DS00540B-page 4

© 1993 Microchip Technology Inc.

Implementing IIR Digital Filters

APPENDIX A: IIR.LST

MPASM B0.54

PAGE

1

Digital IIR Filter Using PIC17C42

TITLE

“Digital IIR Filter Using PIC17C42”

LIST

P=17C42, C=120, T=ON, R=DEC, N=0

;

include “17c42.h”

;

include “17c42.mac”

;

include “17c42iir.mac”

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

;

PIC17C42 MACRO

;

;

Macro For A Bi-Quad IIR Filter

;

2nd order Direct Form (Transposed) Type

;

; Filter co-efficients B0 & B2 are assumed equal

;

; The difference equations for each cascade section is given by :

;

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

;

D(n) = X(n) - A1*D(n-1) -

A2*D(n-2)

;

where X(n) = input sample, Y(n) = output of filter

;

and A1, A2, B0, B1, B2 are the Filter Co-efficients

;

;

The above difference equations are only for 1 section of a

; 2nd order Direct_Form II Filter structure (IIR)

;

;

NOTE :

;

It is possible to design the above structures

;

such that the co-efficients B0 = B2. If this is the

;

case,

;

Y(n) = B0*[D(n) + D(n-2)] = B2*[D(n) + D(n-2)]

;

This way, one multiplication can be avoided

;

;

If a 4th order filter is to be implemented, the output of

; the 1st structure should be input to the 2nd cascade section

;

;

Timing (WORST CASE) :

;

59+4*179 = 775 Cycles

;

(194 uS @ 16 Mhz)

;

Program Memory

:

;

63 locations

;

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

;

The sample filters are desined so that B0=B2

;

This saves 1 multiplication

;

0001

B0_EQUALS_B2

equ

TRUE

;

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

;

Parameters to BIQUAD Macro :

;

Filter Constants A1, A2, B0, B1, B2

;

& D(n), D(n-1), D(n-2), filter stage #

;

BIQUAD

MACRO

Ax1,Ax2,Bx0,Bx1,Dn,Dn_1,Dn_2,stage

;

; Compute Ax2*D(n-2)

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