DSP56300 datasheet(45/108 Pages) FREESCALE

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Expanding the Viterbi Algorithm

Main: Gluing the Pieces Together

Viterbi Decoder Implementation

3-17

Again, we read the next (previous) path into A1 using r0, which points to the previous

page, and n0, which points to the correct path in that page. We extract the new path

pointer information, update r0 to the next previous page, and move the path pointer to

n0. Move the path data to the upper byte, OR in the lower byte from x1, and move the

resulting word into memory using r2. The loop continues until we have traced back to

the beginning, and y:DECOUT points to the start of the decoder output.

3.7

MAIN: GLUING THE PIECES TOGETHER

The only thing left is to put the pieces together. In Examples 3-5 through 3-7, we present

the main routine. Its function is to initialize registers so everything starts properly, and

to invoke the macros at the needed time.

Example 3-5

begins the Viterbi decoding routine by setting equates for the encoder size

and input data length, and initializing address registers. NUMSTATES is set to the

number of encoder states, which for our example is 32. ENCBITS is the number of bits

used to encode. This includes the input bit as well as the state bits. NoOfAcsButt sets the

number of ACS butterflies for the butterfly loop. For our rate 1/2 code, this is

automatically set to half the number of states. The EVEN flag is used by the assembler to

include an extra half-byte of traceback if the number of input bytes of data is odd.

Example 3-5

Main Viterbi Decoding Routine: Initialization

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

;

NUMSTATESequ

ENCBITS

equ

;most cases=log2(NUMSTATES)+1

NoOfAcsButt

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

#>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 butterfly

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

;

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