SX Microcontroller DSP Math Method

8bit 16 step Averageing filter by Andrew Warren

 ; Written by Andrew Warren [fastfwd at ix.netcom.com].
 ;
 ; (NEW - AVE) + 16*AVE
 ; AVE = --------------------
 ; 16
NEW	equ	[ANY FILE REGISTER]	;HOLDS NEW SAMPLE [0-255].
AVE	equ	[ANY FILE REGISTER[	;HOLDS THE AVERAGE [0-255].
AVFRAC	equ	[ANY FILE REGISTER]	;HOLDS THE FRACTIONAL PART
 ;OF THE AVERAGE [0-15].
TEMP	equ	[ANY FILE REGISTER]	;TEMPORARY STORAGE.
 FILTER:
	mov	W, #00001111B	;MASK OFF ALL BUT THE FRACTIONAL
	and	AVFRAC, W	;PART OF THE OLD AVERAGE
 ;(AVFRAC'S HI-NIBBLE CAN
 ;CONTAIN GARBAGE).
	mov	W, AVE	;NEW = NEW - AVE.
	sub	NEW, W	;CARRY = 0 IF THE DIFFERENCE IS
 ;NEGATIVE, CARRY = 1 IF POSITIVE.
	mov	W, <>AVE;HI-NIBBLE OF W = MIDDLE NIBBLE
 ;OF 16*AVE.
	mov	TEMP, W	;LO-NIBBLE OF TEMP = HI-NIBBLE
 ;OF 16*AVE (LO-NIBBLE OF 16*AVE
 ;IS ALWAYS 0000).
 ; NEW CONTAINS (NEW - AVE) AND [TEMP:W] HOLDS 16*AVE.
 ; ADD THEM TOGETHER.
	sb	C	;IF (NEW - AVE) WAS NEGATIVE,
	dec	TEMP	;ADJUST THE HI-NIBBLE OF 16*AVE.
	and	W, #11110000B	;W = LO-BYTE OF 16*AVE.
	or	W, AVFRAC	;(INCLUDE AVE'S FRACTIONAL
 ;PART).
	add	W, NEW	;W = LO-BYTE OF
 ;((NEW-AVE) + 16*AVE).
 ;CARRY = 1 IF SUM > 255,
 ;CARRY = 0 OTHERWISE.
	mov	AVFRAC, W	;LO-NIBBLE OF AVFRAC =
 ;FRACTIONAL PART OF FINAL
 ;RESULT. HI-NIBBLE = GARBAGE.
	and	W, #11110000B	;W = (LO-NIBBLE OF
 ;((15*AVE+NEW)/16)) * 16.
	mov	AVE, W	;AVE = (LO-NIBBLE OF
 ;((15*AVE+NEW)/16)) * 16.
	swap	AVE	;AVE = LO-NIBBLE OF
 ;((15*AVE+NEW)/16).
 ; AVE'S LO-NIBBLE CONTAINS THE LO-NIBBLE OF THE FINAL RESULT.
 ; AVE'S HI-NIBBLE = 0000.
	snb	C	;LO-NIBBLE OF TEMP =
	inc	TEMP	;HI-NIBBLE OF ((15*AVE+NEW)/16).
	mov	W, <>TEMP	;HI-NIBBLE OF W = HI-NIBBLE OF
	and	W, #11110000B	;((15*AVE+NEW)/16).
 ;LO-NIBBLE OF W = 0000.
 ; W'S HI-NIBBLE CONTAINS THE HI-NIBBLE OF THE FINAL RESULT.
 ; LO-NIBBLE = 0000.
	or	AVE, W	;COMBINE THE HI- AND LO-NIBBLES
 ;AND STORE THE FINAL RESULT IN
 ;AVE.
 ; AVE CONTAINS THE NEW AVERAGE.
 See? That wasn't too hard; just 20 words of program space and
 20 instruction cycles... And not a single floating-point
 operation anywhere in there.

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