Xilinx Virtex-6 FPGA User Manual page 260

Chapter 4: Receiver
For the block synchronization state machine shown in
sh_invalid_cnt_max are both constants that are set to 64 and 16, respectively. From the
VALID_SH state, if sh_cnt is less than the value sh_cnt_max and test_sh is High, the next
state is TEST_SH. If sh_cnt is equal to sh_cnt_max and sh_invalid_cnt equals 0, the next
state is GOOD_64 and from there block_lock is asserted. Then the process repeats again
and the counters are cleared to zeros. To achieve block lock, the state machine must receive
sh_cnt_max number of valid synchronization headers in a row without getting an invalid
synchronization header. However, when block lock is achieved, sh_invalid_cnt_max – 1
number of invalid synchronization headers can be received within sh_cnt_max number of
valid synchronization headers. Thus, once locked, it is harder to break lock.
Figure 4-48
RXGEARBOXSLIP numerous times because of invalid synchronization headers before
achieving data alignment.
X-Ref Target - Figure 4-48
block_sync_i
unscrambled_data_i
begin_r
track_data_r
start_of_packet_detected_r
RX_DATA
RXDATAVALID0
RXGEARBOXSLIP0
RXHEADER0
RXHEADERVALID0
RXSTARTOFSEQ0
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260
shows a waveform of the block synchronization state machine asserting
00000000
00000000
Closely spaced slip pulses. State machine
asserts slip as soon as it sees bad header.
0
Figure 4-48: RX Gearbox with Block Synchronization
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Figure 4-47, sh_cnt_max and
Data check on Data alignment
unscrambled data
Slip data
alignment
Virtex-6 FPGA GTX Transceivers User Guide
UG366 (v2.5) January 17, 2011
achieved
UG366_c4_45_051509