Olin wrote:
> > I want to scale an incoming byte 00 - FF into a smaller range of
> > values so that 00 still is 00 but FF gets cut down to say 28h with
> > all the values in between filled in. I though of a few ways to do it;
> > 1) Divide by a constant & round the result.
> >      Maybe the best but I may need to change the curve of the scaling.
> > 2) A 256 byte table.
> >      Very definable but large & a pain to change scale.
> > 3) Lots of compares.
>
>You don't say what machine you're on, how critical code space is, and how
>critical execution speed is.  I think there are two basic solutions, a table
>and a fixed point multiply.

Sorry, forgot to say it's on a 16F876 but may have to jam it into a 16F628
whenever they get released and readily available. (or even a 16F84A)
Not sure how much code space I will need - still early days yet. Execution
needs to be fairly quick as there is the possibility of multiple commands
arriving from the host system in quick succession.

>Since you are multiplying by a number less than 1, this works well when
>expressed in fixed point with 8 fraction bits.  Multiplying this value by
>the original byte yields the scaled value in the high byte.  This solution
>is especially attractive if you are on a machine with a hardware multiplier.

I've never needed to do math before in a PIC - might be time to learn!

>The table is more general and probably the fastest solution if you don't
>have a hardware multiplier, but it can eat up code space.  However, note
>that you don't need to index into the table with all 8 input bits.  If the
>result is only in the 0 - 40 range you should look at using only the high 6
>bits of the input value as the table index for exapmle.  I like to use
>assembler loops to write such tables for me.  It prevents error in typing in
>individual values, and it makes it simple to change the overall scale or
>whatever of the table if needed.

I think the table is probably the most quick & portable solution as present.
Just have to brush up on those page boundary crossing details.  :-)
Regards...

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