Andries Tip wrote:
> ;------------------------------------------------------------------------=
----
>=20
> FIR_256_16BIT                   ; average =3D (255*average + setpoint)/25=
6
>=20
>         movf    average0,w      ; average =3D 256*average - average =3D 2=
55*average
>         sublw   d'0'            ; prepare carry
>         movwf   temp            ; save value for rounding later, otherwis=
e we cannot reach 65535
>         movf    average1,w      ;
>         subwfb  average0,f      ;
>         movf    average2,w      ;
>         subwfb  average1,f      ;
>         movlw   d'0'            ;
>         subwfb  average2,f      ;
>=20
>         movlw   d'255'          ; average =3D round((average + setpoint)/=
256)
>         addwf   temp,w          ;
>         movf    setpoint1,w     ;
>         addwfc  average0,f      ;
>         movf    setpoint2,w     ;
>         addwfc  average1,f      ;
>         clrw                    ;
>         addwfc  average2,f      ;
>=20
> ;------------------------------------------------------------------------=
----

Taking a closer look at your code, I think it has a flaw.

First of all, the comment about "we cannot reach 65535" is simply false. Ev=
en
without temp, with a steady input of 65535, the filter will definitely reac=
h
65535 at the output; it just takes a very long time (i.e., 256 iterations
after it reaches 65534). This is normal behavior for this type of filter.

You're using temp as an extra byte of fractional precision in your accumula=
tor,
but you've made an error in doing that: When you add the new setpoint value=
 to
the accumulator, the operation to initialize the carry with rounding should=
 be
128 + temp, not 255 + temp.

But, like I said at the beginning, this whole business with temp is
superfluous to begin with, and could simply be deleted.

-- Dave Tweed
--=20
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