[sdiy] IIR Shift Filter problem - help with bug spotting in PIC ASM

[Tom Wiltshire]

Hi All,

If you could cast your eyes over this, I'd be grateful. I'm having one of those "can't see the wood for the trees" moments.

The code below is a simple single-pole IIR filter (a digital RC LPF). The INPUT is a 10-bit variable, which is left-aligned, so I have six empty bits below it.

Now, I've actually got this code working (the good version below), but what I don't understand is why the BAD version doesn't do the same thing. I don't see why testing the Borrow flag and sticking the bits in "manually" is really any different from shifting the Borrow flag in. What am I missing, oh wise SynthDIY list? ;)

Thanks for any pointers!
Tom

; The Good Version

WaveformSmoothing:
	; Subtract FILTER from new INPUT 
	movf	FILTER_LO, w
	subwf	INPUT_LO, f			; INPUT = INPUT - FILTER
	movf	FILTER_HI, w
	subwfb	INPUT_HI, f
	
	; Borrow is zero if result required borrow (negative result)
	; Is borrow set?
	bcf		FLAGS, 3
	btfss	BORROW			; Skip if positive
	bsf		FLAGS, 3			; Flag set if negative
	
	; Do the downshifts (x 1/16th effectively)
	lsrf		INPUT_HI, f
	rrf		INPUT_LO, f
	lsrf		INPUT_HI, f
	rrf		INPUT_LO, f
	lsrf		INPUT_HI, f
	rrf		INPUT_LO, f
	lsrf		INPUT_HI, f
	rrf		INPUT_LO, f			; 4 shifts total
	
	; Add the extra bits back in if negative
	movlw	0xF0
	btfsc	FLAGS, 3
	addwf	INPUT_HI, f

	; Add it to FILTER
	movf	INPUT_LO, w
	addwf	FILTER_LO, f
	movf	INPUT_HI, w
	addwfc	FILTER_HI, f
	; We now have a smoothed copy of our INPUT in FILTER


; The BAD Version

WaveformSmoothing:
	; Subtract FILTER from new INPUT 
	movf	FILTER_LO, w
	subwf	INPUT_LO, f			; INPUT = INPUT - FILTER
	movf	FILTER_HI, w
	subwfb	INPUT_HI, f

	; Borrow is zero if result required borrow (negative result)
	 ; Flip the state of the borrow flag
 	movlw	0x01
	xorwf	STATUS, f				; Borrow set if negative
	
	; Do the downshifts (x 1/16th effectively)
	rrf		INPUT_HI, f			; Rotate the borrow flag in
	rrf		INPUT_LO, f
	asrf		INPUT_HI, f			; Propagate it down to lower bits
	rrf		INPUT_LO, f
	asrf		INPUT_HI, f
	rrf		INPUT_LO, f
	asrf		INPUT_HI, f
	rrf		INPUT_LO, f			; 4 shifts total

	; Add it to FILTER
	movf	INPUT_LO, w
	addwf	FILTER_LO, f
	movf	INPUT_HI, w
	addwfc	FILTER_HI, f
	; We now have a smoothed copy of our INPUT in FILTER



If you'd like to know more about this technique, full details are here (I've brought this useful StackExchange post up here before):

http://electronics.stackexchange.com/questions/30370/fast-and-memory-efficient-moving-average-calculation

The crucial part is the filter equation:
	FILT <-- FILT + FF(NEW - FILT)