Digital Electronics: Logic Gates

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Questions and Answers

A combinational circuit is designed to compare two 2-bit binary numbers, A[1:0] and B[1:0], and produce a single output 'greater_than' that is HIGH only when A > B. Which of the following Boolean expressions correctly represents the 'greater_than' output?

  • NOT A[1] AND B[1] OR (A[1] XOR B[1]) AND (A[0] AND B[0])
  • A[1] AND NOT B[1] OR (A[1] XOR B[1]) AND (A[0] AND NOT B[0]) (correct)
  • A[1] AND B[1] OR (NOT A[1] XOR B[1]) AND (A[0] AND NOT B[0])
  • A[1] AND B[1] OR (A[1] XOR B[1]) AND (A[0] AND B[0])

A system requires a digital circuit to indicate when the temperature is outside the range of 60°F to 80°F. If the temperature is represented by a 7-bit binary number, which logic gate arrangement would best accomplish this?

  • Two comparator circuits, one checking if the temperature is less than 60°F and the other if it's greater than 80°F, with the outputs fed into an OR gate. (correct)
  • A comparator circuit that outputs HIGH if the temperature is between 60°F and 80°F, followed by a NOT gate.
  • A single XOR gate with the temperature value as input.
  • A 7-input AND gate where each input represents a bit of the temperature value.

Which of the following statements accurately describes the key difference between synchronous and asynchronous sequential circuits?

  • Synchronous circuits can handle more complex operations than asynchronous circuits.
  • Synchronous circuits are faster but less stable, while asynchronous circuits are slower but more reliable.
  • Synchronous circuits rely on flip-flops, while asynchronous circuits use only logic gates.
  • Synchronous circuits use a clock signal to synchronize state transitions, while asynchronous circuits do not. (correct)

In digital signal processing (DSP), what is the primary purpose of the Nyquist-Shannon sampling theorem?

<p>To ensure that a discrete-time signal accurately represents its analog counterpart, avoiding aliasing. (D)</p> Signup and view all the answers

Consider a JK flip-flop with a clock signal applied. Under what condition will the output toggle its state on each clock cycle?

<p>J = 1, K = 1 (C)</p> Signup and view all the answers

A 4-to-1 multiplexer (MUX) has four data input lines, two selection lines, and one output line. If the selection lines are connected to the outputs of a counter, what function does this circuit perform?

<p>It selects one of the four inputs based on the counter value and passes it to the output, effectively creating a programmable delay line. (C)</p> Signup and view all the answers

Which of the following is the most accurate definition of 'hold time' in the context of flip-flops?

<p>The minimum time the input signal must remain stable after the clock edge to ensure reliable data capture. (A)</p> Signup and view all the answers

A digital system requires a circuit that can detect if a 4-bit binary number is divisible by 3. Which of the following approaches is most suitable for designing this circuit?

<p>A combinational circuit designed using a Karnaugh map (K-map) to simplify the Boolean expression representing the divisibility condition. (C)</p> Signup and view all the answers

In digital signal processing, what is the purpose of convolution?

<p>To combine two signals to produce a third signal that expresses how the shape of one modifies the other. (B)</p> Signup and view all the answers

You are designing a counter using T flip-flops. If you want the counter to increment its count on every rising edge of the clock signal, what should be the input to each T flip-flop?

<p>A constant HIGH signal (logic 1). (A)</p> Signup and view all the answers

Flashcards

Logic Gates

Fundamental building blocks of digital circuits that perform logical operations on inputs to produce a single output.

AND Gate

The output is HIGH only if all inputs are HIGH.

OR Gate

The output is HIGH if at least one input is HIGH.

NOT Gate

The output is the inverse of the input.

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Combinational Circuits

Circuits that produce outputs based solely on the current inputs without memory.

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Multiplexer (MUX)

Selects one of several input signals and forwards it to the output.

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Sequential Circuits

Circuits that produce outputs based on current inputs and past history of inputs, using memory elements like flip-flops.

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Flip-Flops

Basic memory elements that can store one bit of information.

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Digital Signal Processing (DSP)

Manipulating signals represented in digital form to analyze, modify, and synthesize signals.

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Setup Time

Minimum time the input signal must be stable before the clock edge.

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Study Notes

  • Digital electronics deals with signals that are discrete levels.

Logic Gates

  • Logic gates are the fundamental building blocks of digital circuits.
  • They perform basic logical operations on one or more inputs to produce a single output.
  • The basic logic gates include AND, OR, NOT, NAND, NOR, XOR, and XNOR gates.
  • AND gate: The output is HIGH only if all inputs are HIGH.
  • OR gate: The output is HIGH if at least one input is HIGH.
  • NOT gate: The output is the inverse of the input.
  • NAND gate: The output is LOW only if all inputs are HIGH.
  • NOR gate: The output is LOW if at least one input is HIGH.
  • XOR gate: The output is HIGH if the inputs are different.
  • XNOR gate: The output is HIGH if the inputs are the same.
  • Logic gates can be implemented using transistors.
  • Truth tables are used to represent the behavior of logic gates, showing the output for every possible combination of inputs.
  • Boolean algebra is used to analyze and simplify digital circuits, using variables to represent logic signals.

Combinational Circuits

  • Combinational circuits produce outputs based solely on the current inputs.
  • They do not have memory elements, so the output is a direct function of the input.
  • Examples of combinational circuits include adders, subtractors, multiplexers, demultiplexers, encoders, and decoders.
  • Adders: Perform binary addition, including half adders and full adders.
  • Subtractors: Perform binary subtraction.
  • Multiplexers (MUX): Select one of several input signals and forwards it to the output.
  • Demultiplexers (DEMUX): Direct a single input signal to one of several outputs.
  • Encoders: Convert a set of inputs into a coded output.
  • Decoders: Convert a coded input into a set of outputs.
  • Karnaugh maps (K-maps) are used to simplify Boolean expressions and design combinational circuits.
  • Combinational circuits can be described using Boolean equations or truth tables.
  • Propagation delay is a key parameter in combinational circuits, representing the time it takes for the output to respond to a change in the input.

Sequential Circuits

  • Sequential circuits produce outputs based on the current inputs and the past history of inputs (i.e., they have memory).
  • They use memory elements, such as flip-flops, to store the state of the circuit.
  • Examples of sequential circuits include flip-flops, registers, counters, and state machines.
  • Flip-flops: Basic memory elements that can store one bit of information.
  • Registers: Groups of flip-flops used to store multiple bits of data.
  • Counters: Sequential circuits that increment or decrement a count value.
  • State machines: Sequential circuits that transition between different states based on inputs and current state.
  • Sequential circuits can be synchronous or asynchronous.
  • Synchronous circuits: Changes in state occur at specific times, synchronized by a clock signal.
  • Asynchronous circuits: Changes in state can occur at any time, without a clock signal.
  • State diagrams and state tables are used to describe the behavior of sequential circuits.
  • Key parameters for sequential circuits include setup time, hold time, and clock frequency.

Digital Signal Processing

  • Digital signal processing (DSP) involves manipulating signals represented in digital form.
  • DSP techniques are used to analyze, modify, and synthesize signals.
  • Common DSP operations include filtering, convolution, correlation, and Fourier analysis.
  • Filtering: Removing unwanted components from a signal.
  • Convolution: Combining two signals to produce a third signal.
  • Correlation: Measuring the similarity between two signals.
  • Fourier analysis: Decomposing a signal into its frequency components.
  • DSP systems typically include analog-to-digital converters (ADCs) to convert analog signals into digital signals and digital-to-analog converters (DACs) to convert digital signals back into analog signals.
  • Discrete-time signals are sequences of values sampled at discrete points in time.
  • The Nyquist-Shannon sampling theorem states that a signal must be sampled at a rate at least twice its maximum frequency to avoid aliasing.
  • Z-transforms are used to analyze discrete-time signals and systems.
  • DSP is used in a wide range of applications, including audio processing, image processing, telecommunications, and control systems.
  • DSP algorithms are often implemented using specialized hardware, such as digital signal processors (DSPs) or field-programmable gate arrays (FPGAs).

Flip-flops

  • Flip-flops are fundamental building blocks of sequential circuits.
  • They are bistable devices, meaning they have two stable states (0 and 1).
  • Flip-flops are used to store one bit of digital information.
  • Common types of flip-flops include SR, JK, D, and T flip-flops.
  • SR flip-flop: Has set (S) and reset (R) inputs.
  • JK flip-flop: Similar to the SR flip-flop but avoids the invalid state when both inputs are HIGH.
  • D flip-flop: Has a single data (D) input.
  • T flip-flop: Has a toggle (T) input, which causes the flip-flop to change state.
  • Flip-flops can be implemented using logic gates, such as NAND or NOR gates.
  • Flip-flops can be triggered by the rising edge, falling edge, or level of a clock signal.
  • Setup time: The minimum time the input signal must be stable before the clock edge.
  • Hold time: The minimum time the input signal must remain stable after the clock edge.
  • Flip-flops are used in registers, counters, and other sequential circuits.
  • The characteristic equation of a flip-flop describes the next state as a function of the current state and inputs.

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