AR# 8825: 4.1i Virtex MAP - Optimized XORCYs make the carry chain difficult to route.
4.1i Virtex MAP - Optimized XORCYs make the carry chain difficult to route.
Keywords: route, unrouted, carry, XORCY
General Description: A case has been seen where a number of XORCYs in this design are being optimized out because their LI pins are being optimized to GND. The result is that MAP is creating COUT-to-LUT connections that are difficult for PAR to route successfully. This problem can lead to unrouted nets.
In the first major release after 4.1i, a change is planned wherein the XORCY will be kept in order to correctly terminate the carry chain.