# PICMicrocontollerDSPMathMethod

## 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:

MOVLW   00001111B       ;MASK OFF ALL BUT THE FRACTIONAL
ANDWF   AVFRAC          ;PART OF THE OLD AVERAGE
;(AVFRAC'S HI-NIBBLE CAN
;CONTAIN GARBAGE).

MOVF    AVE,W           ;NEW = NEW - AVE.
SUBWF   NEW             ;CARRY = 0 IF THE DIFFERENCE IS
;NEGATIVE, CARRY = 1 IF POSITIVE.

SWAPF   AVE,W           ;HI-NIBBLE OF W = MIDDLE NIBBLE
;OF 16*AVE.

MOVWF   TEMP            ;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.

SKPC                    ;IF (NEW - AVE) WAS NEGATIVE,
DECF    TEMP            ;ADJUST THE HI-NIBBLE OF 16*AVE.

ANDLW   11110000B       ;W = LO-BYTE OF 16*AVE.
IORWF   AVFRAC,W        ;(INCLUDE AVE'S FRACTIONAL
;PART).

ADDWF   NEW,W           ;W = LO-BYTE OF
;((NEW-AVE) + 16*AVE).
;CARRY = 1 IF SUM > 255,
;CARRY = 0 OTHERWISE.

MOVWF   AVFRAC          ;LO-NIBBLE OF AVFRAC =
;FRACTIONAL PART OF FINAL
;RESULT.  HI-NIBBLE = GARBAGE.

ANDLW   11110000B       ;W = (LO-NIBBLE OF
;((15*AVE+NEW)/16)) * 16.

MOVWF   AVE             ;AVE = (LO-NIBBLE OF
;((15*AVE+NEW)/16)) * 16.

SWAPF   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.

SKPNC                   ;LO-NIBBLE OF TEMP =
INCF    TEMP            ;HI-NIBBLE OF ((15*AVE+NEW)/16).

SWAPF   TEMP,W          ;HI-NIBBLE OF W = HI-NIBBLE OF
ANDLW   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.

IORWF   AVE             ;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.

```

See also:

Questions:

• STOPspampauljg25 at spamexcite.com asks:
What is the SKPC mnemonic? I did not see it listed in either of the PIC 12/14/16/17 instruction sets. Is it simply just checking the Carry Flag? Thanks, PJG
Yes. It is a built in macro in MPASM for skip if carry set. +

 file: /Techref/microchip/dsp/av-16w-aw.htm, 4KB, , updated: 2002/11/3 09:21, local time: 2023/12/4 01:12, TOP NEW HELP FIND:  3.235.60.197:LOG IN

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