DSP56300 datasheet(43/108 Pages) FREESCALE

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Expanding the Viterbi Algorithm

Traceback: Obtaining the Decoder Output

Viterbi Decoder Implementation

3-15

For each path stored, then, we start by reading in the path. We move the path 5 bits to

the right, so that A1 has the byte of path data to store right aligned. We mask off all but

the 5 rightmost bits in B1 to clear the stored path bits. The loop finishes by writing both

the path data to be stored and the updated path for the path metric.

The macro finishes by setting r5 for use in the next butterfly, as well as updating N0 to

point to the next page of stored path data during the next storage interval.

Example 3-4

Traceback Output Path Code Listing

;***********************TRACEBACK OUTPUT PATH MACRO*******************

;

FUNCTION: To output the correct data, we begin at the end. We take the

;

output path of the survivor state (0), and place its associated

;

output path in memory as the last output data byte. Then we use

;

bits 3-7 of that data as an offset pointer to the correct traceback

;

data of the next previous path data memory. We continue this until

;

we have traced the data back to the beginning.

;

INPUTS:

;

n0 should point to memory where the path bits are to be stored

;

r5 should point to the latest path metric for state 0

;

OUTPUTS:

;

Decoder output data in Y memory

;

REGISTERS USED:

;

a,b,y,r0,n0,r2,r5

;

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

;

Store off path data in bytes to avoid overflow in path Registers

;

TRACEBACK macro

;

move

N0,R0

;path ptr to r0

move

#0,n0

;prep for traceback

move

#DECOUT+(NUMINPUTS/8/2),r2

;point to end to trace data

move

y:(r5),b

;recall last path

move

#-1,m2

;r2 now linear

move

b1,x:(r0)

;save off last path data

move

#$513,x0

;control word for extract, 16 bit

;

move

#$501d,x0

;control word for extract, 24 bit

;

;*****************BEGIN TRACEBACK*****************************

;

IF (EVEN==0)

move

x:(r0+n0),a

;recall last path

extractu x0,a,b

;bits 3-7 of a1 point to next data

lua

(r0-NUMSTATES),r0

;dec r0 to next earlier state set

lsl

#8,a

;move to upper byte

move

b0,n0

;load as offset for traceback

move

a1,y:(r2)-

;save off

ENDIF

;

Freescale Semiconductor, Inc.

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