© 1993 Microchip Technology Inc.

DS00540B-page 5

Implementing IIR Digital Filters

4

;

MOVFP16

Dn_2,AARG

; D(n-2) = multiplier

MOVFP16

Ax2,BARG

; A2 = multiplicand

call

DblMult

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

;

; Add product to output of 1st section

; Save result in 32 bit Accumulator

;

ADD32

DPX,ACC

;

;

Compute A1*D(n-1)

;

MOVFP16

Dn_1,AARG

; AARG = D(n-2) = multiplier

MOVFP16

Ax1,BARG

; BARG = A2 = multiplicand

call

DblMult

; (ACCd,ACCc) = A1*D(n-1)

;

; Compute A1*D(n-1) + A2*D(n-2) + output of previous section

;

multiplications already done, so simply perform a 32 bit add

;

of previously obtained multiplication results

;

ADD32

DPX,ACC

; ACC = A1*D(n-1)+A2*D(n-2)+(output of 1st

;

;

; save the upper 16 bits

of D(n) from the 32 bit accumulator

; left shift the result by 1, to adjust the decimal point after

; a Q15*Q15 multiplication

;

rlcf

ACC+B1,w

rlcf

ACC+B2,w

movwf

Dn

rlcf

ACC+B3,w

; decimal adjust ( mult by 2)

movwf

Dn+B1

;

; Compute B2 * [D(n) + D(n-2)]

;

if B0_EQUALS_B2

ADD16ACC Dn_2,Dn,AARG

; AARG = Dn + D(n-2) = multiplier

MOVFP16

Bx0,BARG

; BARG = A2 = multiplicand

call

DblMult

; (ACCd,ACCc) = B2*[D(n)+D(n-2)]

MOVPF32

DPX,ACC

else

MOVFP16

Bx0,BARG

MOVFP16

Dn,AARG

call

DblMult

; B0*D(n)

MOVPF32

DPX,ACC

MOVFP16

Bx2,BARG

MOVFP16

Dn_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)

;

movfp

Dn_1,AARG+B0

movpf

AARG+B0,Dn_2

; Shift down D(n-1)

movfp

Dn_1+B1,AARG+B1

movpf

AARG+B1,Dn_2+B1

; AARG = D(n-1) = multiplier

MOVFP16

Bx1,BARG

; BARG = B1 = multiplicand

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

;

ADD32

DPX,ACC

; ACC = B1*D(n-1) + B2*D(n-2) + B0*D(n)

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