© 1993 Microchip Technology Inc.

DS00540B-page 11

Implementing IIR Digital Filters

4

else

MOVFP16 B10,BARG

MOVFP16 Dn1,AARG

call

DblMult

; B0*D(n)

MOVPF32 DPX,ACC

MOVFP16 Bx2,BARG

MOVFP16 Dn1_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)

;

0058 7C22

movfp

Dn1_1,AARG+B0

0059 5C24

movpf

AARG+B0,Dn1_2

; Shift down D(n-1)

005A 7D23

movfp

Dn1_1+B1,AARG+B1

005B 5D25

movpf

AARG+B1,Dn1_2+B1

; AARG = D(n-1) = multiplier

005C 7E32

MOVFP

B11+B0,BARG+B0

; move B11(B0) to BARG(B0)

005D 7F33

MOVFP

B11+B1,BARG+B1

; move B11(B1) to BARG(B1)

005E 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

;

005F 6018

MOVFP

DPX+B0,WREG

; get lowest byte of DPX into w

0060 0F44

ADDWF

ACC+B0

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

0061 6019

MOVFP

DPX+B1,WREG

; get 2nd byte of DPX into w

0062 1145

ADDWFC

ACC+B1

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

0063 601A

MOVFP

DPX+B2,WREG

; get 3rd byte of DPX into w

0064 1146

ADDWFC

ACC+B2

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

0065 601B

MOVFP

DPX+B3,WREG

; get 4th byte of DPX into w

0066 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)

;

0067 6020

MOVFP

Dn1+B0,WREG

; get byte of Dn1 into w

0068 0122

MOVWF

Dn1_1+B0

; move to Dn1_1(B0)

0069 6021

MOVFP

Dn1+B1,WREG

; get byte of Dn1 into w

006A 0123

MOVWF

Dn1_1+B1

; move to Dn1_1(B1)

;

;

; 2nd Biquad filter section

;

BIQUAD

A21,A22,B20,B21,Dn2,Dn2_1,Dn2_2,2

;

; Compute A22*D(n-2)

;

006B 7C2A

MOVFP

Dn2_2+B0,AARG+B0

; move Dn2_2(B0) to AARG(B0)

006C 7D2B

MOVFP

Dn2_2+B1,AARG+B1

; move Dn2_2(B1) to AARG(B1) 006D 7E38

MOVFP

A22+B0,BARG+B0

; move A22(B0) to BARG(B0)

006E 7F39

MOVFP

A22+B1,BARG+B1

; move A22(B1) to BARG(B1)

006F E0AF

call

DblMult

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

;

; Add product to output of 1st section

; Save result in 32 bit Accumulator

;

0070 6018

MOVFP

DPX+B0,WREG

; get lowest byte of DPX into w

0071 0F44

ADDWF

ACC+B0

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

0072 6019

MOVFP

DPX+B1,WREG

; get 2nd byte of DPX into w

0073 1145

ADDWFC

ACC+B1

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

0074 601A

MOVFP

DPX+B2,WREG

; get 3rd byte of DPX into w

0075 1146

ADDWFC

ACC+B2

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

0076 601B

MOVFP

DPX+B3,WREG

; get 4th byte of DPX into w

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