DSP56300 datasheet(96/108 Pages) FREESCALE

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C-4

Viterbi Decoder Implementation

24-Bit Algorithm Program Listing

Example C-1

24-bit Algorithm Program Listing

OPT

mex

;****************56600**24 bit VITERBI DECODER*****************************

;

THIS ROUTINE IMPLEMENTS A CONVOLUTIONAL DECODER USING THE VITERBI ALG.

;

IT IS OPTIMIZED FOR SPEED, WHICH MEANS THAT EVERY ALU REGISTER

;

AND ALL OF THE R REGISTERS ARE USED.

A SIGNIFICANT AMOUNT OF

;

THESE CAN BE FREED BY STORING AND REUSING REGISTERS BETWEEN THE

;

FINDMETRICS ROUTINES AND THE ACS,ACSFlush ROUTINES.

;

INPUT is at INDATA: real in x imag in y.

OUTPUT begins at y:DECOUT

;

GLOBAL REGISTER USE: a,b,x,y,r012345,n025,m245(are set to a modulo mode)

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

;

;*****************BRANCH METRIC MACRO*************************************

;

FUNCTION: Input data and generate branch metrics.

This function

;

subtracts the path metric for state 0 from all branch metrics

;

to provide autonormaliztion. For this decoder, the metric is a scaled

;

sum or difference of the real and imag inputs.

;

INPUTS:

;

r2 should point to the beginning of the branch metric table

;

r5 should point to the latest path metric for state 0

;

r1 should point to the next input XY data pair

;

OUTPUTS:

;

Branch metrics are stored at BRX in XY memory

;

REGISTERS USED:

;

a,b,x01,y01,r1,r2,r5, r5 unchanged,r2 unchanged (modulo req'd)

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

;

FindMetricsmacro

move

x:(r5),a

;st. metric scaling, read last st. 0

neg

l:(r1)+,y ;negate metric|grab dec input

move

#-16,x1

;sign for real component, upper br.

mac

x1,y1,a

a,b

move

y1,x0

;cp metric to b|mv real in to x0

mac

x1,y0,a

a,y1

;y1 gets 0x partial branch

subl

a,b

b,x0

;a has 00 br., b has 11 branch, save st.

tfr

y1,a

a,y1

;swap 0x and 00 to compute 01 branch

mac

-x1,y0,a

b,x1

;a gets 01 br,x1 gets cp of 11 br.

tfr

x0,b

b,y0

;swap st metric,11 branch to y0

subl

a,b

y1,x0

;b gets 10 br, x0 has copy of 00 br.

Freescale Semiconductor, Inc.

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