DSP56300 datasheet(88/108 Pages) FREESCALE

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B-10

Viterbi Decoder Implementation

Extended Algorithm Program Listing

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

;

NUMSTATESequ

ENCBITS

equ

;most cases=log2(NUMSTATES)+1

NoAcs

equ

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

#BRX,r2

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 locations each sym

move

#NUMSTATES*2-1,m5

move

#>1,n5

;ctr/input metrics spacing for each butterfly

;

move

#NUMSTATES/2,n5

;input metrics spacing for each butterfly

move

#PATHOUT,n0

;n0 points to storage for output paths

move

#-1,m3

;set linear mode, traceback ptr

move

#>1,n2

;

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

;preload decisions/trellis start

;

FindMetrics

PreACS

;

ACS

PRELP

move

#NUMSTATES/2,n5

;n5 now serves as offset between fetch

;

states

;

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

#NUMINPUTS/8-2,DATALP;process bytes of output

#8,SYMLP

;8 bits per byte

;

FindMetrics

ACS

SYMLP

STOREPATHS#NUMSTATES

DATALP

Example B-1

Extended Algorithm Program Listing (Continued)

Freescale Semiconductor, Inc.

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