In this tutorial I will explain you the working of a simple multiplexer. At the start a brief and concise introduction to multiplexer specifically a simplest 2×1 multiplexer is provided with the explanation of the output they will show. A multiplexer can also be referred as a mux in short form and 2×1 mux is the simpler of its kind. After that the circuits is simulated using PSPICE and the result are compared with the theoretical discussion provided (which should be same). At the end of the tutorial you are provided with an exercise to do it by yourself, and in the next tutorials I will assume that you have done those exercises and I will not explain the concept regarding them.
Introduction to multiplexer
A simple multiplexer is used to display a number of input signals on a single output line with the help of a select switch. A total of 2n input signals can be displayed on a single output line with the help of n select switch. A simple multiplexer diagram and functionality is shown in the figure below,
Figure 1: Multiplexer
Depending on the value of the select switch one out of all the inputs will be displayed at the output as is obvious from the above figure. To learn the functionality of a multiplexer refer to the simulation example below.
How to design Multiplexer with PSpiceÂ
- Lets’ design a simple digital circuit of a 2×1 multiplexer without using the build in block of an 8×1 multiplexer i.e. using AND, OR and NOT gates as I will explain shortly in this tutorial. Open the PSPICE design manager on your PC by typing design manager in the search bar. From the design manager click on the run schematic button to open a new blank schematic as shown in the figure below,
Figure 2: Opening schematic
- After opening the new schematic before jumping into designing first save the schematic by clicking on the file button at the top left corner and then selecting save as so that we can access it anytime in the future. Refer to the figure below,
Figure 3: Saving schematic
- Click on the get new part icon at the top bar of the schematic window in order to search for the components that are needed for circuit designing.
Figure 4: Getting new part
- In the get new part window, type ‘7404’ it will display a NOT gate available in PSPICE. From that list select a simple NOT gate as shown in the figure below,
Figure 5: Placing NOT gate
- Again open the get new part window and in the part name block type 7408, select the AND gate from the list given and then click on place & close as shown in the figure below
Figure 6: Placing AND gate
- Again open the get new part window and in the part name block type 7432, select the OR gate from the list given and then click on place & close as shown in the figure below
Figure 7: Placing OR gate
- Next step is to place a ground, do the same again and in the part name type Dstm and select the digital source and then click on place & close as shown in the figure below,
Figure 8: Placing digital source
- The placed components in the schematic window are shown in the figure below,
Figure 9: Placed components
- Click on the draw wire icon at the top bar of the schematic window in order to connect the already placed components for circuit designing, as shown in the figure below,
Figure 10: Drawing wire
- Connect all the components to complete the circuit diagram as shown in the figure below,
Figure 11: Complete circuit diagram
- On the top of the schematic window, click on the Voltage/Level Marker button as shown in the figure below,
Figure 11: Voltage marker
- Place it at the output capacitor and at the input node as shown in the figure below,
Figure 1212: Placed voltage marker
- If we are interested in checking the voltage on a specific wire in spite of checking it at a node, double click on the wire and inn the window that appear as a result, type the name of the wire you want to label it with, as shown in the figure below,
Figure 13: Labeling
- We can also change the name of the device i.e. in this case we have 3 identical sources however each of these have separate purpose. Two of the sources are used as inputs of the multiplexer and the other is used for the select line of the multiplexer i.e. which input you want to be displayed on the output. Name one of the source as S0 (select line)
Figure 14: Naming the device
- Next step is to set the attributes of the input digital supply. Double click on the dstm1 supply you connected in the circuit previously and set the input commands of the digital input system as shown in the figure below,
Figure 15: Input attributes of select line
- The commands in the attributes window represents the value of the input at the corresponding time in the command before the space. The COMMAND1 = 0s 0 shows that at time 0s the value of the input dstm is 0 and the COMMAND2 = 1s 1 shows that the value will update to 1 at time 1 second. Similar is the case with COMMAND5 = 4s 0 that at 4 seconds the value of the input will be 0.
- Next step is to set the attributes of the input digital supply. Double click on the dstm2 supply you connected in the circuit previously and set the input commands of the digital input system but these commands may or may not be same to the commands of input 1 as shown in the figure below,
Figure 16: Setting attributes of input 0
- Next step is to set the attributes of the input digital supply. Double click on the dstm3 supply you connected in the circuit previously and set the input commands of the digital input sys
tem but these commands should not be same to the commands of dstm2 i.e. as shown in the figure below,
Figure 17: Setting attributes of input 1
- Next step is to adjust the properties of the simulations in order to produce the graph of the voltage at the marker. Click on analysis and then click on Setup as shown in the figure below
Figure 18: Simulation setup
- A widow will appear, click on the transient block on the window and adjust the properties of the window according to your requirement, refer to the figure below
Figure 19: Transient response
- The final time of the response is 4ms because we have set the commands for only up to 4ms. Now comes the simulation part, click on the analysis at the top bar of the schematic window and then click on simulate as shown in the figure below,
Figure 20: Simulation
- A schematic window will appear showing the voltage at the output wire and the input source as shown in the figure below,
Figure 21: Output
The output of the circuit shows that when the select switch is 0, the value at the input I0 will be displayed at the output Vout. Similarly, when the select switch is 1 then the value at the input line I1 will be mirrored on the output Vout. The select switch will select which out of all the inputs which of the input will be displayed on the single line output.
Exercise:
- Design a 4×1 mux using the same methodology as in case of 2×1 multiplexer (mux)
<<Previous tutorial                        Next tutorial>>