2IntroductionLogic gates are the basic building blocks for forming digital electronic circuitry.A logic gate hasOne output terminal; One or more input terminalsThe outputs is either HIGH(1) or LOW(0) depending on the digital levels at the input terminals and the property of the gateFive basic logic gatesAND, OR NAND, NOR and inverter
33-1 The AND GateThe 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
4Truth table for AND Gate Resultant outputAll possible input combinations
5Figure 3-2 Use AND gate to activate a burglar alarm The output of the AND gate (A) goes HIGH to turn on the alarm, if the alarm activation key (K) is in the ON position AND the front door (D) is opened.
6Figure 3.3 Electrical analogy for an AND gate (a) using manual switches; (b) using transistor switches.
7Boolean equation for AND Gate Boolean Equation: X = A•B or X = ABRead as “X equals A AND B”Can have more than two inputsEquation 3-1Number of combinations in truth table = 2Nwhere N = number of inputs
183-3 Timing AnalysisTiming diagrams are used to analyze the output response to varying inputsOscilloscopeDisplay plots voltage versus timeDual-trace can display two waveforms at a time.Logic analyzerCan display state tables that show binary levels on signal linesCan display up to 16 waveforms at a time
223-4 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 pulsesSolution: Enable four clock pulses to be transmitted, and then disable the transmission from then on.
23Enable function using AND gate Figure 3-16 Clock frequency is 1MHzThe circuit and waveforms to enable four clock pulses. Low -> disable
24Disable function using OR gate Figure 3-17 High -> disable
253-5 Using IC Logic Gates AND and OR gates are available as ICs. Data Manual from manufacturer includesIC pin layout, Logic gate type, Technical specificationsExample ICs7408(74LS08,74HC08): quad two-input AND gate7411(74LS11,74HC11):dual four-input AND gate7432(74LS32,74HC32):quad two-input OR gate
27Figure 3.19 The 7408 quad two-input AND gate IC pin configuration. The top side typically hasIndented circle / notch cut
28Figure 3.20 Using the 7408 TTL IC in the clock enable circuit of Figure 3–17. The clock oscillator signal passes on to the receiving device when the switch is in the enable (1) position, it stops when in the disable (0) position.
29Figure 3.21 Pin configurations for other popular TTL and CMOS AND and OR gate ICs: (a) 7411 (74HC11);(b) 7421 (74HC21);(c) 7432 (74HC32).
303-6 Introduction to Troubleshooting Techniques The procedure used to find the fault or trouble in a circuit.Tools to test ICs and digital circuitsLogic Probe shows digital levels: Low, High, and floating - open circuit, neither high nor lowLogic 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.Indicator lamp glows strongly if highLamp is off if output is lowGlows dimly if floating
31Apply power to Vcc and GND. Check each AND gate with the pulser/probe.When pulser is on pin 12, Vcc is connected with pin 13, the light in the end of the probe flashes at the same speed as the pulser -> good.Reverse the connections to pins 12 and 13 and check the probe again.
32Apply power to Vcc and GND. Check each OR gate with the pulser/probe.When pulser is on pin 2, GND is connected with pin 1, the light in the end of the probe flashes at the same speed as the pulser -> good.Reverse the connections to pins 1 and 2 and check the probe again.