STV0196B Datasheet, PDF (12/23 Pages) STMicroelectronics – QPSK/BPSK DEMODULATOR AND FEC IC

STV0196B
FUNCTIONAL DESCRIPTION (continued)
IX - VITERBI DECODER AND SYNCHRONIZATION (continued)
Other Registers
VSEARCH
Internal Address : Hex06
A/M
F
SN [1..0] TO [1..0] H [1..0]
A/M
: Automatic/manual
F
: Freeze
SN [1..0] : Averaging period. It gives the number of
bits required to calculate the rate error :
SN [1..0]
Number of bits
00
1.024
01
4.096
10
16.384
11
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)
00
16
01
32
10
64
11
128
Reset Value : TO=10 (64K bit periods).
H [1..0]
: Hysteresis value. It programs the
maximum value of the Sync counter.
The unit is the block duration
(204 bytes in Mode A).
H [1..0]
Sync Counter max value
(in blocks periods)
00
forbidden value
01
32
10
64
11
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
channelis 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
CF 0
0 PRF LK
PR [2..0]
CF
: 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
PRF indicates the state of the
puncture rate research : 0 for
searching, 1 when found ; this bit is
irrelevant in manual mode.
LK
: Locked/searchingthe sync word
LK indicates the state of the sync word
research: 0 forsearching, 1 when found.
PR [2..0] : Current Puncture Rate
It hold the current puncture rate indice
with the correspondance :
Punctured Rate
Basic 1/2
Punctured 2/3
Punctured 3/4
Punctured 5/6
Punctured 7/8 (Mode A)
or 6/7 (Mode B)
Regiter Value PR[2..0]
100
000
001
010
011
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