It WAS a typo.  I actually meant Filt =3D (Total + (Factor/2)) / Factor,=20
which rounds the number rather than truncates.  If truncating is good=20
enough, then you can use Filt =3D Total / Factor.

Kerry


Sean Breheny wrote:
> I think you perhaps meant:
>
> Filt=3D(Total)/Factor
>
> In that case, I often use the structure you are talking about. It
> allows you to do only one division for both the output computation and
> the internal filter computation, while avoiding resolution issues with
> fixed-point math.
>
> For example, if we have a filter factor of 16, with Olin's example,
> and using integer math, we have:
>
> FILT<--FILT + (NEW-FILT)/16
>
> If the absolute value of the difference NEW-FILT is less than 16,
> nothing ever happens.
>
> With Kerry's version:
>
> Total<--Total+New-Filt
>
> Any nonzero value of New-Filt causes a change in Total. To scale the
> output, we just divide Total by 16 (filt=3DTotal/16). However, that also
> gives us the value Filt which we need in order to compute the filter's
> difference equation, too.
>
> Sean
>
>
> On Tue, Nov 23, 2010 at 9:30 AM, Kerry Wentworth
> <kwentworth@skunkworksnh.com> wrote:
>  =20
>> I would normally implement such a filter as:
>>
>> Total =3D Total + New - Filt
>> Filt =3D (Total+1) / Factor
>> And I usually  make Factor =3D 2^N so I can divide by right shifting.
>> This keeps everything as integers, and if you want the extra resolution,
>> you can use Total.
>>
>> Kerry
>>
>>
>> Olin Lathrop wrote:
>>    =20
>>> RussellMc wrote:
>>>
>>>      =20
>>>> Simplistic example. If random noise is added to a signal and it reads
>>>> 145 mV 60% of the time and 146 mV 40 % of the time then actual voltage
>>>> MAY be about 145.4 (not 145.6) mV. Take care when arttempting to apply
>>>> this magic. It does work, but ...
>>>>
>>>>        =20
>>> It's pretty easy to apply on the firmware end.  Just low pass filter
>>> multiple A/D readings.
>>>
>>> When you have the cycles and real time available, it's a good idea to
>>> oversample with the A/D, then low pass filter in firmware anyway.  It g=
ets
>>> you a little extra resolution if the right kind of input noise is prese=
nt as
>>> you said, but it also reduces quantization noise and relaxes anti-alias=
ing
>>> filtering requirements on the external hardware.
>>>
>>> Fortunately a single pole low pass filter is easy to compute:
>>>
>>>   FILT <-- FILT + FF(NEW - FILT)
>>>
>>> where FILT is the low pass filter being updated with input value NEW ea=
ch
>>> iteration.  FF is the "filter fraction".  FF =3D 1 is just a pass thru =
filter,
>>> and FF =3D 0 is a infinitely heavy filter.  Useful values are obviously=
 in
>>> between.  In a small system with limited math capabilities, you often
>>> arrange FF =3D 1/(2**N), so that the multiply by FF becomes a right shi=
ft by
>>> N.
>>>
>>>
>>> ********************************************************************
>>> Embed Inc, Littleton Massachusetts, http://www.embedinc.com/products
>>> (978) 742-9014.  Gold level PIC consultants since 2000.
>>>      =20
>>
>>    =20
>
>  =20



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