo
>    movlw   0x01
>    movwf   datahi
>    movlw   0x20
>    movwf   Storlox
>    movlw   0x01
>    movwf   Storhix
>    clrf    TmpReg1
>    clrf    TmpReg2
>
> sub16
>
>         movf    datalo,W        ; subtract lsbs
>         subwf   Storlox,W
>         movwf   TmpReg1
>         movf    datahi,W        ; subtract msbs
>         btfss   status,C        ; including any carry
>         incfsz  Storhix,W
>         subwf   datahi,W
>         movwf   TmpReg2
>         btfsc   status,C        ; new smaller?
>         goto    smaller         ; yes
>         goto    larger          ; no
>
> smaller  movf    TmpReg1,W
>          addwf   TmpReg2,W
>          btfsc   STATUS,Z
>          goto    zero
>          bcf     STATUS,Z
>          decf    Storlox
>          goto    sub16
>
> larger   incf  Storlox
>          goto sub16
>
> zero     incf   Storlox
>          goto   sub16
>
> I am totaly confused with carry's not going well
> this makes me think to do somting in two complement like this
> also totaly confusing
>
>   Storlox     equ   0x10
>   Storhix     equ   0x11
>   ;Storloy    equ   0x12
>   ;Storhiy    equ   0x13
>   TmpReg1     equ   0x14
>   TmpReg2     equ   0x15
>   datalo      equ   0x16
>   datahi      equ   0x17
>
>    movlw   0x30         ; new lowByte
>    movwf   datalo       ;
>    movlw   0x01         ; new highByte
>    movwf   datahi
>    movlw   0x20         ; old lowByte
>    movwf   Storlox
>    movlw   0x01         ; old highByte
>    movwf   Storhix
>    clrf    TmpReg1      ; Store diff
>    clrf    TmpReg2      ; Store diff
>
>     comf    datalo,W     ; two complement lowbyte
>     addlw   0x01
>     movwf   datalo
>     ;btfsc   STATUS,C
>     ;incfsz  datahi
>     comf    datahi,W     ;two complement higbyte
>     addlw   0x01
>     movwf   datahi
>
> cmp16
>     movf    datalo,W     ; datalo>W
>     addwf   Storlox,W    ; add oldlobyte
>     movwf   TmpReg1      ; store difference
>     movf    datahi,W     ; get new higbyte
>     btfsc   STATUS,C     ; test carry ifso
>     incfsz  Storhix,W      ; incf oldhighbyte
>     addwf   Storhix,W
>     movwf   TmpReg2
>     btfss   STATUS,C
>     goto    smaller         ; yes
>     goto    larger          ; no
>
> smaller  movf    TmpReg1,W
>          addwf   TmpReg2,W
>          btfsc   STATUS,Z
>          goto    zero
>          bcf     STATUS,Z
>          decf    Storlox
>          goto    cmp16
>
> larger   incf  Storlox
>          goto   cmp16
>
> zero     incf   Storlox
>          goto   zero
>
> I realy need help on this
>
>  Marcel Elektronic workshop
>  Amsterdam