So, your Filt is a 32 bit number in 20.12 format?  Odd that you didn't=20
mention that before.  Is there any chance of you providing a snippet of=20
code that implements your version of the filter?

Kerry


Olin Lathrop wrote:
> Kerry Wentworth wrote:
>  =20
>> Do you have a math library of some sort to do 10.6 arithmetic?
>>    =20
>
> Not specifically.  I do have various 32 bit math routines for PICs.
>
> The advantage of fixed point is that for much of the processing you can
> treat the numbers as integers.  The machine just does the add, only you k=
now
> that you consider the least significant 6 bits to be fraction bits.
>
>  =20
>> For example, a 1/2 scale left justified A/D reading would be
>>
>> 1000 0000 00.00 0000
>> and 1/16 would be
>> 0000 0000 00.00 0100
>>    =20
>
> Yes.
>
>  =20
>> The desired result is 0000 1000 00.00 0000
>>    =20
>
> Assuming you want to multiply the two numbers, which you didn't say.
>
>  =20
>> Doing a binary multiply would result in a value of 0.
>>    =20
>
> No.  I don't see how you get that no matter how you think multiply works.
>
> If you just multiply fixed point numbers together as if they were integer=
s,
> then the result has the total number of fraction bits in both the input
> numbers, which in this example is 12.  The result of a straight integer
> multiply without any prescaling would be:
>
>                     1000000000.000000
>                   x 0000000000.000100
>                   -------------------
>           00000000100000000000.000000000000
>
> Again, the fact that you consider these numbers to have a binary point
> somewhere in the middle is totally on your end.  The processor just does
> integer arithmetic.
>
>  =20
>> On the other hand, a 1/2 scale right justified A/D reading would be
>> 0000 0010 00.00 0000
>>    =20
>
> That doesn't make sense.  If you are using right justified A/D readings,
> then they would be integers.  I don't see where the fraction bits are
> supposed to come from in this case.
>
>  =20
>> Please pardon my ignorance, I'm here to learn.  Is 10.6 arithmetic
>> something that everybody but me uses?  Is it an assembly language
>> thing?
>>    =20
>
> Fixed point in general is commonly used on small systems like PICs.  10.6
> format is just one example, which has 6 fraction bits in a 16 bit number.
> Fixed point is common when you need to represent fractional values but ha=
ve
> a integer arithmetic processor.  It's more tricky than floating point
> because you have to keep track of where the imagined binary point is.
>
> One way to think of floating point is that it stores the binary point
> information in the number, and updates everything accordingly at run time=
..
> That takes more hardware, which small processors like PICs don't have.
>
>  =20
>> I use a C compiler almost exclusively.
>>    =20
>
> There's the problem.  Wake up and smell the bits.  It is difficult to
> understand what is really going on if you keep a compiler between you and
> what is really going on.
>
>
> ********************************************************************
> Embed Inc, Littleton Massachusetts, http://www.embedinc.com/products
> (978) 742-9014.  Gold level PIC consultants since 2000.
>  =20



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