This error message occurs because BitGen now performs a DRC check to ensure that all differential I/O pins have both the P and N side pins constrained. The Integrated Bit Error Ratio Tester(IBERT) currently only constrains the P side of the differential pair. To work around this issue, follow the solution below:
1. When generating the IBERT core in Core Generator, uncheck the Generate Bitstream option and generate the core.
2.Create the design.
3. Go to the "example_design" directory and open the "chipscope_ibert_top.ucf" file.
4. Find the "System Clock Timing Constraints" section and add a line to constrain the N side of the differential system clock pair. The UCF should now look something like:
INST "U_SYSCLOCK_IBUFDS" IOSTANDARD = LVDS_25;
NET "IBERT_SYSCLOCK_P_IPAD" LOC = AD12;
NET "IBERT_SYSCLOCK_ N_IPAD" LOC = AD11;
5. Make sure that you are locating the N side of the system clock to the appropriate pin for your differential pair.
6. Savethe ucf file and navigate to the "implement" directory. You can now run the implement.sh (for linux) or implement.bat (for windows) to run through an implementation and generate a bit file. Alternate Solution
If, for some reason, you do not want to use theLOC constraint on the N side of the system clock, you can alternatively use the solution below to work around this issue. It is highly recommended that you follow the solution above and use the LOC constraint on the N side of the clock manually.
1. Create the design.
2. Go to the "implement" directory and modify the "implement.sh" (for linux) or "implement.bat" (for windows) script to include the following option: -g UnconstrainedPins:Allow. TheBitGen command in the script file should look something like:
bitgen -d -g UnconstrainedPins:Allow -g GWE_cycle:Done -g GTS_cycle:Done
-g DriveDone:Yes -g StartupClk:Cclk -w example_chipscope_ibert_top.ncd
3. Run the implementation script.
This is a known issue that will be fixed in 13.4.
Please note that this BitGen issue can affect other parts of your design as well if you are only constraining the P side of a differential I/O pair. For more information on this BitGen issue, please see (Xilinx Answer 42678)