Steven Addison <PICLIST@MITVMA.MIT.EDU> wrote:

> I need to check that a value, which will be a slightly variable code
> value, is between an upper and a lower threshold (or not).
>
> The way I have thought of doing it is to subtract it from the upper
> threshold and retain the result, then subtract the lower threshold
> from it and retain that result. If the two results are positive then
> the code is within the limits and is good but if either are less
> than zero then the code is no good.
>
> Here is my problem: How do I check for a <0 (negative) value in a
> PIC register?

Steve:

It's simple.  After a subtraction, the Carry flag is set if the
result is positive, clear if it's negative.  Assuming that all three
values (your code, the upper threshold, and the lower threshold) are
8-bit unsigned numbers, and that you're using a 16Cxx part (NOT a
16C5x), the code to do what you want looks like this:

    MOVF    CODE,W          ;W = the value you're testing.
    ADDLW   -LOWER          ;Is it lower than "LOWER"?
    BNC     BAD             ;If so, it's bad.  Jump.
    ADDLW   -(UPPER-LOWER)  ;Otherwise, is it higher than "UPPER"?
    BC      BAD             ;If so, it's bad.  Jump.

    NOP                     ;Otherwise, it's good.  Do whatever here...

For those of you who care about such things, the standard "This code
has neither been tested nor assembled; you get what you pay for"
disclaimer applies.

-Andy

Andrew Warren - fastfwd@ix.netcom.com
Fast Forward Engineering, Vista, California
http://www.geopages.com/SiliconValley/2499