- Qucs -
Quite Universal Circuit Simulator

Getting Started with
Digital Simulations

Qucs is also a graphical user interface for performing digital simulations. This document should give you a short description on how to use it.

For digital simulations Qucs uses the FreeHDL program (http://www.freehdl.seul.org). So the FreeHDL package as well as the GNU C++ compiler must be installed on the computer.

There is no big difference in running an analog or a digital simulation. So having read the Getting Started for analog simulations, it is now easy to get a digital simulation work. Let us compute the truth table of a simple logical AND cell. Select the digital components in the combobox of the components tab on the left-hand side and build the circuit shown in figure 1. The digital simulation block can be found among the other simulation blocks.
The digital sources S1 and S2 are the inputs, the node labeled as Output is the output. After performing the simulation, the data display page opens. Place the diagram truth table on it and insert the variable Output. Now the truth table of a two-port AND cell is shown. Congratulation, the first digital simulation is done!


Figure 1 - Qucs main window


A truth table is not the only digital simulation that Qucs can perform. It is also possible to apply an arbitrary signal to a circuit and see the output signal in a timing diagram. To do so, the parameter Type of the simulation block must be changed to TimeList and the duration of the simulation must be entered in the next parameter. The digital sources have now a different meaning: They can output an arbitrary bit sequence by defining the first bit (low or high) and a list that sets the times until the next change of state. Note that this list repeats itself after its end. So, to create a 1GHz clock with pulse ratio 1:1, the list writes: 0.5ns; 0.5ns
To display the result of this simulation type, there is the diagram timing diagram. Here, the result of all output nodes can be shown row by row in one diagram. So, let's have fun...

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