Rx Elastic Buffer; Functional Description - Xilinx Virtex-6 FPGA User Manual

Chapter 4: Receiver

RX Elastic Buffer

Functional Description

The GTX RX datapath has two internal parallel clock domains used in the PCS: the PMA
parallel clock domain (XCLK) and the RXUSRCLK domain. To receive data, the PMA
parallel rate must be sufficiently close to the RXUSRCLK rate, and all phase differences
between the two domains must be resolved.
domains, XCLK and RXUSRCLK.
X-Ref Target - Figure 4-34
RX Serial Clock
D
RX
F
EQ
E RX
SIPO
CDR
RX
OOB
Shared
PMA
PLL
Divider
RX-PMA
From TX Parallel
Data (Near-End
PCS Loopback)
The GTX transceiver includes an RX elastic buffer to resolve differences between the
PMACLK and RXUSRCLK domains. The phase of the two domains can also be matched
by using the recovered clock from the transceiver to drive RXUSRCLK and adjusting its
phase to match XCLK (see
these approaches. The costs and benefits of each approach are shown in
Table 4-42: Buffering vs. Phase Alignment
Clocking Options
Initialization
Latency
Clock Correction /
Channel Bonding
www.BDTIC.com/XILINX
238
PMA Parallel Clock
(XCLK)
Over-
sampling
Polarity
PRBS
Checker
RX-PCS
To TX Parallel
Data (Far-End
PMA Loopback)
Figure 4-34: RX Clock Domain
RX Buffer Bypass, page
RX Elastic Buffer
Can use recovered clock or local clock (with
clock correction)
Works immediately
Buffer latency depends on features used (clock
correction and channel bonding)
Required for clock correction / channel
bonding
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Figure 4-34
Comma
Detect
and
Align
10B/8B
Decoder
Loss of Sync
RX Status Control
231). All RX datapaths must use one of
Must use recovered clock
Must wait for all clocks to stabilize then
perform alignment procedure
Lower deterministic latency than using the
RX elastic buffer
Virtex-6 FPGA GTX Transceivers User Guide
shows the two parallel clock
FPGA
Parallel Clock
(RXUSRCLK2)
PCS Parallel
Clock
(RXUSRCLK)
RX
Elastic
RX
Buffer
Gearbox
FPGA
Interface
RX PIPE Control
To TX Parallel
Data (Far-End)
PCS Loopback)
UG366_c4_31_011111
Table 4-42.
RX Phase Alignment
UG366 (v2.5) January 17, 2011
RX