DS00540B-page 6

© 1993 Microchip Technology Inc.

Implementing IIR Digital Filters

;

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

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

;

MOV16

Dn,Dn_1

; Shift down D(n) to D(n-1)

;

ENDM

;

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

;

Second Order Direct Form IIR Filter

;

;

; In the code given below, a 4th order IIR Elliptic Lowpass Filter

; is implemented. Other order filters may be implemented by

; taking the following example code as a basis.

;

;

The specifications of the filter are :

;

;

Sampling Frequency = 2.0 Khz

;

;

Filter Type = 4th Order Elliptic Lowpass Filter

;

;

Band1

Band2

;

Lower Band Edge

0.0

600 Hz

;

Upper Band Edge

500 Hz

1 Khz

;

Nominal Gain

1.0

0.0

;

Nominal Ripple

0.01

0.05

;

Maximum Ripple

0.00906

0.04601

;

Ripple in dB

0.07830

-26.75

;

;

The Filter Co-efficients for the above specifications

; of the filter are computed as follows :

;

;

1st Section :

;

A11 = -0.133331

;

A12 = 0.167145

;

B10 = 0.285431

;

B11 = 0.462921

;

B12 = 0.285431

;

2nd Section

;

A21 = 0.147827

;

A22 = 0.765900

;

B20 = 0.698273

;

B21 = 0.499908

;

B22 = 0.698273

;

;

;

Performance (WORST Case):

;

;

Cycles

= #of Filter Stages*775 + 16

;

= 2*775+16 = 1566 Cycles

;

( 391 uSec)

;

per each sample. Initialization

;

time after reset is not counted

;

Timing measured with B0_EQUALS_B2

;

set to TRUE (see BIQUAD Macro for

;

explanation

;

;

Program Memory :

;

= 16+ # of FilterStages * (BIQUAD

;

+ filter co-efficients)

;

+ multiplier

;

= 16+2*(63+5)+274 = 421 locations

;

(excluding initialization)

;

;

RAM usage = 48 file registers

;

RAM usage/each additional stage = 16 file regs

;

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