Route will issue this error when attempting to route a design:
ERROR:Route:471 -This design is unrouteable. Router will not continue. To evaluate the problem please use fpga_editor. The nets listed below can not berouted:
Where the unroutable net mentioned in the error message is a branched carry chain net with COUT driving multiple CIN loads.
How can I resolve this issue?
This is a known issue and it has been resolved in the next major release of ISE Design Suite, 12.4.
Contact Xilinx Technical Support via a WebCase for a patch that will resolve the issue for 12.3.
Note: This patch only addresses the case where Pack fails to reserve the AX pin for branched carry chains (COUT with multiple CIN loads) so that the AX pin can be used as a route-thru for non-aligned carry connections. This patch does not addressAX pin reservation for broken carry chains (COUT with single non-aligned CIN load) which is a similar but different issue. There is no patch available for broken carry chains, but a fix is scheduled for ISE version 12.4. The workaround is to prevent the break (taller area group range?) or to use pack constraints to reserve the AX pin.