DS00540B-page 8

© 1993 Microchip Technology Inc.

Implementing IIR Digital Filters

0001 B000

movlw

0x00

0002 0140

movwf

X

0003 B07F

movlw

0x7f

; set initial Xn = X(0) = 0x7f00

0004 0141

movwf

X+B1

; test for impulse response

;

NextPoint

0005 E022

call

IIR_Filter

0006 A442

tlwt

_LOW,Y

tracePoint

0007 AE43

tablwt

_HIGH,0,Y+B1

0008 0000

nop

0009 2940

clrf

X

; set X(n) = 0 , n <> 0

000A 2941

clrf

X+B1

; for simulating an Impulse

000B C005

goto

NextPoint

;

000C C00C

self

goto

self

;

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

;

initFilter

;

;

At first read the Filter Co-efficients from Prog. Mem to Data RAM

;

if

LPASS

000D B0C1

movlw

_coeff_lpass

000E 010D

movwf

tblptrl

000F B001

movlw

page

_coeff_lpass

0010 010E

movwf

tblptrh

endif

if

HPASS

movlw

_coeff_hpass

movwf

tblptrl

movlw

page

_coeff_hpass

movwf

tblptrh

endif

if

BPASS

movlw

_coeff_bpass

movwf

tblptrl

movlw

page

_coeff_bpass

movwf

tblptrh

endif

if

BSTOP

movlw

_coeff_bstop

movwf

tblptrl

movlw

page

_coeff_bstop

movwf

tblptrh

endif

;

0011 B02C

movlw

A11

0012 0101

movwf

fsr0

0013 8404

bsf

_fs0

0014 8D04

bcf

_fs1

; auto increment

;

; Read Filter Co-efficients from Program Memory

;

0015 B00A

movlw

NumCoeff

0016 A92C

tablrd

_LOW,_INC,A11

; garbage

NextCoeff

0017 A000

tlrd

_LOW,indf0

0018 AB00

tablrd

_HIGH,_INC,indf0

0019 1700

decfsz

wreg

001A C017

goto

NextCoeff

;

; Initilize “Dn”s to zero

;

001B B020

movlw

Dn1

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