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A-8

Viterbi Decoder Implementation

Basic Algorithm Program Listing

;***************************MAIN***********************************************

NUMSTATESequ

ENCBITS

equ

;most cases=log2(NUMSTATES)+1

NoOfAcsButtequ

NUMSTATES/2

NUMINPUTSequ

168

EVEN

equ

1-(NUMINPUTS/8)%2

;EVEN SET TO 1/0 IF NUMINPUTS IS

;

EVEN/ODD #BYTES

org

p:$400

VITDEC

move

#NUMSTATES/2-1,m2

;r2 points to branch metric table

move

#>3,n2

;increment for branch metric storage

move

#STATE1,r5

;r5 points to current state metric

move

#STATE2,r4

;r4 points to updated state metric

move

#NUMSTATES*2-1,m4

;both modulo to flip loc each sym

move

#NUMSTATES*2-1,m5

move

#NUMSTATES/2,n5

;input metrics spacing for each butter-

fly

move

#PATHOUT,n0

;n0 points to storage for output paths

move

#-1,m3

;set linear mode, traceback ptr

move

#INDATA,r1

;r1 points to input data

;***********************PREPARATION LOOP*************************

;

This loop iterates by the number of bits used in the

;

encoder to pre load the bit decisions.

Thereafter,

;

the paths are updated and stored off in bytes.

We need

;

the preload bits so that the stored path metrics point

;

correctly to their previous paths for traceback

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

#ENCBITS-1,PRELP

;once for each encoder bit

;

FindMetrics

ACS

PRELP

;*****************MAIN LOOP--PROCESS BYTES OF DATA*****************

#NUMINPUTS/8-1,DATALP

;process bytes of output

#8,SYMLP

;8 bits per byte

;

FindMetrics

ACS

SYMLP

STOREPATHS#NUMSTATES

DATALP

;********POSTPROCESSING,LAST INFO BYTE ********************************

;

; ENCBITS-1 PREPROCESSED, SO WE HAVE 8-ENCBITS+1 BITS LEFT.

; FOR THIS EXAMPLE, WE HAVE 3 INPUTS LEFT TO PROCESS

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

;

THE LAST THREE BITS------

#8-ENCBITS+1,FLSH1

;process last 3 bits to get last byte

FindMetrics

ACS

FLSH1

;*******Traceback the path data to obtain the decoder output**********

TRACEBACK

FIN

nop

Example A-1

Basic 16-Bit Implementation of a Viterbi Decoder (Continued)

Freescale Semiconductor, Inc.

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