Computer Systems and Networks

1.     Introduction

The complete digital logic circuit will be designed by the help of the logisim simulator. There are multiple software that are used to design a logic circuit. That are available in network at the commercial and the free manner. But logisim is the efficiently used one. Logisim is a tool that is used to simulate the circuits called digital logic circuits. It was designed by Carl Burch. It is used to learn about the logic circuits in detail manner. More than this tool is mainly used to build the circuits. It is very simple to handle and easy to learn about the circuit designs. To design and simulate the CPU circuit design for the purpose of educational will be done by the help of the logisim. It can be used to construct the big circuit from the given small circuits. And it is also capable to build the bundle of wires with the help of the mouse drag with single action. It is an application based on java so it has the ability to run in the environments Linux and windows workstations. Logisim supports the following simulations like Step simulation, Tick simulation, Simulate enabled. This logisim is used to enable the circuits if it is stateful. This circuit designing tool will be used in many of the purposes like sophomore-level computer organization uses this tool for conducting courses. And then it will be used in many of the computer science surveys.

2.     Logisim Simulator

Logisim simulator is used to build the digital logic circuit. There are two main purposes for designing a digital logic circuit. Because it is used to compare the operations of the different versions in the class. And the second function will be the priority encoder. Logisim is used to simulate the logic circuits. Here the designs will be simulated with the help of the Graphical User Interface (GUI). 

Logisim Simulator window

3.     Features of an Logisim Simulator

Logisim is an open source software.

This circuit designer will be supported in any platforms like MacOS X, Linux and windows.

It has the wiring tool to build a vertical and horizontal wires.

Here the interface for drawing is based on the tool bar called intuitive toolbar.

Once the circuit will be completed then it can be saved in the file format or print this design into a printer and it has the ability to export into a GIF file.

To build the large circuit with the help of the sub circuits. Because it is the easy way to build the circuit.

To perform the operations like Boolean Expressions, Conversion between multiple circuits and truth tables with the help of Combinational analysis.

4.     Working Principles of Logisim

Logisim will worked based on the following methods:

• Use a sub circuit to build a circuit.
• Buses and Splitters in logisim
• Comparators
• Priority Encoder
• Combinational Analysis

Using a Sub circuit: One of the important role for sub circuit is used to build a complex circuits. This complex circuits in logisim using Sub circuit by the following way,

Choose the Add circuit option in the project menu.

Then name the circuit. This newly created circuit names will be shown under the Explorer panel.

Click the black canvas that is associated with the newly generated circuit.

It shows the small icon for the copy of sub circuits. It can be used to build the circuit multiple times.

Example for sub circuit: XOR Gate 

Explorer Panel:

Buses and Splitters in Logisim: Logisim splitter is mainly used split the multiple wire input into single wire for building an output. This multiple wire input will represent the multiple bits of that input. It produces the output by functioning these multiple bits with several wires. It can be implemented in the bread board.

Using Bus in Logisim

Comparators in Logisim: Comparators is used to compare two values in the input design. It compares the values like two’s complement values and unsigned values that is based on the numeric type attribute. It produces the two outputs only that are 0 and 1.Comparison will be performed on the most significant bits. If the undefined values like floating value or error value will be identified the outputs will matches this errors.

Priority encoder: The priority encoder is used to add the multiple inputs in the design. Here there are two important components will be used. That are enable output and enable input. If the input is 0 then it disables the component.so it produces the output in floating points. Whenever the output is 1 then it used to enable the component.

Example for Logisim priority encoder:

Combinational Analysis: It is used to convert the representations like Boolean Expression, Truth tables and Logic circuits in all the directions. It supports only the One-Bits of input and output.It is the easy way to understand the circuits in the better way.

It performs the following steps

Step 1: Opening the Combinational Analysis

Step 2: Editing the truth table

Step 3: Creating expressions

Step 4: Generating the circuits

Combinational Analysis 

5.     Sample circuits

EX-OR Gate

In this above circuit the logic function is implemented by using two inputs. It contains the ability to compare the two logic levels of a circuit.

The expression of this circuit

Q = (A ⊕ B) = A.B + A.B

EX-OR gate Equivalent circuit

EX-OR realization by using NAND gate

The sample logisim circuit

Circuit Description

The exclusive gates are used to perform the arithmetic operations and calculations. It contains the adder and half adder which produce the “carry-bits”. It uses two inputs where one input passes binary input data and passes control signal.

6.     Digital Logic circuit

The Exclusive-OR logic circuit is very useful that can be used in many various computational circuits. It uses standard logical functions, standard Boolean expressions and operational symbols to build and design a circuit. It contains a basic logic comparator which used to produce a logical outputs.

Circuit description

This circuit contains 14 pins.

The first 7 pins are in bottom of circuit design. These pins are split into two or three combined to connected with gates to produce s the proper circuit execution

The pin (1, 2,3) and (4,5,6) are connected with each gates to produce the  required outputs. The pins 8 to 14 are in top of the circuit the pin (8, 9,10) and (11,12,13) also connected with each gates to produce the outputs.

7.     Design a circuits by using logisim

There are several circuits are designed by using logisim library. It contains some steps to design a circuit

Step 1

Build 1×2 multiplexer by using  logisim

Step 2

Design a circuits by using MUX

Step 3

Design a sub circuit using logisim label tip

Step 4

Running overall circuits with wire connection panel

Step 5

Value propagation

Step 6

Final output of the circuit

8.     Circuit Simulation

• creating a circuit to determine equality between two single bits by building an XNOR gate in the following manner
• Begin by placing two AND gates and an OR gate
• Reduce the number of input bits to 2 for each of the gates via the Attribute Table.
• Place a NOT gate in front of one input of each AND gate as well as the output of the OR gate.
• Connect wires by dragging out from each input/output points
• This creates an XNOR gate, which will output a 1 when both inputs are equal

The types of simulation

• Simulate Enabled
• Step Simulation
• Tick Simulation

Simulate Enabled: Let the circuit run based on its inputs. Must be enabled for all tick simulations to work as well.

Step Simulation: Allows the user to simulate a single step at a time. If an input changes in step simulation you must advance the signal through each gate by stepping (ctrl-i).

Tick Simulation Used to tick a clock (found in Explorer Plane Wiring > Clock). This is vital for stateful circuits (e.g., RAM, flip-flops, etc…)

 – Tick Once: Tick the clock once (go from high to low or vice versa)

 – Ticks Enabled: Tick automatically at the rate of tick frequency

– Tick Frequency: How often to tick the clock (measured in Hz)

9.     Conclusion

Digital logic circuit is a combination of digital logic gates. Based on the combination of digital signals, digital logic gates makes the logical decisions. Logical gates generates one output from the two inputs. Our work is done by the logisim simulator. It can run any operating system and it is light weight. One important and useful software for design the circuit is logisim. Logisim simulator is used to create the circuits easily as the traditional drawing programs. We create the exclusively or from NAND gate circuit using the Logisim. Individual IC components should be connected and processed to form a Functional circuit. In our project, we design the exclusively or from NAND gates. Exclusively or gates are built to perform complex arithmetic operations especially half-adder. We use four NAND gates to form the one exclusive or gate. NAND gate operates the AND gate followed by NOT gate. Freezer warning buzzer is one of the most application of NAND gates. The output of the NAND gate is “false” if the both input is “true”. And the output of the exclusively or gate is “true” if the both inputs are “true”. By using logisim, we form the behavior of the XOR gate from the NAND gate. Or gate is mostly used for checking units and parity generator. Process work is fully done on Logisim. Color coded wires are used to debug the some errors.

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