Description

A case has been seen where MAP appeared to be configuring a slice inefficiently because constant signals driving MUXCY direct inputs were sourced external to the slice instead of from the internal constant available in the CY0F/CY0G muxes which determine the DI pin connectivity. This occurred when one MUXCY DI pin was driven by GND and the other VCC. 

NOTE: This issue applies to Virtex, Virtex-E, and Spartan-II devices. This issue does not apply to Virtex-II devices.

Solution

This configuration is necessary due to a hardware limitation that both the CY0F/CY0G muxes in a slice must have the same configuration. This limitation dictates that unless both MUXCY DI pins in a slice are driven by the same constant value, then the CY0F/CY0G muxes must be configured for an external path.