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Some commercial proprietary simulators (such as ModelSim) are available in student, or evaluation/demo editions. These editions generally have many features disabled, arbitrary limits on simulation design size, but are sometimes offered free of charge.

The behavioral simulation works fine, actually the whole elaboration chain I created works perfectly fine as far as the behavioral simulation concerns. However, once I try the Post-translate simulation things start to go wrong: lots of 'X' output signals.

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I followed your advices and cleaned up the FSM by using a single state process. I still have some doubts about "where to put what" but I really like the new implementation. Anyway I now get a clean behavioral simulation but 'X' on all outputs in post translate simulation.What is causing this?I'll post the both the new code and the testbench:

Updated with the last design: this code is not causing any 'X' (can't figure out why, this doesn't but the previous did). However it's not starting (in POST-TRANSLATE simulation) just like the first 3 process machine, and the signal sync_out is stuck at 0 while it should be '1' by default.

I've been looking into the tecnology schematic, starting from the problem of the sync_out=0: it's implemented with a FDS, S is the FSM reset signal, D is coming from a LUT3 with I = state&reset&start and INIT = 45 = "00101101". I've looked for this LUT3 in the simulation and I've noticed that it has INIT = "00000000"!

UPDATE 3It seems that the Post-Translate simulation is buggy in some way, or I'm not configuring it correctly for some reason: the Post-Map and the Post-PAR simulations work and display some outputs.However there is an odd bug: the stored_data register is not updated with the complete data_in vector, after that, the FSM operates correctly and outputs the data stored.I've looked in the tecnology schematic just after synthesis and for some reason the bits 23,22,21,19,18 are not connected to the corresponding data_in bit. You can see the effect in this screenshot from Post-Map simulation. Same happens in Post-PAR, but it seems that this problems comes directly from the synthesis!

One thing to be aware of in post-synth and post-PAR simulations is that their model of time is different from the behavioural model. The behavioural model follows simple rules as I described in this answer and ensures that in a typical design flow you can go straight to hardware - without post-synth simulation, without worry.

I'm new to VHDL and I'm trying to simulate an array multiplier.(I have used verilog before). However in the simulation results it shows 'X' for inputs which used to be '1'.Here is the result:And this is the Testbench:

Then your process assigns something to the same signals, thus, you have multiple drivers which are resolved. But, if you drive 0 at the first place and 1 in the process the resulting value is undefined shown as X in the simulation.

The right answer to your problem isn't the bug that you found, but the process you are missing for finding bugs. You should have a test bench of some type, or a simulation, or a formal model, that will produce a trace with all of your wires and their values/meanings within it. This trace will lead you directly to this type of bug.

I'm more of a Verilog developer, so I use SymbiYosys for generating traces from formal specifications, and Verilator for my simulation. I've also got a means for pulling a trace from a live board as a last resort.

Even better, it's easy to integrate co-simulation into a Verilator based design. Consider this design's use of co-simulation to create an on-screen image (i.e. window) of the VGA output of the given design.

OK, I admit it that there are times when I have the same thoughts ( sometimes I wonder why I do FPGA development at all...) . Of course when I do use Verilog ( you can't simulate MIG designs with VHDL ) I still use Vivado simulator. And sometimes customers require VHDL ( though that's getting less frequent ). As to the examples of "proof" of quality that you offer... I'm not convinced. I can write poor testbenches ( and often do ) that don't expose a flaw and be forced to write a proper one after gaining some insight as to what I need to be looking for as well as anyone else. That's how simulation works. Perhaps your suite of tools shortens this process, or makes it easier to get lucky without having a good understanding of what it is that you are trying to simulate... why wouldn't automating the process intelligently help? What you can;t demonstrate is that ModelSim ( the pricey version that understands Xilinx device behavior ) or Vivado simulator won't do the same thing. I get that you love your development process. I've just never come across it in practice by companies whose living depends on reliable FPGA based products. That's not proof of anything, just an observation.

I've spent my time doing logic simulation, and not so much timing simulations. While I can simulate (logically) any logic running even at multiple clock rates, I've never gotten into the analog side of how/when transitions actually take place. In my opinion, I haven't needed it. Maybe there's something I'm missing. That''s not to say there's no use for it--I just don't feel like I've needed it.

No, that's not timing simulation. Timing simulation, as I pointed out earlier uses a specially formatted post-route netlist representing how your device is going to be configured. It is also, as I pointed out earlier is a vital part of the process when you have to produce reliable products. Neither ISE nor Vivado provide this netlist unless you ask nicely. On reason why people don;t do it as often as they should is because it takes a whole lot longer to simulate than a behavioral simulation. And because your device becomes more of a black box since most of your source signal have been replaced by synthesis optimization ( though you can preserve signal names in your code if you need to..)

I thought that the topic was (turned into by you) developing simulation skills... ( I really want to make a snide comment but ... Ah... it's past... We can agree that the poster need to get started. I can admit that I've not arrived at simulation Nirvana.

If you entered a different socket port specification when you configured the HDL Cosimulation block in Simulink, replace the port number 4449 in the preceding command line with the applicable socket port information for your model. The vsim function informs ModelSim of the TCP/IP socket to use for establishing a communication link with your Simulink model.

This section explains how to use the vsimulink command to load an instance of your VHDL entity for cosimulation with Simulink. The vsimulink command is an HDL Verifier variant of the ModelSim vsim command. It is made available as part of the ModelSim configuration.

Start a Simulink simulation. The value in the Display block changes to 255. Also note the changes that occur in the ModelSim wave window. You might need to zoom in to get a better view of the signal data.

In the Simulink model, change Constant value to 255, save the model, and start another simulation. The value in the Display block changes to 0 and the ModelSim wave window is updated as follows.

In the Simulink model, change Constant value to 2 and Sample time to 20 and start another simulation. This time, the value in the Display block changes to 253 and the ModelSim wave window appears as shown in the following figure.

The purpose of this lab is to introduce you to VHDL simulation and synthesis using the ALDEC VHDL simulator and the Xilinx foundation software for synthesis. There are several defenitions that may be helpful: Simulation is the execution of a model in the software environment. This is done using the ALDEC VHDL simulator. A test bench is a program whose purpose is to verify that the behavior of our system is as expected. The test bench is used in ALDEC to simulate our design by specifying the inputs into the system. Synthesis is the process of translating a design description to another level of abstraction, i.e, from behaviour to structure. We achieved synthesis by using a Synthesis tool like Foundation Express which outputs a netlist. It is similar to the compilation of a high level programming language like C into assembly code.

simulation Complete the code provided. It should describe a 2-bit counter and a 2-4 decoder in VHDL, constructing a top-level structural entity containing these two components. Write a suitable testbench and simulate it to ensure that it is correct (Use ALDEC VHDL). synthesis Wire up the circuit as shown. You may need the circuit diagram or the the manual for the XS40 boards. Make the neccessary additions to your lights.ucf file to reflect the LED connections. Download your program unto the XS40 board.

Modelsim is a program created by Mentor Graphics used for simulating your VHDL and Verilog designs. It is the most widely use simulation program in business and education. This tutorial explains first why simulation is important, then shows how you can acquire Modelsim Student Edition for free for your personal use.

To start your simulation, click on Simulate in the Menu Bar, then click Start Simulation. This opens the Start Simulation Window. Click on the plus sign next to work, then click on the plus sign next to and_gate_tb. Make sure you select and_gate_tb and not and_gate as we want to simulate the design at the test bench level. Once and_gate_tb is highlighted, click OK. 17dc91bb1f