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3-16

Viterbi Decoder Implementation

Expanding the Viterbi Algorithm

Traceback: Obtaining the Decoder Output

We begin the traceback in Example 3-4 by moving the path storage pointer to r0 and

initializing n0 for its use as an offset. Next we position r2 at the end of the buffer that will

contain the decoder output. Again, this is because we will traceback the path from the

end survivor path to the beginning. For our example, we can start at state 0. Start by

taking the current path for state 0 (r5 points to this) and reading into B. We put r2 in

linear addressing mode, then store the last path (the loop will read it again). Finally, we

load x0 with the control code ($513) to extract the pointer to the next traceback location

from the path.

The next section of code is used if the number of bytes of data is odd. It decodes the last

byte, and places it in the most significant byte of decoder output. As this code is like the

last half of the following do loop, discussion of this code is deferred.

The do loop processes the traceback two bytes at a time. In this way, it can assemble the

decoder output into 16-bit words for more efficient storage. Begin by reading the current

path into A1. Then, extract the pointer to the next (actually previous) path using extract.

Recall the control register contains $513, which means we take bits $13-$17 of A or,

equivalently, bits 3-7 of A1. Note that we can load $501d instead and the program will

work in 24-bit mode, although the example data provided would require 00 appended to

every data symbol to test correctly. As noted above, these most significant bits of the

path byte point to the storage address of the path continuation in the previous ÒpageÓ of

memory. By page, we mean the block of stored path bits for each state. There is one page

written each time we store paths (every 8 decoder input periods).

Next, move r0 to point to the previous page, then move the extracted path pointer bits to

n0 to be used in the next traceback read. We end processing of this least significant byte

by moving it to x1.

#NUMINPUTS/8/2,TRCBK

;once for each byte pair

move

x:(r0+n0),a

;recall last path

extractu x0,a,b

;get ptr to next earlier path

lua

(r0-NUMSTATES),r0

;point r0 to next earlier states

move

b0,n0

;save ptr as offset

move

a1,x1

;save out byte in x1

move

x:(r0+n0),a

;do it all again!

extractu x0,a,b

lua

(r0-NUMSTATES),r0

move

b0,n0

lsl

#8,a

;move to upper byte

x1,a

;or in last byte to get 16 bit word

move

a1,y:(r2)-

;store result

TRCBK

endm

Example 3-4

Traceback Output Path Code Listing (Continued)

Freescale Semiconductor, Inc.

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