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Chapter 3 (part 1) Basic Logic Gates 1 1. Introduction Logic gates are the basic building blocks for forming digital electronic circuitry. A logic gate.

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Presentation on theme: "Chapter 3 (part 1) Basic Logic Gates 1 1. Introduction Logic gates are the basic building blocks for forming digital electronic circuitry. A logic gate."— Presentation transcript:

1 Chapter 3 (part 1) Basic Logic Gates 1 1

2 Introduction Logic gates are the basic building blocks for forming digital electronic circuitry. A logic gate has –One output terminal; One or more input terminals –The outputs is either HIGH(1) or LOW(0) depending on the digital levels at the input terminals and the property of the gate –Five basic logic gates AND, OR NAND, NOR and inverter 2

3 3-1 The AND Gate The output, X, is HIGH if input A AND input B are both HIGH. –If A = 1 and B = 1, then X = 1. –If A = 0 or B = 0, then X = 0. Schematic symbol of two-input AND gate 3

4 Truth table for AND Gate All possible input combinations Resultant output 4

5 Figure 3-2 Use AND gate to activate a burglar alarm 5

6 Figure 3.3 Electrical analogy for an AND gate (a) using manual switches; (b) using transistor switches. 6

7 Boolean equation for AND Gate Boolean Equation: X = AB or X = AB –Read as X equals A AND B –Can have more than two inputs Equation 3-1 Number of combinations in truth table = 2 N where N = number of inputs 7

8 Figure 3-4 Multiple-input AND gate symbols 8

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10 3-2 The OR Gate The output at X will be HIGH whenever input A OR input B is HIGH or both are HIGH –If A = 1 and/or B = 1, then X = 1 –If A = 0 and B = 0, then X = 0 Schematic symbol of two-input OR gate 10

11 Truth table for the OR Gate 11

12 Figure 3.6 Electrical analogy for an OR gate (a) using manual switches; (b) using transistor switches. 12

13 Boolean equation for the OR Gate Boolean Equation: X = A + B –Read as, X = A or B Can have more than two inputs Number of combinations = 2 N –N = number of input bits 13

14 Figure 3-7 Multiple-input OR gate symbols 14

15 Figure 3.8 eight-input OR gate symbol 15

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18 3-3 Timing Analysis Timing diagrams are used to analyze the output response to varying inputs Oscilloscope –Display plots voltage versus time –Dual-trace can display two waveforms at a time. Logic analyzer –Can display state tables that show binary levels on signal lines –Can display up to 16 waveforms at a time 18

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22 3-4 Enable and Disable Functions AND and OR gates can be used to enable or disable a waveform from being transmitted from one point to another. Example: Use a 1-MHz clock oscillator to transmit only four pulses –Solution: Enable four clock pulses to be transmitted, and then disable the transmission from then on. 22

23 Enable function using AND gate Figure 3-16 23

24 Disable function using OR gate Figure 3-17 24

25 3-5 Using IC Logic Gates AND and OR gates are available as ICs. Data Manual from manufacturer includes –IC pin layout, Logic gate type, Technical specifications Example ICs –7408(74LS08,74HC08): quad two-input AND gate –7411(74LS11,74HC11):dual four-input AND gate –7432(74LS32,74HC32):quad two-input OR gate 25

26 Common prefixes and suffixes Family designation –LS – Lower power TTL family –HC – High-speed CMOS family –(74LS11,74HC11):dual four-input AND gate Manufacturer –SN: Texas Instruments. E.g., SN7400 Package style (suffix) –N: plastic dual-in-line package (P-DIP), SN7400N –M: Small-outline Integrated Circuit (SOIC) DIP SOIC 26

27 Figure 3.19 The 7408 quad two-input AND gate IC pin configuration. The top side typically has Indented circle / notch cut 27

28 Figure 3.20 Using the 7408 TTL IC in the clock enable circuit of Figure 3–17. 28

29 Figure 3.21 Pin configurations for other popular TTL and CMOS AND and OR gate ICs: (a) 7411 (74HC11); (b) 7421 (74HC21); (c) 7432 (74HC32). 29

30 3-6 Introduction to Troubleshooting Techniques Troubleshooting –The procedure used to find the fault or trouble in a circuit. Tools to test ICs and digital circuits –Logic Probe shows digital levels: Low, High, and floating - open circuit, neither high nor low –Logic Pulser provides digital pulses to a circuit being tested. –Both tools allow you to tell if the pulse signal is getting through the IC as expected. 30

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33 33 Gate 2 Gate 3

34 34 Gate 1 Gate 4


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