ModelSim SE User's Manual - Electrical and Computer Engineering

Simulating VHDL designs UM-81
In this example you have two synchronous processes, one triggered with clk and the other
with clk2. To your surprise, the signals change in the clk2 process on the same edge as they
are set in the clk process. As a result, the value of inp appears at s1 rather than s0.
During simulation an event on clk occurs (from the testbench). From this event ModelSim
performs the "clk2 Runtime Options menu or by modifying the IterationLimit (UM-532)
variable in the modelsim.ini. See "Preference variables located in INI files" (UM-524) for
more information on modifying the modelsim.ini file.
If the problem persists, look for zero-delay loops. Run the simulation and look at the source
code when the error occurs. Use the step button to step through the code and see which
signals or variables are continuously oscillating. Two common causes are a loop that has
no exit, or a series of gates with zero delay where the outputs are connected back to the
inputs.
ModelSim SE User’s Manual