Chapter 3 (part 1)

Basic Logic Gates

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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

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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

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Truth table for AND Gate

All possible input combinations Resultant output

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

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Figure 3.3 Electrical analogy for an AND gate

(a) using manual switches; (b) using transistor switches.

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Boolean equation for AND Gate

• Boolean Equation: X = A•B 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 = 2N

where N = number of inputs

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Figure 3-4 Multiple-input AND gate symbols

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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

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Truth table for the OR Gate

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Figure 3.6 Electrical analogy for an OR gate

(a) using manual switches; (b) using transistor switches.

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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 = 2N

– N = number of input bits

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Figure 3-7 Multiple-input OR gate symbols

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Figure 3.8 eight-input OR gate symbol

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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

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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.

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Enable function using AND gateFigure 3-16

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Disable function using OR gate Figure 3-17

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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

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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

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Figure 3.19 The 7408 quad two-input AND gate IC pin configuration.

The top side typically hasIndented circle / notch cut

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

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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).

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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.

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Gate 2 Gate 3

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Gate 1 Gate 4