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EET 1131 Unit 3 Basic Logic Gates Read Kleitz, Chapter 3. Homework #3 and Lab #3 due next week. Quiz next week. Logic Gates Logic gates are the building blocks in digital circuits. Shown below is a circuit containing ten logic gates. (This is Figure 5-56 on page 187.) Inputs and Outputs This circuit has three inputs (labeled A, B, C) and one output (labeled X). The values at the inputs determine the value at the output. Three Ways of Describing a Circuit To describe a circuit built up from logic gates, we can: 1. Draw a schematic diagram. 2. Write a truth table. 3. Write a Boolean expression. Truth Tables and Boolean Expressions A truth table shows a circuit’s output value for every possible combination of input values. A Boolean expression looks much like an algebraic expression and uses symbols with special meanings. The Inverter A X The inverter performs the Boolean NOT operation. When the input is LOW (0), the output is HIGH (1); when the input is HIGH, the output is LOW. Input Output A X 0 1 1 0 The NOT operation (complement) is shown with an overbar. Thus, the Boolean expression for an inverter is X = A. © 2009 Pearson Education, Upper Saddle River, NJ 07458. All Rights Reserved The Inverter A X Example waveforms: A X © 2009 Pearson Education, Upper Saddle River, NJ 07458. All Rights Reserved The AND Gate A X A B B & X The AND gate produces a HIGH output when all inputs are HIGH; otherwise, the output is LOW. For a 2-input gate, the truth table is Inputs Output A B X 0 0 1 1 0 1 0 1 0 0 0 1 The AND operation is usually shown with a dot between the variables but it may be implied (no dot). Thus, the AND operation is written as X = A .B or X = AB. © 2009 Pearson Education, Upper Saddle River, NJ 07458. All Rights Reserved AND Gate: A Practical Application Many cars have an alarm that sounds if a door is open while the key is in the ignition: Door Open? Key in Ignition? No No No Yes Yes No Yes Yes Sound the Alarm? The AND Gate A B X A B & X Example waveforms: A B X © 2009 Pearson Education, Upper Saddle River, NJ 07458. All Rights Reserved The AND Gate A Multisim circuit is shown. XWG1 is a word generator set in the count down mode. XLA1 is a logic analyzer with the output of the AND gate connected to first (upper) line of the analyzer. What signal do you expect to on this line? The output (line 1) will be HIGH only when all of the inputs are HIGH. © 2009 Pearson Education, Upper Saddle River, NJ 07458. All Rights Reserved The OR Gate A B X A B ≥1 X The OR gate produces a HIGH output if any input is HIGH; if all inputs are LOW, the output is LOW. For a 2-input gate, the truth table is Inputs Output A B X 0 0 1 1 0 1 0 1 0 1 1 1 The OR operation is shown with a plus sign (+) between the variables. Thus, the OR operation is written as X = A + B. © 2009 Pearson Education, Upper Saddle River, NJ 07458. All Rights Reserved OR Gate: A Practical Application Many cars have a “Door Ajar” dashboard light that lights up if any door on the car is open. Driver Door Open? Passenger Door Open? No No No Yes Yes No Yes Yes Turn on the “Door Ajar” Light? The OR Gate A B X A B ≥1 X Example waveforms: A B X © 2009 Pearson Education, Upper Saddle River, NJ 07458. All Rights Reserved The OR Gate A Multisim circuit is shown. XWG1 is a word generator set to count down. XLA1 is a logic analyzer with the output connected to first (top) line of the analyzer. The three 2-input OR gates act as a single 4-input gate. What signal do you expect on the output line? The output (line 1) will be HIGH if any input is HIGH; otherwise it will be LOW. © 2009 Pearson Education, Upper Saddle River, NJ 07458. All Rights Reserved The NAND Gate A A X & X B B The NAND gate produces a LOW output when all inputs are HIGH; otherwise, the output is HIGH. For a 2-input gate, the truth table is Inputs Output A B X 0 0 1 1 0 1 0 1 1 1 1 0 The NAND operation is shown with a dot between the variables and an overbar covering them. Thus, the NAND operation is written as X = A .B (Alternatively, X = AB.) © 2009 Pearson Education, Upper Saddle River, NJ 07458. All Rights Reserved The NAND Gate A B X A & X B Example waveforms: A B X The NAND gate is particularly useful because it is a “universal” gate – all other basic gates can be constructed from NAND gates. © 2009 Pearson Education, Upper Saddle River, NJ 07458. All Rights Reserved The NAND Gate A Multisim circuit is shown. XWG1 is a word generator set in the count up mode. A four-channel oscilloscope monitors the inputs and output. What output signal do you expect to see? The output (channel D) will be LOW only when all of the inputs are HIGH. Inputs © 2009 Pearson Education, Upper Saddle River, NJ 07458. All Rights Reserved The NOR Gate A B X A B ≥1 X The NOR gate produces a LOW output if any input is HIGH; if all inputs are LOW, the output is HIGH. For a 2-input gate, the truth table is Inputs Output A B X 0 0 1 1 0 1 0 1 1 0 0 0 The NOR operation is shown with a plus sign (+) between the variables and an overbar covering them. Thus, the NOR operation is written as X = A + B. © 2009 Pearson Education, Upper Saddle River, NJ 07458. All Rights Reserved The NOR Gate A B X A B ≥1 X Example waveforms: A B X The NOR operation will produce a LOW if any input is HIGH. © 2009 Pearson Education, Upper Saddle River, NJ 07458. All Rights Reserved Enable/Disable Using AND Gate 14 Enable/Disable Using OR Gate 15 Integrated Circuits Cutaway view of DIP (Dual-In-line Package) chip: Plastic Chip case Pins The TTL series, available as DIPs are popular for laboratory experiments with logic. © 2009 Pearson Education, Upper Saddle River, NJ 07458. All Rights Reserved Integrated Circuits DIP chips and surface mount chips Pin 1 Dual in-line package Small outline IC (SOIC) © 2009 Pearson Education, Upper Saddle River, NJ 07458. All Rights Reserved Integrated Circuits Chip Densities: •Small scale integration (SSI): <=10 gates per chip. •Medium-scale integration (MSI): 10 to 100 gates per chip. •Large-scale integration (LSI): 100 to 10,000 gates per chip. •Very large-scale integration (VLSI): 10,000 to 100,000 gates per chip. © 2009 Pearson Education, Upper Saddle River, NJ 07458. All Rights Reserved Programmable Logic Programmable logic devices (PLDs) are an alternative to fixed function devices. The logic can be programmed for a specific purpose. In general, they cost less and use less board space than fixed function devices. © 2009 Pearson Education, Upper Saddle River, NJ 07458. All Rights Reserved Figure 3.50 Pin configuration diagrams for some common gate configurations. Digital Fundamentals, Tenth Edition Thomas L. Floyd Copyright ©2009 by Pearson Higher Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Website for Datasheets Many websites have datasheets for logic chips. The best site is Texas Instruments: www.ti.com Type the chip number (such as 7404) in the Search box. For some chips, the original part number is obsolete and no longer available. In such cases, inserting LS often works. Example: 74LS10. Testing Gates in Multisim Multisim lets you build and simulate circuits containing gates from the 7400 series of chips. Troubleshooting •Troubleshooting means finding and fixing faults in a circuit or system that’s not working correctly. •Your first step in troubleshooting a digital circuit on the breadboard should always be to verify that the voltage at each chip’s power and ground pins are actually +5 V and 0V. •After that, use your knowledge of truth tables to find the gates that are not producing the correct outputs. © 2009 Pearson Education, Upper Saddle River, NJ 07458. All Rights Reserved Multisim Troubleshooting •Multisim lets you insert the following kinds of faults into digital components: •An open pin •Two pins shorted together •A pin shorted to VCC (constant HIGH) •A pin shorted to ground (constant LOW) •Your book has Multisim troubleshooting problems that I’ll assign on some labs and homeworks. •To troubleshoot them, attach switches to input pins and probes to output pins.