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Spice Simulation for Combinational Circuits
In this exercise, digital clock signals will be setup as digital inputs for simulating
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combinational logic circuits that consist of logic gate circuitries.
 | Start with <Ctrl G> to get a part. |
| Knowing the logic Symbols of the different logic gates, use the mouse to
scroll down the different parts at different libraries until you get to the logic
gates: NAND, NOR, AND, OR, and NOT. Designate the part the 74xxx
part numbers. Place and close the part numbers needed to complete the
logic circuit shown at the end of the following demo. Designate the inputs
and outputs by using bubble parts as described in Exercise # 2. |
| In order to simulate digital circuits, digital clock signals are required at the
inputs. At PSpice software, this can be obtained as follows: |
| Set the delay, on, and off times for the different input waveforms at the
attribute window of every digiclock part. The delay time is the time when
the digital clock starts, the ON time is the time of logic high, and the OFF
time is the time of the logic low for the signal. Again, to get to the attribute
window of a digital clock signal, select the digiclock part by clicking on it
with the right button mouse (RBM) and then double click on it by the RBM.
Enter the delay, on, and off times for every waveform. The following is a
demo that shows this sequence. |
| Set up the transient parameters similar to that of Exercise #2. Choose the
stop time to be five times the period of the lowest frequency among the
different inputs. This will allow you to display five cycles for the lowest
frequency waveform and more than five cycles for the others. This is
illustrated as: |
| Run the simulation by hitting <F11>. This will run the Probe to get the input
and output waveforms. Use trace, add, plot, add, trace, add, etc to display
the four inputs and one output of the circuit. This is shown below: |
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