I was trying to add internal device differential termination to my design over pre-existing differential pairs.
I was following the 7 Series SelectIO Resource Guide which said:
"The DIFF_TERM attribute can be specified in the UCF constraints file or by setting the appropriate value in the generic map (VHDL) or in-line parameter (Verilog) of the instantiated IBUFDS, IBUFGDS, IBUFDS_DIFF_OUT, or IOBUFDS_DIFF_OUT primitives."
I tried adding the DIFF_TERM as an INST in the Constraints file but I didn't see anything happen in the .pad report. Turns out that this is a bug and you can actually set DIFF term in the UCF, the .pad report won't ever report it. I found:
http://forums.xilinx.com/t5/7-Series-FPGAs/How-do-I-know-if-DIFF-TERM-is-set-to-be-TRUE-correctly-in-Vivado/td-p/454472
and another link, which both suggested to check the pad's properties to see if it was checked.
I also confirmed that for the pins I did NOT add DIFF_TERM, they did not have the box checked.
Lastly, when I added the DIFF_TERM I made a new entry with INST replacing net for the same name:
http://forums.xilinx.com/t5/Timing-Analysis/NET-vs-INST/td-p/218287
