Dear

You are right, this does *look* simple.
Have you single-stepped through your code with the ICD ?

Patrick J <info@DATECH.SE> on 2001-06-08 09:52:27 AM asked for help:
>   new filt val = <old filt val> - <old filt val>/256 + <input val>/256
...
>with sample-input val of 0:255. Right ? Sample is always in the range of 0-255
>I have simulated this in Excel, works fine there. (not tried as fixed point
math tho)
>
>Must this be done with floating point math ???

simple 2 byte (``fixed point'') math should work fine.

>     ; // calc: 'input/256' //
>  movfw sampleA
>  movwf tempL  ; move high-byte to low-byte makes /256
>  clrf tempH  ; set high byte to 0
>     ; Now 'input/256' is in tempH:tempL
>
>     ; // calc: 'old/256' //
>  movfw FilterAH ; get old value.
>  movwf temp1L  ; move high-byte to low-byte makes /256
>  clrf temp1H  ; set high byte to 0
>     ; Now 'old/256' is in temp1H:temp1L
>

Looks good to me. (After you get it working, it might be possible to improve it
to only use 1 temporary 16 bit register tempL:tempH ... or maybe none at all ).

>
>     ; // calc: 'old/256' + 'input/256'
>  movf tempL, W
>  addwf temp1L, F ; W eller F ?
>
>  movf tempH, W
>  btfsc STATUS, C
>  incfsz tempH, W
>  addwf temp1H, F ; W eller F ?
>     ; Now 'old/256' + 'input/256' is in temp1H:temp1L

Yes, it is true that
  temp1H:temp1L := 'old/256' + 'input/256';
but I think you really want
  temp1H:temp1L := (-'old/256') + 'input/256';
right ?
That's the first partial result from
    new filt val = <old filt val> + ( - <old filt val>/256 + <input val>/256 )
right ?

>     ; // calc: 'old' - ('old/256' + 'input/256') //

Um.... close.

What you want is
  new = <old> - <old>/256 + <input>/256
  new = <old> + (- <old>/256 + <input>/256)
  new = <old> + ( <input>/256 - <old>/256 )

>  movf temp1L, W
>  subwf FilterAL, F
>  movf temp1H, W
>  btfss STATUS,C
>  incfsz temp1H, W
>  subwf FilterAH, F
>     ; Now FINAL RESULT should be in FilterAH:FilterAL !

I hope this isn't optimized to the point of incomprehension:

    ; warning: untested code.
    ; 2001-06-08: David Cary
    ; tempH:tempL := -(old/256);
    clrf tempH
    comf FilterAH,w
    movwf tempL
    incfsz tempL
    decf tempH
    ; tempH:tempL := tempH:tempL +(input/256)
    movfw sampleA
    addwf tempL,f
    skpnc
    incf tempH
    ; FilterAH:FilterAL :=  FilterAH:FilterAL + tempH:tempL;
    ...
    [missing code for 16 bit addition]
    ...
    ; Now FINAL RESULT should be in FilterAH:FilterAL !


--

David Cary

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