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AR# 8825

4.1i Virtex MAP - Optimized XORCYs make the carry chain difficult to route.

Description

Keywords: route, unrouted, carry, XORCY

Urgency: Standard

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.

Solution

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.
AR# 8825
Date Created 03/15/2000
Last Updated 06/13/2002
Status Archive
Type General Article