Configuring The Pcs Internal Datapath And Clocks; Activating And Operating The Oversampling Block - Xilinx Virtex-5 RocketIO GTP User Manual

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Configuring the PCS Internal Datapath and Clocks

Figure 7-9
The RX serial clock runs at the PMA line rate calculated previously. The XCLK runs at the
resulting parallel rate for a 10-bit datapath, PMALineRate/10. The oversampled data from
the SIPO is fed into the oversampling block at the XCLK rate.
RX Serial Clock PMA Parallel
RX RX
SIPO
EQ CDR
PMA
PLL
Divider
From PMA PLL
RX-PMA
Figure 7-9: RX Clock Domains When Using Built-In Oversampling
The oversampling block produces 2 bits of data for every 10 bits received. This data is fed
into the remaining PCS datapath at the required clock rate for the desired line rate, which
is given by
used for RXUSRCLK2, depending on the selected RX datapath width.
When oversampling is used, the oversampled line rate, not the PMA line rate, and the PCS
internal datapath width, which is set by INTDATAWIDTH, must be used for RXUSRCLK
calculations.
between RXUSRCLK and RXUSRCLK2.

Activating and Operating the Oversampling Block

After the PMA line rate and PCS datapath are set, the oversampling block can be enabled
by setting OVERSAMPLING_MODE to TRUE. This attribute affects both transceivers in
the GTP_DUAL tile.
The oversampling block includes a small buffer to hold data before passing it to the PCS.
This buffer can overflow/underflow if the PMA and PCS frequencies are different, such as
if RXUSRCLK stopped temporarily due to other events in the system. If an error in the
oversampling block buffer occurs, the transceiver asserts the RXOVERSAMPLEERR
signal. This error can be cleared by asserting RXRESET or RXCDRRESET.
RXENSAMPLEALIGN is tied High so the oversampling block always attempts to find the
best possible recovered clock and sample point in the incoming data.
Virtex-5 RocketIO GTP Transceiver User Guide
UG196 (v1.3) May 25, 2007
shows the clock domains of the GTP RX datapath when oversampling is used.
Clock
(XCLK)
Over-
Polarity
Sampling
PRBS
Check
RX-PCS
Equation 7-5. This PCS rate is used for RXUSRCLK. A multiple of the rate is
"FPGA RX Interface," page 182
www.xilinx.com
Oversampling Block
Recovered Clock
(RXRECCLK)
Comma
10B
Detect
/
&
8B
Align
Loss of Sync
RX Status Control
RX Pipe Control
provides more details about the relationship
Oversampling
RX Interface
Parallel Clock
(RXUSRCLK2)
PCS Parallel
Clock
(RXUSRCLK)
Elastic
Buffer
FPGA
Logic
UG196_c7_09_102306
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