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You are using an out of date browser. It may not display this or other websites correctly. You should upgrade or use an alternative browser. Modelsim vs. Xilinx ISim. Thread starter spman Start date Jan 21, Status Not open for further replies. Hi Which one is better? Isim is more convenient and easy for me. I don't know why it is said that Modelsim is more professional.
What does Modelsim have more than Isim? Perhaps I don't know Modelsim capabilities! And another question! I didn't find any useful information at model. Thanks in advance. TrickyDicky Advanced Member level 5. Modelsim is more powerful.
But if all you want to do is similate basic testbenches, ISIM is fine. Isim indeed has the convenience going for it, and it's fine for basic testbenches. Modelsim however supports System Verilog, isim doesn't. I also get the feeling Isim wont support all of VHDL like protected types as I got a few bugs in modelsim fixed, and modelsim is usually the best for supporting all language features. But then no one uses protected types except me Modelsim SE is no longer offered. All versions of Modelsim support all of SystemVerilog except constrained random generation, functional coverage and assertions.
Simulation of the design takes a long time in behavioral mode. About 10 min! But some errors appear during compilation. For examples Modelsim doesn't accept redeclaration of an output as register or wire in verilog! If the simulation of your module produces any errors, the time of the error, the current output and the expected output will be displayed in the ISE console, along with a count of the total number of errors.
If your simulation has no errors, a message will be displayed telling you so. Don't just take the absence of error messages to mean that your module is working perfectly. An error in your simulation code may have prevented your tests from even being executed, or if there is an error in your module design and the value of a wire in your simulation is unknown indicated by the value "x" and a red line in your waveforms , Isim may assume that any comparisons against this wire are true. Therefore, you should always check your waveforms in addition to the ISim console.
If your simulation has errors, you can go back, fix your module and re-use the same test fixture to test the module again.
It is a good idea to write your test fixture before you design your module as long as you know the interface , because this will force you to think carefully about the expected outputs of the module and corner cases that should be tested.
Once you have written a complete and accurate test fixture, it can continually be used to test your module. Immediately after adding the signals, they will not have any waveforms, and will have a value of "No Data" if you move the cursor. In order to read these signals, ISim must run the simulation again.
To do this, first click on the Restart button on the toolbar, leave all of the boxes checked on the subsequent prompt and click Restart. ModelSim will reload the simulation, but will not re-run it; click the Run -All button on the toolbar to start the simulation again.
Waveforms will now appear for t1, t2 and t3. Restarting and running the simulation again will not incorporate any changes you have made to your module or test fixture. There are other simulation controls that may be useful. The Break button stops a running simulation. The ContinueRun button continues a stopped simulation. The Run button runs the simulation for the amount of time entered in the box to the left of the button.
Designing and Testing an Adder. After entering a project name and location, you'll be prompted for the project properties. Note: If you accidentally select a simulator other than ISE Simulator for your project, or if you open a previous project that had a different simulator selected, you can change the simulator by right-clicking on xc2vpff in the Sources window in ISE and selecting Properties.
This tutorial will use a full adder that is the same as the one you created in Lab 0. The file will be added to your project. ISE creates a skeleton test fixture a. The inputs to the module are registers "reg" because they are assigned in a procedural block the section between "begin" and "end". The outputs from the module are wires. The test simulation begins after the "initial begin" line.
The values are initialized, and your code goes under the "Add stimulus here" line. After saving your test fixture, it will not immediately appear in the Sources window. To find the test fixture, click on the "Sources for:" drop-down box, and select Behavioral Simulation. Note: Make sure your test fixture file, not the file containing your actual Verilog module, is selected in the Sources window before running ISim.
Once ISim starts, check the Transcript window carefully for warnings and errors. Note: Your output signals may have "false" transitions at times when more than one input signal changes.
Advanced Functional Simulation Instead of visually checking your simulation outputs every time you run the simulation, you can write test code that will change your inputs and check the outputs for you. Test code that does this is called a "task. The task changes the inputs to the module, waits for the changes to propagate, then checks that the simulated outputs match the expected outputs. The task prints an error to the simulation console if the outputs do not match. Simulation errors displayed in the console.
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