Rx Elastic Buffer - Xilinx 7 Series User Manual

Chapter 4: Receiver
It is necessary to start the RX phase alignment after RX CDR is locked to ensure that the RX
recovered clock and RXUSRCLK are stable and ready to be used for alignment.
When the RX elastic buffer is bypassed, data received from the PMA can be distorted due to phase
differences after conditions such as a GTP transceiver reset or rate change. If the received data
evaluated at the fabric interface is invalid on any lane, the RX alignment procedure needs to be
repeated for the interface after RX CDR is locked on all lanes.

RX Elastic Buffer

Functional Description
The GTP transceiver 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.
RXUSRCLK.
X-Ref Target - Figure 4-43
Clock from
PLL0 or PLL1
RX
Clock
Dividers
RX EQ
SIPO
RX CDR
RX OOB
RX Serial
Clock
The GTP transceiver includes an RX elastic buffer to resolve differences between the XCLK and
RXUSRCLK domains. The phase of the two domains can also be matched by using the RX
recovered clock from the transceiver to drive RXUSRCLK and adjusting its phase to match XCLK
when the RX buffer is bypassed (see
of these approaches. The costs and benefits of each approach are shown in
186
Send Feedback
From TX Parallel To TX Parallel
Data (Near-End Data (Far-End
PCS Loopback) PMA Loopback)
Comma
Detect
Polarity
and
Align
PRBS
Checker
PMA Parallel
Clock
(XCLK)
Figure 4-43: RX Clock Domains
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Figure 4-43 shows the two parallel clock domains: XCLK and
RX PIPE
Control
RX Status
Control
8B/10B
Decoder
RX
Elastic
Buffer
RX Buffer Bypass, page 174). All RX datapaths must use one
7 Series FPGAs GTP Transceivers User Guide
To TX Parallel
Data (Far-End PCS
Loopback)
FPGA RX
Interface
RX
Gearbox
PCS Parallel FPGA Parallel
Clock Clock
(RXUSRCLK) (RXUSRCLK2)
UG482_c4_25_112911
Table 4-32.
UG482 (v1.9) December 19, 2016