© 1993 Microchip Technology Inc.

DS00540B-page 9

Implementing IIR Digital Filters

4

001C 0101

movwf

fsr0

001D B00C

movlw

6*FltStage

NextClr

001E 2900

clrf

indf0

001F 1700

decfsz

wreg

0020 C01E

goto

NextClr

;

0021 0002

return

;

;*******************************************************************

;

1st Cascade Section

;*******************************************************************

;

IIR_Filter

;

; Compute

D(n) = X(n) + A1*D(n-1) + A2*D(n-2)

;

Since the filter constants are computated in Q15 format,

;

X(n) must be multiplied by 2**15 and then added to the

;

other terms.

;

; Move Input to accumulator after proper scaling

;

0022 8804

bcf

_carry

0023 1941

rrcf

X+B1

0024 1940

rrcf

X

0025 2900

clrf

wreg

; Scale the input X

0026 1900

rrcf

wreg

0027 0145

movwf

ACC+B1

0028 6040

movfp

X,wreg

0029 0146

movwf

ACC+B2

002A 6041

movfp

X+B1,wreg

002B 0147

movwf

ACC+B3

; ACC = scaled input : X*(2**15)

;

; 1st Biquad filter section

;

BIQUAD

A11,A12,B10,B11,Dn1,Dn1_1,Dn1_2,1

;

; Compute A12*D(n-2)

;

002C 7C24

MOVFP

Dn1_2+B0,AARG+B0

; move Dn1_2(B0) to AARG(B0)

002D 7D25

MOVFP

Dn1_2+B1,AARG+B1

; move Dn1_2(B1) to AARG(B1)

002E 7E2E

MOVFP

A12+B0,BARG+B0

; move A12(B0) to BARG(B0)

002F 7F2F

MOVFP

A12+B1,BARG+B1

; move A12(B1) to BARG(B1)

0030 E0AF

call

DblMult

; (ACCd,ACCc) = A2*D(n-2)

;

; Add product to output of 1st section

; Save result in 32 bit Accumulator

;

0031 6018

MOVFP

DPX+B0,WREG

; get lowest byte of DPX into w

0032 0F44

ADDWF

ACC+B0

; add lowest byte of ACC, save in ACC(B0)

0033 6019

MOVFP

DPX+B1,WREG

; get 2nd byte of DPX into w

0034 1145

ADDWFC

ACC+B1

; add 2nd byte of ACC, save in ACC(B1)

0035 601A

MOVFP

DPX+B2,WREG

; get 3rd byte of DPX into w

0036 1146

ADDWFC

ACC+B2

; add 3rd byte of ACC, save in ACC(B2)

4-137