STV0196 datasheet(12/23 Pages) STMICROELECTRONICS

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IX - VITERBI DECODER AND SYNCHRONIZATION (continued)

FUNCTIONAL DESCRIPTION (continued)

Other Registers

VSEARCH

Internal Address : Hex06

A/M

SN [1..0]

TO [1..0]

H [1..0]

A/M

: Automatic/manual

: Freeze

SN [1..0] : Averaging period. It gives the number of

bits required to calculate the rate error :

SN [1..0]

Number of bits

1.024

4.096

16.384

65.536

Reset Value : SN=01 (4096 bits)

The SN[1..0] bits also inpacts the C/N

indicator (see paragraph VII).

TO [1..0] : Time out value. It programs the

maximum durationof the synchro word

research in automatic mode; if no sync

is found within this duration, the phase

is incremented.

TO [1..0]

Time out

(in 1024 bit periods)

128

Reset Value : TO=10 (64K bit periods).

H [1..0]

: Hysteresis value. It programs the

maximum value of the Sync counter.

Th e uni t is t he bl oc k dur at i o n

(204 bytes in Mode A).

H [1..0]

Sync Counter max value

(in blocks periods)

forbidden value

128

Reset Value : H=01 (32 blocks).

In Mode A, the sync word is 47hex and it is com-

plemented to B8hex for every 8th block.

An Up/Down Sync counter counts whenever a sync

word is recognized with the good timing, and

counts down for each missing sync word ; this

counter is bounded by a programmable maximum

value; when this value is reached, the LK bit

(”locked”) is set in VSTATUS register; when the

event counter counts down until 0, this flag is reset.

VSEARCH bit 7 (A/M) and bit 6 (F) programs the

automatic/manual(or computer aided) search mode :

- if A/M =0 and F=0 : automatic mode; successive

enabled punctured rates are tried with all possible

phases, until the system is locked and the block

synchro found ; this is the default (reset) mode.

- if A/M=0 and F=1, the current puncture rate is

frozen, if no sync is found, the phase is incre-

mented, but not the rate number; this mode al-

lows to shorten the recovery time in case of noisy

conditions: the puncture rate is not supposed to

change in a given channel.

In a typical computer aided implementation, the re-

search begins in automatic mode; the micro reads

the error rate or the PRF flag in order to detect the

captureof a signal; thenit switches F to 1, until a new

channel is requested by the remote control.

- if AM=1 : manual mode; in this case, only one

puncture rate should be validated, the system is

forced to this rate, on the current phase, ignoring the

time-out registerandtheerror rate;in this mode, each

0 to 1 transition of the bit F leads to a phase incre-

mentation, allowing full control of the operation by an

external micro by choosing the lowest error rate :

ResetValue: A/M=0, andF=0;automaticsearchmode

VERROR (Read only register)

Internal Address : Hex07

ERROR RATE

At any time, the last value of the error rate may be

read in this register (unlike VTH, the possible range

is 0 to 255/256).

VSTATUS (Read only register)

Internal Address : Hex08

PRF

PR [2..0]

: Carrier Found flag (see carrier recovery)

CF when set, indicates that a QPSK

signal is present at the input of the

Viterbi decoder.

PRF

: Puncture Rate Found

P RF indicat es t he sta te of th e

pu n ct ure rat e re search : 0 fo r

searching, 1 when found ; this bit is

irrelevant in manual mode.

: Locked/searchingthe sync word

LK indicates the state of the sync word

research: 0 for searching, 1 when found.

PR [2..0] : Current Puncture Rate

It hold the current puncture rate indice

with the correspondance :

Punctured Rate

Regiter Value PR[2..0]

Basic 1/2

100

Punctured 2/3

000

Punctured 3/4

001

Punctured 5/6

010

Punctured 7/8 (Mode A)

or 6/7 (Mode B)

011

STV0196B

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