I found the two stage filter to be too fast for my application so I added 6
more
stages (cascaded). The responsetime is approx 1 sec now, still way too fast
!
As I add more and more stages I notice the responsetime gets a little
longer for
each stage. Wonder if that is because the mainloop gets bigger or its due
to the
filter gets more stages... hmm... I'd expect the responsetime to increase
very much
with so many stages.

As u can see I use a new variable for each stage, is that really necessary
?
All my attemts to use a sub or just re-use the same variables seems to make
the filter response faster than if using many variables! Could be error in
the
calc ?

I also need slower decay when the sensor reads 0, and not as now symetrical
responstime. The fill-up-time is 2-4 sec, but the time until empty is 20-30
secs.

With a simple box filter I could put more 'weight' on the 1:s by adding 2
or 3
to the filter for each 1 that is read. That would yield a faster response
for 1 and
keep same response for the 0 level. Cant really see howto do that using a
cascaded single pole filter.


From: "Olin Lathrop
> of the code.  The two stage filter would yield a cleaner and less noisy
> value than the single stage filter.  The step response of the two stage
> filter would be a little slower, but still well within your requirements
as
> I understand them.  The 50% step response time would be a bit over 400
> samples, and the 80% response time at about 760 samples.

My implementation:

FILTER_A    ; new = old - old/256 + input/256
  ; *** FILTER: STEG 1
  ;
  ;   Subtract <FILT1/256> from FILT1
  ;
  movf FilterA1H, W ;get FILT1/255
  subwf FilterA1L, F ;make the new low byte
  btfss STATUS, C ;no borrow from high byte ?
  decf FilterA1H, F ;propagate the borrow
  ;
  ;   Add <input/256> to the filter
  ;
  btfss   sample, plutt ;the input bit is 1 ?
  goto done_inbitA ;input bit is 0, skip this section
  movlw h'FF'  ;get value implied by input bit / 256
  addwf FilterA1L, F ;make new low byte
  btfsc STATUS, C ;no carry to high byte ?
  incf FilterA1H, F ;propagate the carry
done_inbitA    ;done dealing with input bit contribution


  ; *** FILTER: STEG 2
  ;
  ;   Subtract <FILT2/256> from FILT2
  ;
  movf FilterA2H, W ; get <old/256>
  subwf FilterA2L, F ; make the new low byte
  btfss STATUS, C ; need to borrow from high byte ?
  decf FilterA2H, F ; yes, propagate the borrow
  ;
  ;   Add <FILT1/256> to the filter
  ;
  movf FilterA1H, W ;get FILT1/256
  addwf FilterA2L, F ;make new low byte
  btfsc STATUS, C ;no carry to high byte ?
  incf FilterA2H, F ;propagate the carry


  ; ---- FILTER: STEG 3 ----
  ;
  ;   Subtract <old/256>
  ;
  movf FilterA3H, W ; get <old/256>
  subwf FilterA3L, F ; make the new low byte
  btfss STATUS, C ; need to borrow from high byte ?
  decf FilterA3H, F ; yes, propagate the borrow
  ;
  ;   Add <input/256> to the filter.
  ;
  movf FilterA2H, W ; get <input/256> (the output from the filter above)
  addwf FilterA3L, F ; make new low byte
  btfsc STATUS, C ; no carry to high byte ?
  incf FilterA3H, F ; propagate the carry


repeat same as above with new variables for each stage...
use the high byte of the last set of variables as output.
  ; ---- FILTER: STEG 4 ----
  ; ---- FILTER: STEG 5 ----
  ; ---- FILTER: STEG 6 ----
  ; ---- FILTER: STEG 7 ----

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