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AVR Assembler (Mega8) - Test if a value has changed more than a given amount

Discussion in 'Embedded' started by Mike Warren, May 7, 2008.

  1. Mike Warren

    Mike Warren Guest

    Can someone please offer a suggestion for a way to check if a value
    has changed up or down by at least some threshold. It doesn't matter
    how long it takes but code size and register use are important.

    I'm not sure how to handle values lower than ChangeThreshold or
    higher then 255 - ChangeThreshold.

    ..equ ChangeThreshold = 6
    ..def OldValue = R16
    ..def NewValue = R17

    Available registers R10, R11, R12, R13, R19, R24

    ; Determine if NewValue > OldValue + ChangeThreshold
    ; or NewValue < OldValue - ChangeThreshold
    rjmp HasChanged
    Mike Warren, May 7, 2008
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  2. Mike Warren

    Mike Warren Guest

    This is what I have so far. Does it seem reasonable?

    ChOld should contain the new value(ChNew) if ChNew is different to
    ChOld by at least TrigThresh.

    ..def A = R19
    ..def B = R24

    push A
    push B

    cp ChNew,ChOld
    brlo CheckForLower

    ldi A,TrigThresh
    add A,ChOld
    ; skip if overflowed
    cpi A,TrigThresh+1
    brlo EndCheckForChange
    ; test higher
    cp A,ChNew
    brlo HasChanged
    rjmp EndCheckForChange

    ldi B,TrigThresh
    mov A,ChOld
    sub A,B
    ; skip if underflowed
    cpi A,255-TrigThresh
    brsh EndCheckForChange
    ; test lower
    cp ChNew,A
    brlo HasChanged
    rjmp EndCheckForChange

    mov ChOld,ChNew
    pop B
    pop A
    Mike Warren, May 7, 2008
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  3. Mike Warren

    Arlet Ottens Guest

    how about this:

    sub r17, r16 ; diff = new - old
    brcc new_bigger ;
    neg r17 ; if (new < old) diff = old - new
    cpi r17, 6 ; if (diff >= 6 )
    brsh HasChanged ; has changed
    Arlet Ottens, May 7, 2008
  4. Mike Warren

    Mike Warren Guest

    That looks great. Thank you.
    Mike Warren, May 7, 2008
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