Forget the RAMs, I can't get Quartus to use the cascade chains!

I got my issues with the rams worked out and I belive I have an inferred
design that will work in both Spartan 3 and ACEX chips with few
changes.  I am trying to compile this design in Quartus II 3.0 and need
for the software to use the cascade chain to provide fast bus muxes with
lots of inputs.  

Right now my critical speed path (and one of the LE hogs) is a four
input mux which I expect to be able to do in 2*n (where n is the data
bus width) LEs.  Each bus has a select signal which is gated with all
the bits of that bus and then all of the buses are logically OR'd to
complete the mux.  This OR can be done by the cascade chain if the logic
is inverted appropriately.  Rather than depend on the software to invert
all this for me, I coded the mux as an AND of ORs rather than the normal
OR of ANDs.  Instead of using the simple structure I am trying to get,
it is use short cascades, but not in a regular way to minimize logic and
delays.  

Other than instantiation, is there a good way to "encourage" the Quartus
tools to use the cascade chains in a structured way?  If I instantiate
the cascade backbone, how do I get that to simulate in Modelsim?  

-- 

Rick "rickman" Collins

rick.collins@XYarius.com
Ignore the reply address. To email me use the above address with the XY
removed.

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