Chapter 3 (part 1) Basic Logic Gates 1

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

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

Truth table for AND Gate Resultant output All possible input combinations

Figure 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.

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

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

Figure 3-4 Multiple-input AND gate symbols

Column A lists eight 0s, then eight 1s; Column B lists four 0x, four 1s, four 0s, four 1s.

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

Truth table for the OR Gate

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

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

Figure 3-7 Multiple-input OR gate symbols 8 input OR gate has 256 entries.

Figure 3.8 eight-input OR gate symbol

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

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.

Enable function using AND gate Figure 3-16 Clock frequency is 1MHz The circuit and waveforms to enable four clock pulses. Low -> disable

Disable function using OR gate Figure 3-17 High -> disable

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

Common prefixes and suffixes DIP 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) SOIC

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

Figure 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.

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).

3-6 Introduction to Troubleshooting Techniques 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. Indicator lamp glows strongly if high Lamp is off if output is low Glows dimly if floating

Apply 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.

Apply 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.

Gate 2 Gate 3 Gate 2 Gate 3

Gate 1 Gate 4 Gate 1 Gate 4