David Duffy <PICLIST@MITVMA.MIT.EDU> wrote:

> I need to implement the following equation in a 16F876.
> RESULT = (255 / A) * B
> A and B are bytes (each have possible values of 01 to FF)
> RESULT is a word (unsigned) for a timer routine counter.
> Speed is not an issue. Code space is not really either.
> ....
> Actually, I can probably make the constant 256 which
> means that I could put B in the high byte and then divide
> by A. The error would be acceptable for the intended usage.

    Yeah, you could do that... Or you could leave it at 255, and do
    the multiplication like this:

        MOVF    B,W
        MOVWF   PRODUCT_HI
        SUBLW   0
        MOVWF   PRODUCT_LO
        DECF    PRODUCT_HI

    That's only two instructions more than what you'd need to do a
    multiply by 256.

> So all I really need is a 16 bit / 8 bit divide routine then?
> That will give me a 16 bit result for my counter won't it?

    Yes.  If you decide to make the constant 256, though, you can do
    the division in about half the time that a regular 16/8 division
    would take... But you already said that speed isn't an issue, so
    you might as well use 255 and do a full 16/8 divide.

    -Andy


=== Andrew Warren - fastfwd@ix.netcom.com
=== Fast Forward Engineering - San Diego, California
=== http://www.geocities.com/SiliconValley/2499

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