Why do I see incorrect simulation results when using ModelSim version 6.2 and above?

You may see incorrect simulation results when simulating Quartus® II software generated netlists in the  ModelSim® SE or PE or QuestaSim™ software versions 6.2 and 6.3f, because there is an optimization problem.

For the ModelSim and QuestaSim software v6.2 use one of the following methods to disable the optimization and work around this problem:

• Edit the VoptFlow setting in your modelsim.ini:
  VoptFlow = 0;  (default is 1)
  (Quartus II NativeLink may overwrite the modelsim.ini file)
  
• Specify the -novopt; option on the vsim command line:
  vsim -novopt work.rtl_conf
  
• In the the ModelSim or QuestaSim GUI, turn off Optimization in the Start Simulation window
  

For the ModelSim and QuestaSim software v6.3f, there is a problem with the Altera® ALTSYNCRAM model.

To workaround the problem, turn off optimization when compiling the Altera VHDL libraries and use the -opt=-clkOpt ; with the vcom command. For example:

vcom -opt=-clkOpt -work altera_mf altera_mf.vhd;

Note:  If you start a simulation by double-clicking the top-level file from the workspace viewer, ModelSim runs optimization by default. For this reason, you should start simulations via the simulation menu or the command line.