L64777 A LSI Logic Corporation, L64777 A Datasheet - Page 10

• Manufacturer Part Number: L64777 A
• Description: Communications, DVB QAM Modulator
• Manufacturer: LSI Logic Corporation

Sync Acquisition Phase
Sync Tracking Phase
FIFO Clock Conversion
10
In the sync acquisition phase, the number of sync detections required for
sync and loss is programmable from 3 to 5. TS is the designation for the
number of track steps. After TS error-free consecutive detections of the
sync byte S at the correct locations, the L64777 declares synchroniza-
tion; if a mismatch occurs, it goes back to the search state.
Validating the detection of the sync word three times ensures a
probability of false alarm equal to P
detection of the sync word five times insures a probability of false alarm
equal to P
The sync tracking phase checks the detection of S at the correct location
(i.e., every P bytes). TS
mismatch the L64777 declares a loss-of-sync and goes back to state S0
to look for new synchronization.
The L64777 uses a dual-ported RAM to implement the circular buffer
FIFO function. The circular buffer has a write pointer driven by ICLK and
a read pointer driven by the Symbol clock, OCLK/4. The device does not
prevent collisions of the pointers; rather, the PLL-VCO follow-up time and
proper initial setup of the pointer distance must guarantee this.
For FIFO initialization, the L64777 loads a user-programmable pointer
distance of 0 to 127 cycles (the FIFO delay value of Register 2 in
Group 2) into the read address pointer (after each microprocessor delay
register access) and sets the write address pointer to zero (see
Figure
ICLK frequency relationship determines how the read and write pointers
advance.
To allow outside watching of the asynchronous pointers, an alarm
comparator indicates when both pointers are equal. Because both
counters are Gray Code counters (in which changes occur only in one
bit) the spikes and glitches of the asynchronous signals are minimized.
L64777 DVB QAM Modulator
7). After this initialization, both pointers run free, and the OCLK to
fa
= (2
8
)
5
= 9*10
1 mismatches are tolerated, but at the last
13
.
fa
= (2
8
)
3
= 6*10
8
. Validating the