© 1993 Microchip Technology Inc.

DS00540B-page 13

Implementing IIR Digital Filters

4

MOVFP16 B20,BARG

MOVFP16 Dn2,AARG

call

DblMult

; B0*D(n)

MOVPF32 DPX,ACC

MOVFP16 Bx2,BARG

MOVFP16 Dn2_2,AARG

call

DblMult

; B2*D(n-2)

ADD32

DPX,ACC

endif

;

; Shift down D(n-1) to D(n-2) after D(n-2) usage is no longer required.

; This way in the next iteration D(n-2) is equal to the present D(n-1)

;

0097 7C28

movfp

Dn2_1,AARG+B0

0098 5C2A

movpf

AARG+B0,Dn2_2

; Shift down D(n-1)

0099 7D29

movfp

Dn2_1+B1,AARG+B1

009A 5D2B

movpf

AARG+B1,Dn2_2+B1

; AARG = D(n-1) = multiplier

009B 7E3C

MOVFP

B21+B0,BARG+B0

; move B21(B0) to BARG(B0)

009C 7F3D

MOVFP

B21+B1,BARG+B1

; move B21(B1) to BARG(B1)

009D E0AF

call

DblMult

; (ACCd,ACCc) = B1*D(n-1)

;

; Compute Output Y = B1*D(n-1) + B2*D(n-2) + B0*D(n)

;

= B1*D(n-1) + B0*[D(n) + D(n-2)]

; Since all multiplications are already done, simply perform a

; 32 bit addition

;

009E 6018

MOVFP

DPX+B0,WREG

; get lowest byte of DPX into w

009F 0F44

ADDWF

ACC+B0

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

00A0 6019

MOVFP

DPX+B1,WREG

; get 2nd byte of DPX into w

00A1 1145

ADDWFC

ACC+B1

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

00A2 601A

MOVFP

DPX+B2,WREG

; get 3rd byte of DPX into w

00A3 1146

ADDWFC

ACC+B2

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

00A4 601B

MOVFP

DPX+B3,WREG

; get 4th byte of DPX into w

00A5 1147

ADDWFC

ACC+B3

; add 4th byte of ACC, save in ACC(B3)

;

; Shift down D(n) to D(n-1) so that in the next iteration, the new

; D(n-1) is the present D(n)

;

00A6 6026

MOVFP

Dn2+B0,WREG

; get byte of Dn2 into w

00A7 0128

MOVWF

Dn2_1+B0

; move to Dn2_1(B0)

00A8 6027

MOVFP

Dn2+B1,WREG

; get byte of Dn2 into w

00A9 0129

MOVWF

Dn2_1+B1

; move to Dn2_1(B1)

;

;

; The filter output is now computed

; Save the Upper 16 Bits of 32 bit Accumulator into Y after

; adjusting the decimal point

;

MOV16

ACC+B2,Y

00AA 6046

MOVFP

ACC+B2+B0,WREG

; get byte of ACC+B2 into w

00AB 0142

MOVWF

Y+B0

; move to Y(B0)

00AC 6047

MOVFP

ACC+B2+B1,WREG

; get byte of ACC+B2 into w

00AD 0143

MOVWF

Y+B1

; move to Y(B1)

;

00AE 0002

return

; Output Y(n) computed

;

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

;

Set SIGNED/UNSIGNED Flag Before Including 17c42MPY.mac

;

include

“17c42MPY.mac”

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

;

Low Pass Filter Co-efficients

;

;

4-141