I have a design that uses on-chip LVDS differential termination. The Virtex-4 User's Guide states that I should use the DIFF_TERM attribute to achieve this. When I do that, however, I receive the following error:
"WARNING:ConstraintSystem:11 - In file: signalconversion.ucf(2): Loading a 'DIFF_TERM' constraint from file is not supported at this time. This constraint specification will be ignored."
This appears to be a major issue because I NEED the differential termination.
A sample section from my UCF file is:
NET "adcIn_N<0>" LOC = "D17" | IOSTANDARD = LVDS_25 | DIFF_TERM="TRUE";
NET "adcIn_P<0>" LOC = "C17" | IOSTANDARD = LVDS_25 | DIFF_TERM="TRUE";
How do I use the DIFF_TERM?
The DIFF_TERM is not allowed to be used in UCF now. Try to instance IBUFDSand add attribute in the code other than use UCF. For example:
.DIFF_TERM("FALSE"), // Differential Termination (Virtex-4 only)
.IOSTANDARD("DEFAULT") // Specify the input I/O standard
) IBUFDS_inst (
.O(O), // Clock buffer output
.I(I), // Diff_p clock buffer input (connect directly to top-level port)
.IB(IB) // Diff_n clock buffer input (connect directly to top-level port)
// End of IBUFDS_inst instantiation
Please refer to the Library Guide or (Xilinx Answer 23579) for more Information.