ECE 109 Spring Launch Logisim by opening a terminal window and entering the command:

Transcription

1 Problem Session 3 Overview Welcome to Logisim! Logisim is a logic simulator that allows you design and simulate digital circuit using a graphical user interface. It was written by Carl Burch. Logisim comes with libraries containing basic gates, memory chips, multiplexers and decoders, and other simple components. In later problem sessions, you will use many of these components, but for now, we will guide you through the process of creating basic logic gates using transistors. Part 0: Preparation Begin by downloading Logisim from the SourceForge page to your Linux account. There are three choices of release to download, but the system should automatically detect that you are using a Linux machine and offer you a.jar file as your download. Save the.jar file to your desktop. Change the name of the.jar to something easy to type like logisim.jar. Launch Logisim by opening a terminal window and entering the command: java jar logisim.jar & Part 1: An Inverter Logisim is a simple tool to use, most of the features you will need are well documented in the reference document. You can obtain the guide from the Logisim Documentation page. We'll begin by working with a very simple circuit. 1. Start by downloading (right-click and select Save As) this Logisim file containing an inverter constructed using P-type and N-type transistors. Select File->Open from the menu bar at the top of the Logisim window to open the file. You should see the following in your Logisim window. Jan,

2 2. Select the "Poke" tool on the top tool bar and click on the input pin in your schematic. Observe what happens to the output. Does this match what you think an Inverter should do? 3. An inverter is defined by a truth table. Select Project->Analyze Circuit from the menu bar at the top of the Logisim window. Observe the truth table for your inverter under the Table tab. We will learn about the other tabs later. 4. Return to the Logisim main window, and click the "Select and Add Wires" tool on the top tool bar. Select each of the two transistors in turn and notice the information displayed in lower-left corner of Logisim window. Notice that you can change the Type and orientation (i.e., Facing) of each transistor. 5. Now try to build the inverter transistor circuit yourself. Select File->New from the menu bar at the top of the Logisim window to open a new schematic window. Try to recreate the inverter schematic that you were given. Here s a condensed guide to the functionality you ll need. Be sure to ask your instructor for help when you need it. 1. Click the "Input Pin" button on the menu bar to add inputs. 2. Click the "Output Pin" button on the menu bar to add outputs. 3. Click the transistor symbol found in the Wiring folder to add transistors. Note, you will have to change the orientation and type of the transistors to build your circuit. Jan,

3 Also note that for both N-type and P-type transistors, the arrow points in the direction of the output when they are properly orientated. 4. The power and ground symbols are also located in the Wiring folder. 5. Click the "Select and Add Wires" tool on the menu bar to add wire connections. Click and drag to connect the input pin to the gate of the transistors. You can only draw vertical and horizontal wires but you can connect wires to each other and to the inputs and outputs of components. 6. Use the Poke tool to exercise your inverter and then check the truth table of your newly constructed inverter to verify that it is correct. 7. Note the following: a. The P-type transistor is connected to high voltage. It connects high voltage to output when the input to its gate is 0. b. The N-type transistor is connected to ground. It connects ground to output when the input to its gate is 1. Show your schematic and truth table to your instructor before proceeding. Be prepared to explain how the circuit works. Part 2: An NAND We can perform a logical function by connecting two switches we just demonstrated that by building an inverter. Additional logic functions such as AND and OR are created by placing the N-type and P-type transistors either in series, or in parallel as shown below. N-type transistors P-type transistors Figure 1: Series connection with N-type & P-type transistors Jan, N-type transistors P-type transistors Figure 2: Parallel connection with N- type & P-type transistors

4 We will build CMOS logic gates which always have an equal number of P-type and N-type transistors. The inverter required just one of each, but to build more complicated functions we will need two or more transistors. 1. Open the Logisim file containing an NAND gate constructed using P-type and N-type transistors. You should see the following in your Logisim window. 2. Notice the following about this schematic: 1. There are two inputs and each input is connected to one P-type and one N- type transistor. 2. The P-type transistors are always connected to high voltage and the N-type transistors are always connected to ground. 3. The arrows on both the P-type and N-type transistors point towards the output. 4. The output separates the P-type and N-type transistors. 5. The P-type transistors are arranged in parallel and the N-type transistors are arranged in series. 3. Use the Poke tool to change the inputs. Verify that the behavior you observe is consistent with its truth table (Project->Analyze Circuit then Truth tab). NAND stands for Not AND. Write down the truth table for AND (write it on a piece of paper), and verify that the NAND truth table in Logisim is the inverse of your hand-written AND truth table. Show your AND truth table to your instructor before proceeding. Be prepared to explain why NAND is implemented with the P-type transistors in parallel and the N- type transistors arranged in series. Jan,

5 Part 3: An NOR Construct the schematic for NOR (Not OR). HINT: Start by writing down the truth table for OR and then inverting it to produce the truth table for NOR. Based on the NOR truth table, determine if the N-type or the P-type transistors are in series; the other transistor type will be in parallel. Show your NOR truth table and schematic to your instructor. Be prepared to explain why you choose your implementation (i.e., which transistors are in series and which are in parallel). Part 4: An OR Combine a NOR circuit and an inverter circuit to produce the OR functionality. Show your OR schematic to your instructor. Part 5: What s wrong with this picture? Why is each of the implementations shown below incorrect? Explain why each of the implementations above is incorrect and then go home. Jan,