IO standards vs instantiated primitives

As I said in the previous post, I took the IO specifications from the reference design included with the ML605. What I wasn't expecting was that these IO specifications have to work with specific Xilinx primitives.

Here is an example:
DIFF_SSTL15_T_DCI = IOBUFDS_DIFF_OUT

IOBUFDS_DIFF_OUT took a while to find as it doesn't appear in the Virtex 6 HDL Libraries Guide. It does appear in the Virtex 6 FPGA SelectIO Resources User Guide (ug361.pdf) with a caveat that it is for experienced Xilinx designers for usage in DDR2 / DDR3 applications.

I have yet to figure out why you need a separate tristate for this primitive, but hopefully I'll find that answer as I move along on this project.

Note: When you use a DCI IO standard, you must adhere to the DCI rules. For the ML605, you have to put the CONFIG_CASCADE constraint in so that ISE will successfully generate the bitstream:

//For ML605

CONFIG DCI_CASCADE = "36 35";

CONFIG DCI_CASCADE = "26 25";