Tutorial 2 – Performing transient simulations
For this tutorial we will modify the basic circuit we built in tutorial 1 and make it into an RC circuit. Then we will perform a transient analysis on the circuit to see the RC delay.
1. First let’s create an RC circuit using the steps we learned in tutorial 1.
2. Create a new circuit file and name it basic_sim2
3. Open the existing basic_sim_top.va file (FileàOpen...; change the file type to Verilog source files. This will make it easier to find)
4. Instead of creating a new top file we will simply add a new module to the existing one.
5. Create a new module and name it basic_sim2_top
6. In this new module we need 3 components: a pulse voltage source, a resistor, and a capacitor. If you follow the circuit schematic of Figure 1.24, you will get an idea of how to setup your RC circuit.
7. You will need three nodes this time: vin, vout, vss. Connect the vpulse source to vin and vss. Connect the resistor between vin and vout, and connect the capacitor between vout and vss.
8. For the voltage pulse source to work, we need to feed it the proper parameters. For this demo enter the following set of numbers in this order: # (0, 5, 2n, 10p, 10p, 10n, 20n). This data string of data basically sets the parameters of the source. Based on these numbers the source is set to pulse from 0 to 5 volts, with a time delay of 2ns, with a rise and fall time of 10p, with a pulse width of 10ns and a 20ns period. Remember, if you are never sure what order to pass parameters in always refers to the component library that holds the module for it.
9. Once you have completed the new module save the file and close it.
10. Now open the new circuit files (basic_sim2) and add your component libraries and top file to it.
11. Compile your circuit file (remember we do this by saving it, closing it, and opening up again) to make sure it has no errors. If successful you are now ready to perform a transient analysis on the circuit.
12. To perform a transient analysis select AnalysisàTransientàParameters...
13. The menu will open up and you can select your end time and step size of your simulation. Pressing the Run button will instantly perform the simulation. Pressing the Apply button will save the parameters settings you have selected and place them in the circuit file. Lets select APPLY first.
14. After selecting Apply you will see the spice commands that perform a transient analysis. This is useful since SMASH does support command line spice (i.e. you can just type what you want here and have it execute). The best suggestion is to use the menu to perform simulation and edit the spice commands later for faster changes. Now let’s go back to the transient menu and select Run this time.
IMPORTANT!: when selecting your step size (print step parameter) you must make sure that it is not greater than or equal to the max Time step control. The Time step control acts as an override meaning if you wanted to use 1n for the steps and the time control list 1m as the max, your simulation would default to 1ms. To change this open the advanced parameters section to change these settings.
15. A simulation window will open and you will be prompted to add which signals you want to simulate. These signals are located on the left panel in two areas. The above (purple) names will give you the current that flows through those components. The below (ones with signals next to their name) provide voltage of the given names. These names generally match the node names that were placed in the top file. To add them select the node you desire and press the add button. If successful you will see a line popup in the background. This signifies a new graph for the new signal. When you are finished adding signals press the close button.
16. After closing the previous menu your simulation should begin immediately. If it is complex you will see a green status bar on the bottom right of the screen signifying the progress of the simulation. Once it is complete your signals will appear on the screen in their respective graphs
17. Now that your simulation is done you can modify you graph by adding more signals or by combining multiple signals onto the same graph. To add signals select ProbesàTrace Nets And Signals... Once you add more signals you will need to run the simulator again (press the play button or go to DebugàGo).
18. To combine signals grab the signal name with your mouse, drag and drop it into the graph you wish to combine it with.