The data inputs (I0 & I1) of the MUXF6 can come only from the outputs of a MUXF5. This is a physical limitation of using the MUXF6. If the data inputs of a MUXF6 do not come from a MUXF5, a user will receive a message from map(A1.5p) of the following form:
"ERROR: xvkma F6MUX <instance name of muxf6> fed from illegal source through net/signal: <netname that does not originate from muxf5> (only legal sources are F5MUXes)"
The select input for the MUXF6 doe not have to originate from a muxf5.
If you run into this problem from a synthesized HDL design, which does not instantiate the MUXF6, the synthesis tool has incorrectly inferred the use of a MUXF6. Contact your synthesis vendor for help.
If you run into this problem with a synthesized(HDL) or schematic design,
and you have instantiated the MUXF6, there two several options:
(1) The easiest option is to remove the MUXF6 altogether and replace it with either a MUXF5, or RTL.
(2) Instead of directly connecting a signal to the inputs of a MUXF6, indirectly
connect it to the MUXF6 by routing the signal through a MUXF5 first.