\nyou may find yourself asking your hardware to do too much work. In those cases,
\nyou need to understand what your hardware is and how it works. So,
\nlet’s start by looking at the lowest level: the lowly
\nlogic gate. To that end, let’s use a software package named Logisim<\/a>
\nin order to play with logic gates in various groupings.<\/p>\n
Logisim should be available in most distributions’ package management
\nsystems. For example, in Debian-based distros, install it
\nwith the following command:<\/p>\n
sudo apt-get install logisim<\/p>\n
You then can start it from your desktop environment’s menu,
\nor you can open a terminal, type logisim and press
\nEnter. You should see a main section of the application
\nwhere you can start to design your logic circuit. On the left-hand side,
\nthere’s a selection pane with all of the units you can use for your
\ndesign, including basic elements like wires and logic gates, and
\nmore complex units like memory or arithmetic units.<\/p>\n
![\"""\"](\"https:\/\/www.linuxjournal.com\/sites\/default\/files\/styles\/max_650x650\/public\/u%5Buid%5D\/logisim1.png\")
<\/p>\n
Figure 1. When you first start Logisim, you get a blank project where To learn how to start using Logisim, let’s look at how to set up one of Figure 2. You easily can add logic gates to your circuit to model If you click the On the front side of the The last step is to Figure 3. You can add extra items, like inputs and outputs, to your By default, the input pins While you can build your circuits up from first principles and see how Figure 4. You can build up your logic circuits in reverse by defining The first step is to provide a list of The last step Once Figure 6. By using the Combinational Analysis window, you can create You can click on the inputs to toggle them and Once you finish your circuit, you can save it in a .circ You also can export the circuit itself This is just a brief introduction, but I hope Logisim helps you learn a bit more
\nyou can start to design your first logic circuit.<\/em><\/p>\n
\nthe most basic logic circuits: an AND gate.<\/p>\n![\"""\"](\"https:\/\/www.linuxjournal.com\/sites\/default\/files\/styles\/max_650x650\/public\/u%5Buid%5D\/logisim2.PNG\")
<\/p>\n
\ncomputations.<\/em><\/p>\n
\nGates entry on the left-hand side, you’ll see a full list of available
\nlogic gates. Clicking the AND gate allows you to add them to the design
\npane by clicking on the location where you want them added. At the bottom
\nof the left-hand side, you’ll see a pane that displays the attributes
\nof the selected gate. You can use this pane to edit those attributes to
\nmake the gate behave exactly the way you want. For this example,
\nlet’s change the number of inputs value from 5 to 2. The next
\nstep is to add an output pin in order to see when the output is either
\n1 or 0. You can find pins in the wiring section.<\/p>\n
\nAND gate, you’ll want to add pins so you can control input. In the
\nattributes for each of the pins, you’ll see that you can change whether
\nthe pin is supposed to be an output pin. You also can set whether
\nthe pin is supposed to be a three-state pin.<\/p>\n
\nconnect all of these pieces by simply clicking and dragging
\nbetween the separate components.<\/p>\n![\"""\"](\"https:\/\/www.linuxjournal.com\/sites\/default\/files\/styles\/max_650x650\/public\/u%5Buid%5D\/logisim3.PNG\")
<\/p>\n
\nlogic circuit.<\/em><\/p>\n
\ncurrently are set to 0, so once the wires are connected, you should see
\nthat the output is set to 0. In order to toggle the input pins, you first need
\nto select the toggle tool from the toolbar at the top of the window
\n(the one shaped like a pointing hand). Once you have selected this tool,
\nyou can click on the input pins to change their states. Once both inputs
\nare set to 1, you should see the output flip to 1 also.<\/p>\n
\nthey behave, Logisim also lets you define the behavior first and generate
\na circuit that gives you the defined behavior. Clicking the
\nWindow\u2192Combinational Analysis menu item pops up a new window where you can
\ndo exactly that.<\/p>\n![\"""\"](\"https:\/\/www.linuxjournal.com\/sites\/default\/files\/styles\/max_650x650\/public\/u%5Buid%5D\/logisim4.PNG\")
<\/p>\n
\nthe behavior you wanted first, then allowing it to generate a circuit that
\ngives you this required behavior.<\/em><\/p>\n
\ninputs. You simply add a series of labels, one for each input. For this
\nexample, you’ll define an x, y and z. Next, you’ll need to click
\nthe outputs tab and do the same for the number of outputs you want to
\nmodel. Let’s just define a single output for this example.<\/p>\n
\nis actually to define the behavior linking the inputs to the outputs. This
\nis done through a logic table. So here, you’ll have the output
\nas 0, unless either x and z or y and z are high.<\/p>\n![\"""\"](\"https:\/\/www.linuxjournal.com\/sites\/default\/files\/styles\/max_650x650\/public\/u%5Buid%5D\/logisim5.PNG\")
<\/p>\n
\nFigure 5. Logisim includes a tool that allows you to generate logic circuits
\nbased on a truth table that you define to handle the computation you’re
\ninterested in modeling.<\/em><\/p>\n
\nyou’re happy with the definition, click the Build Circuit
\nbutton at the bottom of the window. This pops up a new dialog window
\nwhere you can define the name and select the destination project, as
\nwell as choosing whether to use only NAND gates or only 2-input
\ngates.<\/p>\n![\"""\"](\"https:\/\/www.linuxjournal.com\/sites\/default\/files\/styles\/max_650x650\/public\/u%5Buid%5D\/logisim6.PNG\")
<\/p>\n
\nmore complex circuits based purely on their expected behavior.<\/em><\/p>\n
\nverify that everything behaves as you had planned.
\nThe Combinational Analysis window has two other tabs: Expression and Minimized. The
\nExpression tab shows you the logical mathematical expression that
\ndescribes the truth table you defined. You can edit your
\ncircuit further by editing this equation directly. The minimized tab gives you
\nthe logical equation as either the sum of products or the product of sums.<\/p>\n
\nfile. These files define a complete circuit that can be reused as a single
\nunit. When you do want to reuse them in a larger, more complex circuit,
\nclick Project\u2192Load Library\u2192Logisim Library and
\nselect the saved file. This allows you to build up very
\ncomplicated computing circuits rather quickly.<\/p>\n
\nby clicking File\u2192Export Image. This allows you
\nto save the circuit as an image that you can use in a report or
\nsome other process.<\/p>\n
\nabout the fundamentals of computing and logical structures.<\/p>\n