General Description: F5/F6 MUX usage is similar to carry chains in that the place can only handle them if all elements are contained in a macro or all elements are not contained in a macro.
Problems arise when one of the muxes gets combined into a macro that is being constructed for reasons other than to align the muxes. In this case, one of the muxes was packed with a shift register; then, the placer built a macro to deal with the shift register functionality, leading to the error:
ERROR:Place:1802 - Components abc] and xyz are using the F5/F6MUX resources. The components should be in a single common RPM. One component is not in the RPM. Please either remove the constraints or correct them.
A workaround is to RLOC the three F5/F6 MUX pairs, or use some other form of map constraint to prevent the mux and shift register combination.