Xilinx Virtex-6 FPGA User Manual page 147

Even though the Virtex-6 FPGA package pinout eliminates the crosstalk between the
SelectIO interface and the GTH transceiver pins due to pin adjacency, it is still possible to
induce crosstalk on the PCB. Therefore, when routing signals on the PCB:
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Virtex-6 FPGA GTH Transceivers User Guide
UG371 (v2.0) February 16, 2010
Routing of GTH transceiver signals and SelectIO interface signals on adjacent layers
should be eliminated. If these signals are routed on adjacent layers there is potential
for broadside coupling.
Isolation of the return current paths should be maintained for both the SelectIO
interface signals and the GTH transceiver signals, including both traces and vias.
SelectIO interface signals should not be routed over or through the GTH transceiver
power islands. The power islands for the GTH transceivers are also a potential source
for SelectIO interface induced noise.
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SelectIO Interface Usage Guidelines
147