Digital Systems

What is the difference between combinational and sequential circuits?

Combinational: output depends only on current inputs; Sequential: output depends on current and past inputs.

What enables real logic gates to store state?

Transient delays allow for feedback, enabling memory in sequential circuits.

Name four important sequential logic elements.

Bistables, Latches, Flip-Flops, State Machines.

What is a bistable circuit?

A circuit with two stable states due to feedback.

What does a Gated SR Latch do?

Adds an enable input to control set/reset functionality.

What is a D-type Flip-Flop?

A flip-flop that transfers D input to Q output on a clock edge.

What are the actions for J-K Flip-Flop inputs (J=1, K=1)?

Toggle the output.

Which flip-flop is ideal for counters?

J-K Flip-Flop.

What distinguishes Moore and Mealy machines?

Moore: output depends only on state; Mealy: output depends on state and input.

What are the steps in designing a state machine?

Define states, create excitation table, simplify logic, implement circuit.

What is an FPGA?

Field Programmable Gate Array - hardware reconfigurable logic device.

What are CLBs in FPGAs?

Configurable Logic Blocks - contain LUTs, Flip-Flops, and multiplexers.

What does VHDL stand for?

VHSIC Hardware Description Language.

What are common VHDL signal values?

'0', '1', '-', 'X', 'U'

What is the purpose of signals in VHDL?

To define internal connections and intermediate logic.

What is MTBF?

Mean Time Between Failures - average time between repairable failures.

What does the 'bathtub curve' represent?

Lifecycle of a product: early failures, stable use, wear-out.

What does the Arrhenius equation estimate?

Accelerated failure rate based on temperature.

What is traceability in electronics?

Tracking parts' origins and specifications for quality control.

How do you prevent ESD damage?

Use grounded mats, wrist straps, and ESD-safe packaging.

Signal

A function that conveys information about a physical system; can be continuous or discrete.

Continuous-Time Signal

A signal defined for all values of time, denoted x(t).

Discrete-Time Signal

A signal defined only at discrete time instants, denoted x[n].

Time Shifting

Delaying or advancing a signal: x(t - t1) shifts right, x(t + t1) shifts left.

Time Scaling

Compressing or expanding a signal in time: x(bt), where b > 1 compresses and 0 < b < 1 expands.

Time Reversal

Flips the signal about the vertical axis: x(-t).

Amplitude Scaling

Multiplies signal by a constant: y(t) = a * x(t).

Even Signal

Symmetric about the origin: x(t) = x(-t).

Odd Signal

Antisymmetric about the origin: x(t) = -x(-t).

Periodic Signal

Repeats over time: x(t) = x(t + T) for some T > 0.

Real Exponential Signal

x(t) = A * e^(λt), where A and λ are real constants.

Sinusoidal Signal

x(t) = A cos(ωt + θ); A is amplitude, ω is angular frequency, θ is phase

Complex Exponential

x(t) = A * e^(jωt) = A(cos(ωt) + j sin(ωt))

Unit Impulse (CT)

An ideal signal δ(t) with infinite height, zero width, and unit area.

Unit Impulse (DT)

δ[n] = 1 when n = 0, and 0 otherwise.

Unit Step Function

u(t) = 0 for t < 0, and 1 for t ≥ 0

Causal System

Output depends only on present and past inputs.

Memoryless System

Output depends only on current input.

Time-Invariant System

System behavior doesn't change over time.

Linear System

Follows superposition principle: y = a1y1 + a2y2.

LTI System

Linear and time-invariant system, fully described by impulse response.

Fourier Series

Represents periodic signals as a sum of sinusoids.

Convolution (CT)

y(t) = ∫ x(τ) h(t - τ)

Convolution (DT)

y[n] = Σ x[k] h[n - k]

Fourier Transform

Extends Fourier Series to non-periodic signals: X(ω) = ∫ x(t) e^(-jωt) dt

Convolution Property (FT)

Convolution in time = multiplication in frequency: Y(ω) = X(ω)H(ω)

What distinguishes digital from analogue electronics?

Digital uses binary 0s and 1s, while analogue uses continuous voltages and currents.

List two advantages of digital electronics.

Better noise immunity, supports logical operations and DSP.

Name three basic logic gates.

AND, OR, NOT.

What are universal logic gates?

NAND and NOR.

What is Boolean algebra used for in digital electronics?

To describe logic operations.

What does TTL stand for?

Transistor-Transistor Logic.

What makes LSTTL different from TTL?

Lower power consumption.

Why is CMOS preferred over TTL in some cases?

It consumes less power and supports a wider voltage range.

Match the integration level: VLSI

10,000 to 100,000 gates.

Which logic gate type is used in a 7400 chip?

NAND gate.

What is fanout in digital logic?

The number of inputs a single output can drive.

Typical fanout for TTL logic?

40

Name two noise sources in digital circuits.

RF interference and thermal noise.

What is a totem pole output?

A push-pull configuration that actively drives both high and low outputs.

What is the advantage of Schottky TTL over standard TTL?

Faster switching by avoiding deep saturation.

What does an open collector output require?

An external pull-up resistor.

What logic device is used to connect two different Vcc logic families?

Open collector output with pull-up resistor.

What does the gate of a MOSFET control?

The current flow from drain to source.

What happens when both inputs are high in a CMOS NAND gate?

Output goes LOW.

What does the 'B' in 4011B indicate?

Buffered output.

Name one open-drain CMOS logic device.

74HC03.

What protects CMOS inputs from ESD?

Diode clamps.

What is Binary Coded Decimal (BCD)?

A 4-bit representation of decimal digits 0-9; values above 9 (like 1010) are invalid.

What voltage levels define HIGH and LOW in TTL logic?

HIGH: 2V-5V, LOW: 0V-0.8V

What does a truth table represent?

The output of a logic gate or circuit for all possible input combinations.

Which gates are considered universal gates?

NAND and NOR

What is DeMorgan's first theorem?

¬(A + B) = ¬A · ¬B

What is a Sum of Products (SOP) expression?

ORed terms formed from ANDed variables (e.g., AB + A'C)

What is the purpose of Karnaugh Maps (K-maps)?

To visually simplify Boolean expressions by grouping terms.

What are the steps in designing a combinational circuit?

Truth Table → SOP/POS → Simplify → Implement

What does "Active-LOW" mean in digital logic?

A function is enabled when the input is 0 (represented by a bubble).

What is the output of a majority circuit with 3 inputs?

X = AB + AC + BC

What is the XOR gate's output when inputs differ?

1 (True)

What does a Half-Adder compute?

Sum = A ⊕ B, Carry = AB

What does an Encoder do?

Converts multiple inputs into a smaller number of outputs (e.g., 4-to-2 encoder).

What does a Decoder do?

Converts fewer inputs into multiple outputs (e.g., 2-to-4 decoder).

What is the function of a Multiplexer (Mux)?

Selects one of many inputs to pass to the output.

What is the function of a Demultiplexer (Demux)?

Routes a single input to one of many outputs.