Horizontal Sample Point Shift - Xilinx Virtex-5 RocketIO GTP User Manual

Chapter 7: GTP Receiver (RX)

Horizontal Sample Point Shift

One advanced feature of the CDR of the GTP transceiver is the built-in horizontal
sampling point shift. During normal operation, the CDR finds the transition points in
incoming data and uses them to recover the frequency of the incoming clock.
The transition points are also used to select the optimal time to sample data. To minimize
the chance of error, the CDR attempts to sample data as far from transition points as
possible (that is, at the time when the bit value is most stable). This position is the center of
the data eye (see
A rough measure of the quality of the incoming signal, as seen by the CDR, can be obtained
by determining how close to the transition points of the eye the sampling point can be
before the number of received bit errors increases significantly.
example of a possible scan with an eye from clean data (good signal integrity) versus a scan
of bad data.
This functionality is supported through the PMA_CDR_SCAN attribute. Dynamic scans
are performed by changing PMA_CDR_SCAN using the DRP while receiving known data,
such as a PRBS pattern to determine the number of bit errors.
PMA_CDR_SCAN has an 8-bit field that controls the position of the data sampling point
relative to the first transition point in the eye. Resetting the field to 0 puts the sampling
point at the first transition point. Setting the field to 127 puts the data sampling point at the
180º point, the second transition point of the eye (see
field is 64, the nominal 90º sampling point.
There are three fields to accommodate the different possible PLL_RXDIVSEL_OUT
settings.
140
Figure 7-6).
0 Degrees
(Transition Point) (Nominal Sampling
Figure 7-6: Normal CDR Operation
Widest Scan Possible
Clean Data Eye
Figure 7-7: Scanning to Evaluate the Data Eye
Table 7-12 shows how to set PMA_CDR_SCAN for PLL_RXDIVSEL_OUT = 1.
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Data Eye
90 Degrees 180 Degrees
(Transition Point)
Point)
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Bit Errors Increase
in Transition Regions
Figure 7-6). The default value for each
Virtex-5 RocketIO GTP Transceiver User Guide
Figure 7-7 shows an
Narrower Scan
Bad Data Eye
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UG196 (v1.3) May 25, 2007
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