Question 1

Frequency counter front end: role of the Schmitt trigger In a digital frequency counter, what is the primary function of the Schmitt trigger at the input stage?

Accepted Answer

Introduction / Context: Frequency counters must transform messy, analog-like inputs into clean logic-level pulses for accurate counting. A Schmitt trigger input stage provides hysteresis, eliminating spurious transitions due to noise or slow edges. Given Data / Assumptions: Incoming signals may be sine, triangular, or noisy square waves. Counters require fast, debounce-free edges at logic thresholds. Schmitt trigger adds two thresholds (VT+ and VT−) to create hysteresis. Concept / Approach: Signal conditioning converts the analog-ish input into a robust digital waveform. Schmitt triggers reshape slow or noisy transitions into sharp edges by switching only when input rises above VT+ or falls below VT−, greatly improving measurement accuracy. Step-by-Step Solution: Apply hysteresis to reject noise near the threshold.Square up edges to produce uniform pulses.Feed conditioned pulses to the counting logic.Ensure repeatable triggering within the timing gate interval. Verification / Alternative check: Oscilloscope comparison before/after a Schmitt trigger shows reduced chatter and consistent pulse widths. Why Other Options Are Wrong: A/C are partial descriptions; “signal conditioning” is the comprehensive function encompassing noise reduction and waveform squaring. D: The display unit uses decoded count data, not the raw input signal. Common Pitfalls: Using a plain comparator without hysteresis can cause multiple counts from small noise excursions. Final Answer: To condition the input signal

Question 2

Digital clock timing: required division of the basic frequency In a standard digital clock application, to generate the “tick” that advances the seconds counter, the basic frequency must be divided down to what rate?

Accepted Answer

Introduction / Context: Whether your time base is the mains frequency (50/60 Hz) or a crystal oscillator (for example, 32.768 kHz), a digital clock requires a precise 1 Hz pulse to advance the seconds counter once per second. Given Data / Assumptions: Seconds increment once each second. Higher-frequency references must be divided (prescaled). Stable division ensures accurate timekeeping. Concept / Approach: Use a frequency divider so that the output is 1 tick per second. For a 60 Hz input, divide by 60; for 32.768 kHz, divide by 32,768. The exact divider topology may use cascaded binary or BCD counters in HDL. Step-by-Step Solution: Identify the reference frequency.Compute division factor to reach 1 Hz.Implement counters in HDL to realize the division.Feed the 1 Hz output to the seconds counter. Verification / Alternative check: Simulate and measure output period; confirm 1 second between rising edges. Why Other Options Are Wrong: 60 Hz/100 Hz/1000 Hz correspond to upstream references or debounce rates, not the seconds tick. Common Pitfalls: Neglecting oscillator tolerance and temperature; consider calibration if needed. Final Answer: 1 Hz.

Question 3

HDL project scoping: which item is not strictly a scope-definition step? When defining the scope of a hardware description language (HDL) project, identify which item below does not belong to initial scope definition and is more about detailed imple…

Accepted Answer

Introduction / Context: Project scope establishes boundaries and requirements before implementation. In HDL work, scope definition clarifies interfaces, performance, and data widths, enabling modular design and estimation. Detailed “how” decisions typically follow in the design phase, not in scope-setting. Given Data / Assumptions: Scope focuses on “what” and “why” more than “how.” Key scope elements: I/O polarity, performance targets, word sizes. Implementation details (specific techniques) are subsequent steps. Concept / Approach: Distinguish requirements from solutions. Scope should define signals, throughput/latency limits, and resource ranges. Devising the exact internal mechanisms for each block is a design activity after scope agreement. Step-by-Step Solution: Review each option and classify as requirement or implementation.I/O polarity → requirement (scope).Speed requirements → requirement (scope).Data width (# of bits) → requirement (scope).“How to make each block work” → implementation detail (not scope). Verification / Alternative check: Standard development lifecycles: requirements → architecture → design → implementation → verification. Why Other Options Are Wrong: A/C/D are direct, scoping-level requirements. Common Pitfalls: Locking into a specific solution too early can preclude better architectures discovered later. Final Answer: A clear vision of how to make each block work

Question 4

Frequency counter data path: when is the new count transferred to the display register? Consider the timing of a gated frequency counter in HDL. Identify when the fresh count value is latched into the display register for stable presentation.

Accepted Answer

Introduction / Context: In a gated frequency counter, the measurement cycle consists of opening a timing gate, counting input pulses, closing the gate, and then transferring the stable result to a display register while the next measurement begins. This requires coordinated timing and control. Given Data / Assumptions: A dedicated timing/control FSM (finite state machine) orchestrates gate, count, and latch operations. Display register must capture the count only when stable (post gate-close). Transfer may coincide with a latch/LD signal generated by control logic. Concept / Approach: The transfer is explicitly commanded by the timing/control block after the counting window ends. This guarantees that the displayed value does not glitch while counting continues internally for the next interval. Step-by-Step Solution: Gate opens → counter accumulates pulses.Gate closes → count is stable.Control asserts “load display” → value moves to display register.Display remains steady until next cycle’s transfer. Verification / Alternative check: Simulations show clean latch timing edges aligning with end-of-gate signals. Why Other Options Are Wrong: A/C describe phases but not the authoritative transfer event. B: “Buffer full” is not a standard event in simple counters. Common Pitfalls: Capturing mid-count causes flicker or incorrect readings. Final Answer: When the timing and control block has put it there

Question 5

Digital clock minutes section: input rate of the MOD-6 (tens-of-minutes) counter In a 12-hour digital clock minutes chain (units-of-minutes: MOD-10; tens-of-minutes: MOD-6), what is the correct input event rate for the tens-of-minutes (MOD-6) counte…

Accepted Answer

Introduction / Context: Digital clocks represent minutes with two cascaded counters: a MOD-10 for units (0–9) and a MOD-6 for tens (0–5). Understanding the carry/rollover behavior between these counters is essential to design correct timing chains in HDL. Given Data / Assumptions: Seconds counter provides a 1 pulse per minute output to the minutes-units counter (MOD-10). The minutes-units counter generates a carry when it rolls over from 9 to 0. The tens-of-minutes counter (MOD-6) should advance on that carry event. Concept / Approach: The tens-of-minutes counter must increment once for every ten minutes elapsed, because the units-of-minutes runs from 0 to 9 each minute. Thus, its input rate is one-tenth the minutes-units input rate. Step-by-Step Solution: Minutes-units (MOD-10) input: 1 pulse per minute.Units roll over every 10 minutes → emit a carry pulse.Tens-of-minutes (MOD-6) increments on that carry.Therefore, tens-of-minutes input = 1 pulse every 10 minutes (i.e., 6 per hour). Verification / Alternative check: Timing diagram over one hour shows six increments of the tens-of-minutes counter at 00, 10, 20, 30, 40, and 50 minutes. Why Other Options Are Wrong: A: Incorrect; that rate applies to the minutes-units counter, not tens. C: Physically impossible; tens counter does not increment multiple times per minute. D: It increments six times per hour, not once. Common Pitfalls: Feeding the tens counter directly with the 1-minute tick rather than the units’ carry line leads to an illegal 0–5 sequence every minute. Final Answer: 1 pulse every 10 minutes

Question 6

Frequency counter design math: dividing a 100 kHz system clock to 1 Hz A frequency counter uses a 100 kHz system clock as its timing reference. How many cascaded decade (/10) counters are required to obtain an accurate 1 Hz timing signal?

Accepted Answer

Introduction / Context: Gate-time generation is central to frequency counters. If the system clock is 100 kHz, the designer must divide it down to 1 Hz to define a 1-second measurement interval. Decade counters are a common HDL-friendly choice for divisions by powers of ten. Given Data / Assumptions: System clock f_sys = 100 kHz. Target gate frequency f_gate = 1 Hz. Available blocks: decade (/10) counters. Concept / Approach: To go from 100,000 Hz to 1 Hz requires an overall division of 100,000. Using only /10 counters, we need N such that 10^N = 100,000. Step-by-Step Solution: Compute division ratio: 100,000 / 1 = 100,000.Express as power of 10: 100,000 = 10^5.Therefore, number of /10 stages N = 5.Implement five cascaded decade counters to produce a 1 Hz enable. Verification / Alternative check: Simulation: observe one output rising edge per 100,000 input cycles; period = 1 s at 100 kHz input. Why Other Options Are Wrong: 6: Would divide to 0.1 Hz (10 seconds period). 4: Divides to 10 Hz. 3: Divides to 100 Hz. Common Pitfalls: Mixing decimal and binary divisors; be consistent with chosen counter modulus. Final Answer: 5

Question 7

Digital clock indicator logic: when should the PM indicator assert (go HIGH)? In a 12-hour digital clock, identify the exact time transition at which the PM indicator should change state from LOW to HIGH.

Accepted Answer

Introduction / Context: A 12-hour clock toggles its AM/PM indicator at noon and midnight. Correctly placing the transition is important to avoid off-by-12-hours display bugs when minutes and hours roll over. Given Data / Assumptions: AM corresponds to times 12:00:00 through 11:59:59 of the first half-day. PM corresponds to times 12:00:00 through 11:59:59 of the second half-day. We consider the exact second-level transition. Concept / Approach: The PM indicator asserts at the instant the clock moves into the afternoon period: precisely at 12:00:00 noon (following 11:59:59 AM). It deasserts at midnight (12:00:00 AM) when the cycle restarts. Step-by-Step Solution: Track time through 11:59:59 AM.Increment seconds → 12:00:00 noon.Toggle AM/PM flip-flop → PM goes HIGH at this boundary.Next toggle at 11:59:59 PM → 12:00:00 AM (PM goes LOW). Verification / Alternative check: Truth table of AM/PM over a 24-hour period confirms toggles only at the two 12:00:00 transitions. Why Other Options Are Wrong: A: The indicator certainly toggles twice per day. C: 12:59:59 to 01:00:00 does not change AM/PM. D: Refers to a signal edge rather than the time-state boundary. Common Pitfalls: Incorrectly toggling at 11:59:59 or 12:00:01 introduces a one-second display error. Final Answer: Going from 11:59:59 to 12:00:00

Question 8

Strategic planning for HDL development: identify the item that is not a strategic-planning step During upfront strategic planning of an HDL system, which of the following is not a strategic step, but rather a detailed specification activity?

Accepted Answer

Introduction / Context: Successful HDL projects begin with strategy: verification approach, integration plan, and functional definitions. Some details—such as exact signal names—are important, but they belong to lower-level specification and coding standards rather than strategy. Given Data / Assumptions: Strategic planning emphasizes testability, modular integration, and clear functional definition. Signal naming is crucial but is a documentation/coding convention step. We aim to separate strategic concerns from implementation details. Concept / Approach: Strategy ensures that each block can be verified independently and integrated cleanly. Functional behavior must be understood to prevent rework. Exact names can be assigned later or evolve with the design without changing the strategy. Step-by-Step Solution: Identify strategic items: testing approach, system partitioning, functional definitions.Identify detailed specification item: precise signal names.Select the non-strategic item as the answer.Retain the rest as strategic planning components. Verification / Alternative check: Typical design flows: strategy → requirements/architecture → interface spec (names) → RTL coding → verification. Why Other Options Are Wrong: A: Test plans and DFT are strategic. B: Architectural fit and interfaces are strategic. D: Thorough functional definition is strategic. Common Pitfalls: Confusing naming conventions with architectural strategy; keep documentation flexible early on. Final Answer: The names of each input and output must be known.

Question 9

Frequency counter behavior at high input frequencies In a digital frequency counter, what typically happens when the input frequency is very high and the sampling (gate) interval is chosen too long relative to the counter's register capacity and des…

Accepted Answer

Introduction / Context: A frequency counter counts the number of input cycles during a fixed gate (sampling) interval, then computes frequency as counts / gate_time. At very high input frequencies, the number of pulses during a long gate can exceed the counter’s register capacity, leading to overflow. Understanding this limitation is essential when selecting gate times and counters for RF or high-speed digital measurements. Given Data / Assumptions: Counter measures frequency by counting input edges over a fixed gate time. The counter uses a finite-width register (for example, 24 or 32 bits). Input frequency can be high enough that counts in a long gate exceed the register’s maximum value. No prescaler or divide-by-N front-end is assumed unless stated. Concept / Approach: Let N_max be the maximum count a register can hold. During a gate time T_g, the expected count is N = f_in * T_g. If N > N_max, the digital counter wraps around (overflow), producing an invalid reading. Designers mitigate this by shortening T_g, adding a prescaler, or using reciprocal counting methods at high frequencies. Step-by-Step Solution: Define capacity → N_max determined by register width.Compute expected count → N = f_in * T_g.Compare → if N > N_max, overflow occurs.Conclusion → With too long T_g at very high f_in, the counter overflows. Verification / Alternative check: Check the specified maximum frequency without prescaling for a chosen gate time. Many instruments also provide overflow indicators or error flags when the count exceeds capacity. Why Other Options Are Wrong: The counter works fine: ignores finite register limits. Undercounts: long gate typically increases counts; overflow, not undercount, is the failure mode. Less precise: long gates usually increase precision; precision loss is not the primary issue here. Common Pitfalls: Assuming longer gate time is always better. At high frequencies, use prescalers or shorter gates to prevent overflow. Final Answer: The counter overflows.

Question 10

Keypad scanner timing — effect on dav after key release In the keypad application, just after 4 ms the simulation releases the key (column returns to F hex), causing the d output to enter Hi-Z. On the next rising clock edge after this release, what …

Accepted Answer

Introduction / Context: Many keypad interface state machines assert a data-available (dav) flag when a stable key code is present at the data bus d. When the key is released, the column lines revert (F hex), the encoder tri-states d (Hi-Z), and the controller clears dav on a defined clock edge. Understanding this timing prevents software from falsely reading stale data. Given Data / Assumptions: At t ≈ 4 ms, the column lines return to the idle pattern F hex, indicating release. The output d goes Hi-Z after release. dav is a registered flag updated on the rising clock edge. No key debounce or hold latch is keeping dav asserted after Hi-Z. Concept / Approach: dav indicates “valid data present.” Once d is Hi-Z, there is no valid key code. On the next clock edge, the FSM or register bank samples the absence of a valid key and deasserts dav. Therefore, dav transitions LOW at the next rising edge following the release event. Step-by-Step Solution: t ≈ 4 ms → key released → columns = F hex.Encoder action → d → Hi-Z (no valid code).Synchronous update → next rising edge clocks state.Since d is not valid → dav is cleared → dav goes LOW. Verification / Alternative check: Timing diagrams in typical keypad scanner IP show dav asserted only while a stable decoded key is latched. When the matrix returns to idle, dav is cleared on the next clock edge. Why Other Options Are Wrong: Goes HIGH: contradicts loss of valid data. Goes to Hi-Z: dav is a driven status flag, not a tri-stated bus. Goes to 1111H: dav is a single-bit signal, not a 4-bit value. Common Pitfalls: Confusing the tri-stated data bus with status flags; only the bus lines go Hi-Z, not control flags. Final Answer: It goes LOW.

Question 11

Frequency counter gating accuracy: In a digital frequency counter, the measured accuracy depends on the exact width of the enable (gate) signal that allows input pulses to be counted. Evaluate this statement.

Accepted Answer

Introduction / Context: Digital frequency counters measure how many cycles of an input signal occur during a precisely defined time window, often called the gate interval. The number of counted pulses divided by the gate time yields the measured frequency. Therefore, any error in the gate width directly translates into a frequency error. Given Data / Assumptions: Frequency f_meas ≈ N / T_gate, where N is the counted pulses, T_gate is the enable interval. Gate timing is derived from a reference time base (for example, a crystal oscillator). Counter input stages are properly conditioned (e.g., with a Schmitt trigger) to ensure clean edges. Concept / Approach: The measurement is only as good as the accuracy and stability of the time base and the definition of the gate interval. If the gate pulse width deviates by delta_T, then the fractional frequency error is approximately delta_T / T_gate. Consequently, a stable, accurate gate interval is critical for accurate frequency measurements. Step-by-Step Solution: Define measurement: f_meas = N / T_gate.Relate gate error to frequency error: delta_f / f ≈ delta_T / T_gate (assuming large N and negligible count quantization error).Infer: tighter control of T_gate improves accuracy; longer T_gate also reduces relative error.Conclusion: the statement is correct. Verification / Alternative check: In reciprocal counters and high-precision instruments, the time base specification (ppm accuracy, temperature coefficient) and gate width programming are explicitly documented because they set the accuracy floor. Why Other Options Are Wrong: Incorrect: Contradicts the N / T_gate principle.Depends only on input duty cycle: Duty cycle affects triggering, not the fundamental T_gate accuracy requirement.Cannot be determined without oscillator Q: Time-base Q is not the same as its calibrated accuracy specification. Common Pitfalls: Ignoring time-base tolerance and temperature drift; forgetting count quantization error for very short gate times; neglecting input conditioning that may cause missed pulses. Final Answer: Correct

Question 12

Definition of a frequency counter: A frequency counter is a digital circuit that measures and displays the frequency of an input signal by counting cycles over a known time base. Evaluate this definition for correctness.

Accepted Answer

Introduction / Context: Frequency counters are ubiquitous test instruments in labs and embedded systems. They rely on precise time bases to translate counts of periodic events into a frequency value that can be displayed numerically. Given Data / Assumptions: The input signal is periodic with a frequency in the counter’s operating range. A stable reference oscillator defines the measurement gate time. Input conditioning circuitry shapes the signal to logic levels so edges can be counted reliably. Concept / Approach: By counting the number of input cycles N within a gate interval T_gate, the instrument computes f ≈ N / T_gate. Alternatively, some counters measure the period and invert it, which improves resolution at low frequencies (reciprocal counting). Both approaches fulfill the stated definition. Step-by-Step Solution: Shape the input signal to clean digital transitions.Open the measurement gate for a precise interval T_gate.Count the input edges during this interval to obtain N.Compute frequency as f_meas = N / T_gate and display the result. Verification / Alternative check: Datasheets for bench frequency counters and MCU-based counters outline exactly this method, sometimes adding averaging, prescalers, or reciprocal techniques for improved resolution. Why Other Options Are Wrong: Incorrect: Denies standard instrument operation.Only correct for sine waves / duty-cycle-dependent: Frequency counting works for any periodic waveform with clean edges. Common Pitfalls: Measuring noisy or non-periodic signals; inadequate input conditioning causing false counts; relying on short gate times that magnify quantization error. Final Answer: Correct

Question 13

Digital clock HDL design detail: In a typical digital clock implemented in HDL, a 1 pulse-per-second (1 pps) signal is used as the synchronous clock that drives all counter stages, which are synchronously cascaded. Assess this statement.

Accepted Answer

Introduction / Context: Digital clocks increment seconds once every second, then roll over to minutes and hours. In HDL (VHDL/Verilog) implementations, a clean 1 pps tick derived from a crystal time base or external reference is often used to clock the synchronous counters that hold seconds, minutes, and hours. Given Data / Assumptions: A time base (e.g., 32.768 kHz crystal) is divided down to 1 Hz (1 pps). Seconds, minutes, and hours counters are synchronous (all flip-flops share the same clock edge). Carry/enable logic creates next-stage increments at the correct rollovers (59→00 for seconds/minutes; 23→00 for hours in a 24-hour clock). Concept / Approach: Using a single synchronous clock simplifies timing closure and eliminates ripple delays inherent in asynchronous chains. The 1 pps tick serves as the sole clock, while enable conditions inside each counter determine whether a given stage increments on that tick (for example, minutes increment when seconds = 59 then wraps). Step-by-Step Solution: Generate a clean 1 pps from a stable oscillator using a divider.Use the 1 pps as the shared clock for all counters (seconds, minutes, hours).Implement synchronous enable logic to step minutes/hours at the correct rollover events.Maintain synchronous resets for well-defined startup states. Verification / Alternative check: Simulation waveforms show all counters advancing on the same clock edge; synthesis reports confirm a single global clock domain, reducing skew problems compared to ripple chains. Why Other Options Are Wrong: Incorrect: Synchronous 1 pps clocking is a standard, robust approach.Only correct if ripple-cascaded / only for minutes: Misunderstands that “synchronous” means shared clock, not asynchronous ripple behavior. Common Pitfalls: Deriving 1 pps with asymmetric duty or jitter; mixing clock domains inadvertently; failing to debounce external references. Final Answer: Correct

Question 14

Input conditioning in the digital clock project: A 60 Hz mains signal is conditioned through a Schmitt trigger to produce clean logic pulses at 60 pulses per second (pps). Does this circuit create “sine wave pulses,” or square logic pulses?

Accepted Answer

Introduction / Context: Schmitt triggers are used to clean up slow or noisy input waveforms by adding hysteresis. When a sinusoidal mains-derived signal (for example, 60 Hz) is fed into a Schmitt trigger, the output becomes a crisp digital square wave at the same fundamental rate, suitable for clocking counters. Given Data / Assumptions: Input: 60 Hz sine or rectified waveform after isolation and scaling. Conditioner: a Schmitt-trigger buffer or inverter with defined upper and lower thresholds. Output: logic-level transitions whenever the input crosses thresholds. Concept / Approach: A Schmitt trigger does not output sine waves; it outputs near-rail square pulses determined by the device thresholds and supply rails. The purpose is to remove indecision around the switching point and prevent chatter due to noise. Therefore, describing the output as “sine wave pulses” is incorrect. Step-by-Step Solution: Convert the analog 60 Hz input to a logic-level using a Schmitt trigger.Observe: output toggles between logic LOW and HIGH, producing a square wave (or pulse train).Conclusion: statement claiming “sine wave pulses” is false.Correct description: square logic pulses at 60 pps. Verification / Alternative check: Oscilloscope traces of Schmitt outputs show crisp edges at rail levels, not sinusoidal shapes, with edge timing established by threshold crossings. Why Other Options Are Wrong: Correct / Depends: The device always outputs digital-level pulses; it does not emulate sine pulses.“Depends on LED color”: Irrelevant to signal shape. Common Pitfalls: Confusing the input waveform with the conditioned output; assuming analog filtering is present when the intent is digital edge cleaning. Final Answer: Incorrect

Question 15

HDL syntax and control structures: Nesting one CASE construct inside another is referred to as a “nested CASE,” not a loop construct. Evaluate the statement that this is called a “do-loop.”

Accepted Answer

Introduction / Context: HDLs such as VHDL and Verilog provide conditional constructs (if/elsif/else), selection constructs (case), and iteration constructs (for/while; VHDL also has generate). Clear terminology avoids confusion during code reviews and when communicating design intent. Given Data / Assumptions: CASE is a selection statement mapping enumerated or discrete choices to actions. Loops (e.g., for, while) repeat statements; “do-loop” is a software term, not used in VHDL, and distinct from CASE. Nested constructs are allowed in both VHDL and Verilog. Concept / Approach: Placing a CASE statement inside another CASE is simply “nested CASE.” It does not create a loop; no iteration occurs unless explicitly coded using loop constructs. Therefore, calling it a “do-loop” is incorrect. Step-by-Step Solution: Identify construct: CASE selects among alternatives based on an expression’s value.Nesting behavior: inner CASE executes once when control reaches it, not repeatedly.Loop definition: repetition until a condition changes; not applicable here.Conclusion: the statement is false. Verification / Alternative check: Language reference manuals define loops and selection separately; synthesis guides use “nested CASE” as standard terminology. Why Other Options Are Wrong: Correct: Contradicts language semantics.Only correct in VHDL / only for ROMs: No HDL uses “do-loop” to mean nested CASE; ROM inference is unrelated. Common Pitfalls: Assuming software terminology maps 1:1 to HDL; confusing nested decision trees with iterative constructs. Final Answer: Incorrect

Question 16

Role of testbench scenarios in HDL simulation: When simulating an HDL design, the engineer must craft representative input scenarios (testbench stimuli) and must know the correct expected responses to verify behavior. Assess this statement.

Accepted Answer

Introduction / Context: Simulation is the first line of defense against design bugs in HDL projects. A design without a robust testbench can appear to function yet conceal corner-case failures that only emerge in hardware, when fixes are more expensive. Given Data / Assumptions: A testbench applies input vectors, clocks, resets, and sequences to the Device Under Test (DUT). Expected outputs or assertions define pass/fail criteria. Coverage (code/functional) indicates how thoroughly scenarios exercise the design. Concept / Approach: Creating scenarios means modeling normal, boundary, and error conditions. Knowing the proper response allows the use of self-checking mechanisms (assertions, scoreboards) so that simulations fail automatically when behavior deviates from the specification. Step-by-Step Solution: Translate the specification into test cases and expected outcomes.Drive stimuli via a testbench that clocks and resets the DUT appropriately.Add assertions to automatically detect violations.Review coverage to identify untested states and sequences. Verification / Alternative check: Industry best practices (UVM, constrained random testing) formalize this approach; regressions rely on known expected results to guard against regressions after code changes. Why Other Options Are Wrong: Incorrect: Simulation without expected results cannot prove correctness.Only necessary post-layout / “waveforms look clean”: Functional errors occur long before layout; visual inspection is unreliable and non-scalable. Common Pitfalls: Overlooking reset/initialization conditions; failing to simulate worst-case timing; not creating self-checking tests leading to subjective pass/fail judgments. Final Answer: Correct

Question 17

Stepper-motor drive modes and torque at moderate speeds Evaluate the statement: “The wave-drive sequence of a stepper motor has more torque and operates more smoothly than the full-step sequence at moderate speeds.”

Accepted Answer

Introduction / Context: Stepper motors can be driven using several phase-energizing patterns: wave drive (one phase ON), full-step (two phases ON in many motors), half-step (alternating one and two phases), and microstepping (sinusoidal current shaping). Torque output and smoothness depend on how many windings are energized and how current is shared between them. Given Data / Assumptions: Wave drive energizes a single phase at a time. Full-step commonly energizes two phases simultaneously in hybrid and PM steppers. Moderate speed region means operation well below resonance issues but above creep. No current limiting or advanced current profiling beyond the named modes is assumed. Concept / Approach: Electromagnetic torque in a stepper is proportional to the vector sum of phase currents and the motor’s torque constant. When two phases are driven at the same magnitude, the resultant torque vector is larger than when only one phase is energized. Additionally, having two phases ON provides better detent against load disturbance, improving positional stiffness and reducing vibration compared with single-phase excitation. Step-by-Step Solution: Compare excitation: wave drive uses 1*I in one phase; full-step uses two phases with I each.Resultant magnetic vector in full-step is approximately √2 times a single-phase vector (idealized), yielding higher holding and running torque.More torque generally enables smoother motion under load because the motor is less likely to stall or resonate.Therefore, the claim that wave drive has more torque and is smoother than full-step is not supported; the opposite is typical. Verification / Alternative check: Practical datasheets and application notes consistently show full-step torque curves above wave-drive curves. Microstepping can be smoother than both, but that is not the comparison being made here. Why Other Options Are Wrong: “Correct” reverses the well-known torque relationship. “True only with microstepping drivers” mixes modes; microstepping is a different approach. “Applies only to variable-reluctance motors” misattributes a general torque principle to a subset of motors. Common Pitfalls: Confusing smoothness improvements from microstepping with wave drive. Assuming less electrical power (one phase) somehow yields more torque. Final Answer: Incorrect

Question 18

HDL project scoping best practices One of the first steps in a hardware description language (HDL) project is to define the scope by explicitly naming and documenting each input and output of the design entity or top module.

Accepted Answer

Introduction / Context: Successful HDL projects begin with a clear statement of what the design must do and how it will interface with the outside world. Clear I/O definitions drive verification planning, pin assignments, timing constraints, and downstream integration with boards, IP, and testbenches. Given Data / Assumptions: An HDL “top” encapsulates the external contract: inputs, outputs, and optionally inouts. Constraints, timing analysis, and verification depend on this contract. Team members may work in parallel, so a shared I/O spec prevents ambiguity. Concept / Approach: Begin with a short specification that lists ports, direction, width, reset polarity, clock domains, and basic protocols (e.g., valid/ready). This informs both the design and testbench scaffolding. Early I/O clarity also exposes integration risks such as voltage level translation, clocking architecture, and asynchronous crossings. Step-by-Step Solution: Define the design boundary: top entity/module name and purpose.Enumerate ports with direction and bit widths (for buses).Specify control semantics: resets, enables, handshakes, clock domains.Share the I/O list with stakeholders; use it to seed constraints, testbench stubs, and documentation. Verification / Alternative check: A quick sanity check is to stub a testbench with the proposed ports; if connections feel forced or unclear, refine the interface before RTL coding deepens. Why Other Options Are Wrong: “Incorrect” ignores standard methodology. “Only needed after synthesis” is backwards; synthesis requires a stable interface. “Skip if using auto-I/O inference” risks unpredictable tooling behavior and poor documentation. Common Pitfalls: Letting I/O drift mid-project; failing to document clock/resets; overlooking active-low conventions; changing bus widths without updating testbenches and constraints. Final Answer: Correct

Question 19

Planning for verification in HDL projects In HDL-based development, a key strategic planning tactic is to ensure every block can be independently tested (for example, with self-checking unit testbenches).

Accepted Answer

Introduction / Context: Complex digital systems are easier to deliver on time and with quality when verification is planned from day one. Breaking the design into testable units lowers integration risk and accelerates debug by localizing defects before they ripple across the system. Given Data / Assumptions: The project contains multiple HDL modules/entities arranged hierarchically. Each module has a clear interface (ports, parameters/generics). Simulators and verification frameworks (assertions, coverage) are available. Concept / Approach: Unit-level verification validates functionality against the specification, independent of other blocks. Self-checking testbenches compare outputs to expected results and flag mismatches automatically, improving repeatability. This approach complements system-level tests and hardware bring-up with chips or FPGAs. Step-by-Step Solution: Partition the design into coherent blocks with stable interfaces.Create a self-checking testbench per block, including stimulus, assertions, and pass/fail checks.Automate regression runs to catch regressions early as code evolves.Promote proven blocks to higher-level integration, reducing late-stage surprises. Verification / Alternative check: Add functional coverage and constrained-random stimulus where appropriate; verify corner cases and reset/clock transitions to ensure robustness. Why Other Options Are Wrong: “Incorrect” contradicts established hardware verification practice. “Only applies to software projects” conflates two domains; HDL benefits equally from modular testing. “Unnecessary if FPGA has on-chip debug” is false; instrumentation helps but cannot replace exhaustive pre-silicon verification. Common Pitfalls: Relying solely on top-level testing; leaving unit tests for the end; lacking self-checks, which turns tests into manual waveform inspection. Final Answer: Correct

Question 20

Understanding top-down design in digital systems Top-down design starts at the highest level of hierarchy, treating the system initially as a black box with defined inputs and outputs before decomposing it into lower-level subsystems.

Accepted Answer

Introduction / Context: Top-down methodology helps teams control complexity by focusing first on behavior and interfaces at the system level. Once the external contract is stable, designers iteratively refine the internal architecture into subsystems, modules, and leaf cells with progressively detailed specifications. Given Data / Assumptions: The system has clear stakeholders and requirements. I/O, performance targets, and clock/reset strategies can be articulated early. Lower-level details (state machines, datapaths) will be derived from the system model. Concept / Approach: Treat the top as a black box: define ports, protocols, and timing first. Then allocate functionality to subblocks, define their interfaces, and continue refinement until modules are implementation-ready. This process pairs naturally with bottom-up reuse of known IPs, but the initial control point remains the top-level intent. Step-by-Step Solution: Capture the system specification and external interfaces.Partition functionality into subsystems aligned with performance and reusability.Define interfaces between subsystems to minimize coupling.Iterate until leaf modules are well specified and testable. Verification / Alternative check: Build a top-level behavioral model to validate requirements quickly; use it as a reference for RTL conformance during integration. Why Other Options Are Wrong: “Incorrect” denies a widely used system-engineering method. “Only valid for software” is wrong—HDL flows embrace this. “Opposite: begin at gate-level” leads to local optimizations that may miss system goals. Common Pitfalls: Insufficient top-level clarity, changing interfaces mid-project, and neglecting non-functional requirements (throughput, latency, power). Final Answer: Correct

Digital System Projects Using HDL Questions