Here's something germain: http://www.embedded-computing.com/pdfs/Xilinx.Sep05.pdf It mentions Virtex-4 though...something called ChipSync is what Xilinx brags about doing > 1GBps data... with differential DCM clock output. Spartan3 could be in trouble needing external help. adrian.cosoroaba at xilinx.com was the author... John G