AVR-Chat

Jim,

Thanks. This seems to work ok.

Geert

jimmcguffin wrote:
> 
> The problem is the "negate" step. You cannot negate the two bytes
> separately. Negation is the same as a ones-complement followed by
> adding one. This is covered in Atmel's application note #202. They
> do it like this:
> 
> ;***** Register Variables
> .def ng1l = r16
> .def ng1h = r17
> 
> ;***** Code
> ng16:     
> com    ng1l            ;Invert low byte       ;Calculated by
> com    ng1h            ;Invert high byte      ;inverting all
> subi   ng1l,low(-1)    ;Add 0x0001, low byte  ;bits then adding
> sbci   ng1h,high(-1)   ;Add high byte         ;one (0x0001)
> ;Expected result is 0xCBEE
> 
> Jim
> 
> 
> --- In AVR-Chat@yahoogroups.com, "Leon Heller" <leon.heller@d...>
> wrote:
>  > ----- Original Message -----
>  > From: Geert De Pecker
>  > To: AVR-Chat@yahoogroups.com
>  > Sent: Sunday, January 02, 2005 10:21 PM
>  > Subject: [AVR-Chat] newbie absolute value
>  >
>  >
>  > Hi,
>  >
>  > I have a question about abtaining the absolute value. I want to
>  > substract a 1 16bit values from eachother, but when the result is
>  > negative, set a bit and get the absolute value of the difference.
>  >
>  > I thought of the following, but that doesn't seem to work:
>  >
>  > ...
>  >    ldi direction, 0
>  > ; Substract values
>  >    sub moveL, curPosL
>  >    sbc moveH, curPosH
>  > ; If result positive, ok
>  >    brcc PM1
>  > ; Result negative, take absolute value and set flag
>  >    neg moveL
>  >    neg moveH
>  >    ldi direction, 1
>  > PM1:
>  >
>  > Any hints?
>  >
>  > I'd compare them first, to find which is the bigger of the two,
> then
>  > subtract them appropriately with two separate bits of code,
> setting the flag
>  > as necessary.
>  >
>  > Leon
>  >
>  >
>  >
>  > --
>  > No virus found in this outgoing message.
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> 30/12/2004
> 
> 
> 
> 
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