Debugging Iteration Limit error in VHDL Modelsim

Question

I'm writing VHDL code for a d-flip-flop on Modelsim and I get an error when I try to simulate it:

Error: (vsim-3601) Iteration limit reached at time 400 ps.

I'm not sure what it means, but I've looked through much of my source code for errors to no success. Can anyone guess what the problem might be?

Answer 1

You need to add breakpoints in you code and single step until you see the loop. Another technique, maybe more productive, is a good code review with a close look at you iterations and sensitivity lists.

Answer 2

If your iteration limit is reached, that means the system hasn't stabilized. Most likely it is something like:

a <= b;

--- and then later...

b <= a;

Answer 3

One problem most people have with VHDL or any other HDL languages is that they do not understand that this is not a sequential code. EVERYTHING you have inside a process happens in parallel. The example from Ahmed is a good one:

PROCESS (count)

BEGIN

count <= not count;

END PROCESS;

An HDL simulator tries to set the value of the count to "not count" after each simulation tick, and the change will trigger another tick since the value of the count is changed and it keeps going on until it either crashes or gives you the above problem.

For an HDL to work properly you must use delays, either in form of a clock or if it is not for synthesis, to use an actual valued delay.

By changing the above code to

PROCESS (count)
BEGIN
   count <= not count after 1 ns;
END PROCESS;

The simulation will work and the count will toggle every 1 ns.

Answer 4

As stated the issue is that the signals aren't stabilizing. While the likely issue is two combinational logic signals continually replacing each other there are a couple of other possibilities that I want to highlight for posterity's sake.

As documented by Xilinx in Answer Record #19068 it can also be caused by a process that changes a signal in it's sensitivity list.

Another check to make that finally solved my issue here was to make sure your simulation resolution is small enough. Mine was orders of magnitude too high, and the clock on my test bench was running too many times in a single simulation step.

Answer 5

This error usually indicates that ModelSim is stuck in an infinite loop. In VHDL, this can happen when a signal is placed in the sensitivity list and this signal is changed in the process. The signal changes, triggering the process, which changes the signal, which again triggers the process and the cycle continues.

The following is a simple example of a process that causes an infinite loop:

PROCESS (count)

BEGIN

count <= not count;

END PROCESS;