Question 1

Stepper motor control in HDL — why provide more than one operating mode? In a hardware description language (HDL) stepper motor design, what is the main reason for supporting multiple modes (for example, wave drive, full-step, half-step, and directi…

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Introduction / Context: Stepper motors advance in discrete steps by energizing stator phases in specific sequences. HDL implementations for FPGAs or CPLDs often include several modes to adapt torque, smoothness, speed range, and direction. A configurable controller lets one hardware platform serve many mechanical requirements without rewiring. Given Data / Assumptions: Common mode choices: wave drive (one phase on), full-step (two phases on), and half-step (alternates one/two phases). Speed is set by step frequency; direction by phase sequence order. “Direct drive” refers to producing coil drive patterns from logic (typically via external power stages). Concept / Approach: Different modes trade torque and resolution. Full-step maximizes torque (two coils on), wave drive lowers power and torque (one coil on), and half-step doubles position resolution and reduces resonance. Direction control is achieved by reversing the sequence. Frequency control provides the RPM setting and acceleration ramps. Thus, exposing modes covers speed, direction, and the pattern used to energize windings. Step-by-Step Solution: Define a phase sequence table for each mode.Select direction: forward uses 0→1→2→3; reverse uses 3→2→1→0.Set speed by changing the step period or clock divider.Output coil enables through drivers or H-bridges appropriate to motor current. Verification / Alternative check: Simulation waveforms show mode-dependent coil patterns and confirm that changing the sequence order reverses direction while altering the step interval changes speed. Why Other Options Are Wrong: Options a and b are each partial truths; the design usually enables both speed and direction. Option c alone ignores that “direct drive” still requires speed/direction control. Option e is unrelated to core HDL step sequencing. Common Pitfalls: Driving coils directly from logic without current drivers; always use proper power stages. Final Answer: All of the above

Question 2

Digital frequency counter — which is NOT a typical control/timing signal? In a standard frequency counter block diagram, the control and timing section produces signals such as Gate/Enable, Reset/Clear, and Store/Latch. Which of the following is NOT…

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Introduction / Context: A basic frequency counter counts input pulses during a precise time window. A control and timing block (often driven by a time base) generates key control signals: open/close the counting gate, reset the counters, and store the measured count for display. Understanding these signals clarifies how accuracy and display stability are achieved. Given Data / Assumptions: Common control names: Gate (Enable), Reset, and Store (Latch). Naming conventions vary, but functions are consistent. “Clear” and “Reset” are often synonyms; designs usually choose one. Concept / Approach: The control and timing section opens the counting interval (Gate/Enable), then closes it, commands a Store/Latch so the display updates without disturbing the next count, and asserts Reset to zero the counters before the next measurement window. Because “Clear” duplicates the meaning of “Reset,” designs typically implement only one of those as a distinct signal, not both. Step-by-Step Solution: Time base defines a window (for example, 1 second).Gate/Enable goes HIGH to allow pulses into the counter chain.At the end of the window, Gate goes LOW and Store captures the count into a display register.Reset clears counters to zero for the next cycle. Verification / Alternative check: Block diagrams in textbooks typically show three lines from control: Gate (or Enable), Reset, and Store/Latch. “Clear” is either the same as Reset or unused; rarely is it provided as an extra, separate control in minimal counters. Why Other Options Are Wrong: Enable/Gate: always required to define the counting interval. Reset: needed to zero counters between measurements. Store: used to transfer count to the display register. Common Pitfalls: Not separating Store from Reset; storing before resetting avoids display flicker and race conditions. Final Answer: Clear

Question 3

Digital clock project — identify the three basic counter blocks When implementing a 12-hour digital clock from counters, which set of counter moduli represents the three basic building blocks used to form the seconds, minutes, and hours counts?

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Introduction / Context: A digital clock divides time into seconds, minutes, and hours. Practical hardware uses cascaded modulus counters to realize the decimal-looking outputs needed for display. Understanding which moduli are required keeps the design minimal and modular, simplifying debugging and reuse. Given Data / Assumptions: Seconds and minutes each count 0–59. Hours (12-hour format) count 1–12 (or 0–11 internally). BCD displays prefer decade-based counters for easy digit driving. Concept / Approach: To get 0–59, hardware commonly uses a MOD-10 counter for the ones digit (0–9) and a MOD-6 counter for the tens digit (0–5). Chaining these two yields MOD-60. For hours in a 12-hour clock, a MOD-12 counter is used (often realized as a decade counter with additional logic or a dedicated MOD-12). Thus, the fundamental building blocks are MOD-10, MOD-6, and MOD-12. Step-by-Step Solution: Construct seconds: MOD-10 (ones) cascaded into MOD-6 (tens) → counts 00–59.Reuse the same pair for minutes: MOD-10 + MOD-6 → 00–59.Use MOD-12 for hours to cycle 1–12 (or 00–11) with appropriate display mapping.Therefore the necessary primitive moduli are MOD-10, MOD-6, and MOD-12. Verification / Alternative check: Many textbooks and lab kits implement seconds/minutes using a 7490/4510 decade counter feeding a 7492/4511 MOD-6 stage; hours use a MOD-12 realized with gating or a dedicated counter, confirming the chosen set. Why Other Options Are Wrong: Option a/c: omit one needed modulus; you need three building blocks. Option b: MOD-5 is not used; tens digit must roll 0–5, not 0–4. Option e: arbitrary moduli unrelated to clock division. Common Pitfalls: Confusing “final modulus” (60) with the underlying cascaded moduli (10 and 6). Final Answer: MOD-6, MOD-12, and MOD-10 counters

Question 4

Keypad scanning with a ring counter — what does the preset state define? In a scanned-matrix keypad application that uses a ring counter to step through columns, what does the preset state of the ring counter determine at power-up?

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Introduction / Context: Matrix keypads are scanned by driving one axis (rows or columns) and reading the other. A ring counter conveniently cycles one active line at a time across the driven axis. Correct initialization ensures the scan begins on a known line so the encoder/decoder that interprets key positions operates predictably from the first cycle. Given Data / Assumptions: Ring counter outputs correspond to individual column lines. One output is asserted (or de-asserted) at a time in sequence. An encoder maps the active column/row to a binary code. Concept / Approach: Presetting the ring counter forces a specific bit to be active on startup, aligning the scanning sequence with the column encoder’s expectations. If the first active line were ambiguous, early keypresses could be misinterpreted because the encoder would not know which column index corresponds to the instantaneous ring state. Step-by-Step Solution: Use preset to assert the first column line (for example, C0) at power-up.Advance the ring counter each scan tick so the active line walks C0→C1→…Feed the active column index into the column encoder for position decoding.Sample the row lines; a pressed key closes a row–column contact at the current column index. Verification / Alternative check: Practical firmware/HDL often synchronizes a debouncer and a timer with the ring counter so that the encoder index equals the known column position at each sample time—reliant on a deterministic preset state. Why Other Options Are Wrong: NANDing of rows/columns: gate choice is implementation detail, not what the preset defines. Row encoder output: the preset controls the driven axis (columns in this setup), not the sensed axis. Pull-up strength: unrelated to ring preset. Common Pitfalls: Forgetting to preset can yield a rotated scan pattern, breaking key mapping tables. Final Answer: The proper output of the column encoder

Question 5

Project management for digital design — which item is NOT typically listed as a formal step? Among the following activities commonly performed during a hardware project, identify the one that is not usually named as a distinct “step” in standard pro…

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Introduction / Context: Digital design projects follow a sequence: define requirements, partition the system, implement (synthesize), verify, integrate, and release. While documentation is essential, most process diagrams treat it as a continuous activity accompanying each phase rather than as a single, isolated step. Recognizing this distinction helps students map tasks onto standard lifecycles. Given Data / Assumptions: Typical steps include definition/specification, synthesis/implementation, verification/testing, and integration. Documentation is performed throughout and finalized at release. Terminology varies, but core steps remain consistent. Concept / Approach: Project management frameworks (from V-model to iterative Agile hybrids) emphasize traceable requirements and validated implementations. Documentation supports each step—requirements docs, design docs, test plans, user manuals—but it is not typically a stand-alone phase in the same sense as synthesis or integration. Therefore, when asked for the “not a step,” documentation is the best choice among otherwise step-like items. Step-by-Step Reasoning: Overall definition → capture goals and constraints.Synthesis and testing → implement RTL/netlists and verify behavior.System integration → assemble subsystems and perform system-level validation.Documentation → occurs across all phases, not as a single checkpoint step. Verification / Alternative check: Reviewing common course syllabi and industry flow diagrams shows documentation deliverables associated with each milestone rather than isolated as the only focus of a phase. Why Other Options Are Wrong: Overall definition is the kickoff step that frames the project. Synthesis and testing are core engineering phases. System integration is required to achieve a working whole. Risk assessment (option e, if present) is often a formal step or recurring activity; it is more defensible as a step than documentation alone. Common Pitfalls: Treating documentation as an afterthought; it should evolve continuously to avoid gaps. Final Answer: System documentation

Question 6

VHDL counters — two ways to remember the current state In synthesizable VHDL for a counter or state machine, which pair of VHDL objects are used to hold (remember) the current state across clock cycles?

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Introduction / Context: VHDL provides multiple object classes—signals, variables, constants—that behave differently with respect to time and simulation. For sequential logic like counters, designers must understand which objects store values across cycles and how synthesis tools map them to flip-flops and registers. Given Data / Assumptions: Signals are the primary means of modeling wires and registered state in RTL. Variables exist inside processes and procedures and update immediately within that scope. Both can represent state that persists, depending on coding style and clocked processes. Concept / Approach: A common pattern uses a variable within a clocked process to compute the next state, then assigns that value to a signal for registered output at the rising edge. Alternatively, a signal alone can hold the current state if assigned in a synchronous process. Thus, both SIGNAL and VARIABLE are valid constructs for remembering state; the difference lies in update semantics and scope. Step-by-Step Solution: Create a clocked process sensitive to CLK.Use a VARIABLE (e.g., tmp_state) to compute intermediate results in the same delta cycle.Assign the VARIABLE back to a SIGNAL (e.g., state) which synthesizes to flip-flops.On the next clock, the SIGNAL presents the remembered current state. Verification / Alternative check: Post-synthesis netlists show that clocked signal assignments generate registers. Variables used only for intermediate computation do not create extra storage unless written to signal/registers or mapped into memories by the tool. Why Other Options Are Wrong: Functions/processes are structural elements, not storage objects. “Counters and timers” are design concepts, not VHDL objects. “bit types” are data types, not storage mechanisms by themselves. FILE/TEXTIO are for simulation I/O, not hardware state. Common Pitfalls: Assuming variables update like signals; variables update immediately within a process and can simplify sequential arithmetic. Final Answer: With SIGNAL and VARIABLE

Question 7

Full-step drive with two flip-flops — step angle per state advance In a simple educational model with two flip-flops sequencing four coils (four distinct states) in a full-step pattern, how far does the stepper motor advance per step?

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Introduction / Context: Textbook stepper-motor demonstrations often abstract mechanical detail and map digital states directly to “positions.” With two flip-flops you can encode four states, commonly used to energize four distinct coil combinations in a full-step sequence. In that simplified model, a full cycle of four steps corresponds to a full revolution, so each step equals one quarter turn. Given Data / Assumptions: Two flip-flops → 2^2 = 4 states. Four states energize four unique coil patterns in sequence. Educational assumption: 4 steps = 360° rotation for the model. Concept / Approach: The question frames a conceptual teaching model, not a real motor with many poles/teeth. Under the model’s mapping, each of the four states represents an equal angular increment. Therefore, the step angle is 360° / 4 = 90°. Real stepper motors typically have much smaller step angles (for example, 1.8°), but the simplified digital demonstration uses quarter-turn increments to illustrate sequencing, direction reversal, and state encoding. Step-by-Step Solution: Count available states with two flip-flops → 4.Assume one state transition corresponds to one mechanical step.Assume four steps make one revolution in the model.Compute angle per step = 360° / 4 = 90°. Verification / Alternative check: Exam/lab problems that show only four coil states and ask for the “step angle” typically intend this simple division, reinforcing digital sequencing concepts rather than mechanical pole counts. Why Other Options Are Wrong: 45°, 30°, 15°, 7.5° imply 8, 12, 24, or 48 steps per revolution; those do not match the 4-state abstraction given. Common Pitfalls: Applying real-world stepper specifications (e.g., 200 steps/rev) to a pedagogical four-state model; always read the problem’s implied mapping. Final Answer: 90°

Question 8

Digital clock seconds counter — how to count 00 through 59? In the digital clock project, which counter configuration is typically used to count seconds from 00 to 59 for easy BCD display driving?

Accepted Answer

Introduction / Context: Human-readable time uses decimal digits, so digital clocks usually build 00–59 with two cascaded counters: one for the ones place (0–9) and one for the tens place (0–5). Implementations that align with BCD simplify driving seven-segment displays and reduce glue logic. Given Data / Assumptions: Seconds must roll 00→59, then back to 00. Ones digit cycles 0–9 (MOD-10 or BCD). Tens digit cycles 0–5 (MOD-6). Concept / Approach: Use a decade (BCD) counter for the ones digit so that decoding to a display is straightforward. Cascade its ripple-carry (or decoded terminal count) into a MOD-6 counter for the tens digit. When the ones digit reaches 9 and wraps to 0, the tens digit increments; when the tens digit reaches 5 and the ones digit wraps, the chain resets to 00, yielding 60 unique states (MOD-60 overall). Step-by-Step Solution: Choose a BCD (MOD-10) counter for the least significant digit.Feed its carry into a MOD-6 counter for the most significant digit.Reset the chain when the pattern 59 advances to 00.Drive seven-segment decoders directly from each BCD digit. Verification / Alternative check: Many reference designs implement seconds/minutes as BCD+MOD-6, while hours use MOD-12 or MOD-24. This modular approach is proven and minimizes decoding complexity. Why Other Options Are Wrong: MOD-60 (single block) is less common as an off-the-shelf counter and makes display decoding harder. MOD-6 or BCD alone cannot span 00–59. Two MOD-30 blocks do not map cleanly to decimal digits. Common Pitfalls: Forgetting to synchronize resets so 59 rolls to 00 without transient invalid display codes. Final Answer: BCD followed by a MOD-6

Question 9

HDL stepper-motor controller workflow In a hardware description language (HDL) design for a stepper-motor controller, once the up/down step counter has been implemented and verified, what is the next logical design block to create so that the motor …

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Introduction / Context: Designing a stepper-motor controller in HDL typically follows a pipeline: timing generation, counting, sequence mapping, and finally output driving. After you have a reliable up/down counter that represents the step index, you must convert that index into the correct coil energizing pattern. This is the role of a sequencer. Given Data / Assumptions: The up/down counter produces a step index (for example, 0, 1, 2, 3 for full-step). The motor requires a specific excitation sequence (full-step, half-step, or microstep). Outputs ultimately drive a power stage (H-bridges or drivers). Concept / Approach: The sequencer maps the counter value to the phase pattern. For a 4-phase unipolar motor in full-step mode, index 0..3 maps to patterns like 1000, 0100, 0010, 0001. In half-step, the mapping alternates single-coil and dual-coil patterns. The sequencer is best implemented as a case statement or lookup table so that stepping direction simply increments or decrements the index. Step-by-Step Solution: Confirm counter direction control (up for CW, down for CCW).Define the desired stepping mode (full, half, microstep).Implement a sequencer (case/with-select) that maps count → coil pattern.Route sequencer outputs to the driver interface and add enable/brake control as needed. Verification / Alternative check: Simulate by sweeping the counter and verifying that outputs follow the correct phase order. In hardware, observe coil drive with a logic analyzer and confirm proper motion without missed or reversed steps. Why Other Options Are Wrong: Test it on a simulator: Testing is essential, but it is not the design block that follows the counter. Test the decoder: The key functional block after the counter is the sequencer; the term “decoder” is ambiguous here. Design an intermediate integer variable: Variables are implementation details, not the required functional block. Common Pitfalls: Hard-coding patterns without considering direction; neglecting disable states that prevent coil heating when idle; forgetting to parameterize for different step modes. Final Answer: Build the sequencer

Question 10

HDL keypad scanner behavior In an HDL keypad-scanning application that uses a ring counter to scan columns, what should the ring counter do when a key press is detected?

Accepted Answer

Introduction / Context: Matrix keypads are commonly scanned by activating one column (or row) at a time and reading the opposite dimension. A ring counter is ideal for generating one-hot scan patterns. Correct behavior on a key press is crucial for debouncing and stable detection. Given Data / Assumptions: A ring counter cycles active columns (for example, 0001, 0010, 0100, 1000). Rows are read back to detect which key is pressed. Once a press is found, we want to capture deterministic column/row data. Concept / Approach: When any key closes, the current column and the detected row together identify the pressed key. If the ring counter keeps advancing, the sensed column will change during the same physical press, creating ambiguity. Freezing the ring counter stabilizes the active column while the system debounces and encodes the key. Step-by-Step Solution: Scan columns with the ring counter one at a time.Detect a press by sensing an asserted row line.Freeze the ring counter at the active column to lock the column identity.Debounce and then encode the row/column into a key code for downstream logic. Verification / Alternative check: Scope the column lines and a row line: during a press, the active column should hold steady until data valid (DAV) is generated and the system acknowledges. Why Other Options Are Wrong: Count to find the row/column: The ring counter scans columns; rows are sensed inputs, not counted. Start the D flip-flop: Flip-flops may store states, but the fundamental action needed is to freeze scanning. Common Pitfalls: Failing to freeze can cause ghosting or multiple detections; neglecting debounce can produce spurious keystrokes. Final Answer: Freeze

Question 11

Minimum functional blocks for an HDL stepper-motor design A robust HDL stepper-motor controller must, at minimum, include which pair of functional blocks to translate step commands into coil drive patterns?

Accepted Answer

Introduction / Context: Stepper-motor control is about mapping a desired step index (position increment) into a known pattern of coil energization. In HDL, this is usually split into two core parts: counting and decoding. Given Data / Assumptions: An up/down counter represents the current step position. A decoder/sequencer maps that count to coil drive patterns (full-step, half-step, etc.). Additional blocks (timers, PWM for current control) may be added, but are not strictly minimal. Concept / Approach: The counter keeps track of where you are in the sequence. The decoder (often implemented as a case statement) translates the numerical count into phase lines to energize the motor coils. Without both, you cannot produce a deterministic, repeatable stepping sequence. Step-by-Step Solution: Implement a step counter with direction control and optional enable.Create a decoder that maps each count value to a coil pattern.Integrate timing (step rate) so that count updates at controlled intervals.Verify in simulation and then in hardware that coils energize in the intended order. Verification / Alternative check: Simulate a sequence of steps and record the output bits; compare to a reference table for your stepping mode. In hardware, check with a logic analyzer or LEDs on phase lines. Why Other Options Are Wrong: Variables/processes; types/bits: These are HDL constructs, not functional blocks. Sequencers and multiplexers: A “sequencer” is often synonymous with the decoder here, but without the counter there is no step index; the canonical minimal pair is counters and decoders. Common Pitfalls: Forgetting to wrap the counter correctly (modulus), or mismatching count order with the physical motor wiring, causing jitter or reverse rotation. Final Answer: Counters and decoders

Question 12

HDL keypad encoding In a matrix-keypad HDL application, what does the final data signal (often presented to the system as a key code) represent?

Accepted Answer

Introduction / Context: Matrix keypads are scanned to determine which intersection of row and column lines is closed by a pressed key. HDL logic must translate the raw scan into a compact code usable by firmware. Given Data / Assumptions: Columns are driven one at a time via a ring counter or sequencer. Rows are read as inputs to detect closures. A debounced, valid “data” signal is produced when a key is confirmed. Concept / Approach: The “data” output is typically an encoded representation of the row and column of the detected key. This may be a binary code (for example, 4 bits) or an ASCII mapping, depending on the application. Internally, the ring counter and row capture produce the raw indices; the encoder combines them. Step-by-Step Solution: Scan columns using a one-hot pattern.When a row input asserts, freeze the scan and debounce.Encode the (row, column) pair into a key code (for example, 0–15 for a 4x4 keypad).Assert data-valid (DAV) and output the encoded data to the system. Verification / Alternative check: Press each key and verify the output code matches the expected table. Use a logic analyzer to confirm correct row/column capture and timing of DAV. Why Other Options Are Wrong: Ring encoded data / ring counter data: These are intermediate scan signals, not the final key code. Freeze locator data: “Freeze” is a control action, not the content of the data bus. Common Pitfalls: Not handling multi-key presses or ghosting; failing to deassert DAV correctly, causing repeated characters. Final Answer: The row and column encoded data

Question 13

HDL design best practices When developing a digital system in HDL, which of the following is the worst mistake from a verification and maintainability standpoint?

Accepted Answer

Introduction / Context: Complex HDL projects benefit from incremental development and continuous verification. The cost of bugs grows exponentially the later they are found. A disciplined workflow avoids compounding errors. Given Data / Assumptions: Multiple modules (counters, sequencers, interfaces) must integrate cleanly. Simulation, synthesis checks, and on-hardware tests are available. Version control and documentation are recommended. Concept / Approach: The worst practice is to pile on untested code. Each new block can mask earlier bugs and multiply the debugging space. Instead, follow “build a little, test a little”: unit-test modules, then integrate gradually with self-checking testbenches and assertions. Step-by-Step Solution: Write a small module and immediately create a focused testbench.Run simulation and add checks (assertions, scoreboards).Synthesize early to catch coding-style issues that affect hardware.Integrate modules only after they pass unit tests to keep failure surfaces small. Verification / Alternative check: Track bug origin by commit history; teams that add untested code see longer integration times. Continuous integration (CI) with regression tests mitigates this risk. Why Other Options Are Wrong: Concluding a block works perfectly: Overconfident, but not as damaging as skipping testing entirely. Failing documentation: Harmful to maintainability, yet testing still finds functional bugs. Overlooking a variable: A minor coding slip compared to systemic lack of testing. Common Pitfalls: Skipping corner cases, not modeling resets and metastability, and ignoring synthesis warnings—each magnifies when untested blocks accumulate. Final Answer: Adding blocks of code prior to testing them

Question 14

Keypad HDL tri-state behavior In the keypad scanner, when all columns read HIGH (no key pressed), the ring counter is enabled and counting, and dav is LOW (no data available), what should be the state of the data outputs d[ ] that feed the system bu…

Accepted Answer

Introduction / Context: Shared buses require that only one device drive the lines at a time. In keypad HDL designs, the encoded key data should be driven only when valid and requested. Otherwise, the outputs must not contend with other devices. Given Data / Assumptions: No key press detected (all columns HIGH). Ring counter continues scanning (normal idle behavior). dav (data available) is LOW, meaning no valid key code. Concept / Approach: When data is not valid, the keypad interface should tri-state its data outputs so that other peripherals can use the bus. Driving a pattern during idle risks contention or false reads by a host that samples multiple devices. Step-by-Step Solution: Use dav as an output-enable qualifier for the data buffer.If dav = 0, assert OĒ so that the output buffer is disabled.Disabled buffer → data lines in high-impedance (Hi-Z) state.Only when dav = 1 should the code be driven onto the bus. Verification / Alternative check: On a logic analyzer, observe that when no key is pressed, the data lines float (pulled by bus resistors if present) and only drive when dav asserts. Why Other Options Are Wrong: On/Off: Ambiguous and not an electrical state for a bus interface. 1011: Arbitrary pattern; driving any pattern with no valid data is incorrect. Common Pitfalls: Forgetting to tri-state in idle can cause multi-driver conflicts; failing to debounce can briefly drive spurious codes before dav is qualified. Final Answer: Hi-Z

Question 15

Frequency counters: sampling interval vs. measurement quality How does the sampling (gate) interval of a frequency counter affect the displayed result?

Accepted Answer

Introduction / Context: Digital frequency counters count the number of input cycles within a fixed gate time (sampling interval). The choice of this interval affects resolution, precision, and update rate of the displayed frequency. Given Data / Assumptions: Counter measures cycles within a sampling window Tgate. Displayed frequency f_meas ≈ N / Tgate, where N is counted cycles. Timebase is accurate and stable. Concept / Approach: For a given input frequency, increasing Tgate allows more cycles to be counted, reducing the relative quantization error (±1 count). The fractional resolution improves approximately as 1 / N, so longer gate times give finer precision, albeit with slower updates. Step-by-Step Solution: Let N be the integer cycles counted.Quantization error is roughly ±1 count, so relative error ≈ 1 / N.Since N = f * Tgate, increasing Tgate increases N linearly.Therefore, a longer sampling interval → more precision (smaller relative error) but slower refresh. Verification / Alternative check: Observe the stability of the last digits: with a 1 s gate, a 10 MHz signal yields N ≈ 10,000,000; with 0.1 s, N ≈ 1,000,000, so the least significant digits vary more. Why Other Options Are Wrong: Directly/Inversely with sampling interval: These do not specify what aspect; the key relationship is precision improving with longer gate time. Less precision with longer interval: Opposite of true behavior. Common Pitfalls: Confusing precision with accuracy; accuracy depends on the timebase oscillator, while precision (resolution) depends on gate time. Final Answer: More precision with longer sampling interval

Question 16

Major building blocks of an HDL keypad emulator When emulating a matrix keypad in HDL, which major block is essential for generating the one-hot scanning pattern that steps through columns?

Accepted Answer

Introduction / Context: Keypad emulation or scanning requires driving one column at a time while monitoring rows for closures. HDL designs accomplish this with a compact pattern generator to step through columns in a loop. Given Data / Assumptions: Matrix keypad with multiple rows and columns. Goal is to minimize simultaneous column drive to avoid ghosting. Need a deterministic, cyclic one-hot pattern. Concept / Approach: A ring counter is perfect for producing a circulating one-hot vector (for example, 0001 → 0010 → 0100 → 1000 → 0001 ...). This directly drives columns and simplifies the logic required to identify the active column during a key press. Step-by-Step Solution: Instantiate a ring counter of width equal to the number of columns.Seed it with a 1 at reset and rotate left/right each tick.Use the counter outputs to drive column lines; read rows synchronously.On detection, freeze the ring counter while encoding and debouncing. Verification / Alternative check: Simulate the ring counter sequence and confirm only one column is asserted at a time. In hardware, LEDs on columns will “walk” visibly. Why Other Options Are Wrong: Sequencer: Sometimes used generically, but the specific, efficient block is a ring counter. Clock: Necessary, but not the scanning logic itself. Multiplexer: Useful for routing signals, not for generating the one-hot pattern. Common Pitfalls: Forgetting to handle rollover or to synchronize row sampling to avoid metastability; not freezing the scan during a press. Final Answer: A ring counter

Question 17

Why hardware testing matters for HDL stepper designs Beyond simulation, why should you perform a real hardware functional test of an HDL stepper-motor controller?

Accepted Answer

Introduction / Context: Simulations validate logic, but electromechanical systems add real-world variables. Stepper motors draw phase currents that depend on driver topology, supply voltage, winding resistance/inductance, and stepping profile. Verifying current in hardware protects the motor and driver. Given Data / Assumptions: HDL generates coil-enable patterns (full/half/microstep). A real driver stage (for example, chopper driver/H-bridge) powers the motor. Motor current must stay within datasheet ratings to avoid overheating. Concept / Approach: Logic may be correct, yet current can be excessive due to wiring errors, wrong decay mode, insufficient current limiting, or incorrect microstep tables. Hardware tests with a current probe or sense resistor confirm that phase currents track the intended limits during acceleration, steady stepping, and hold states. Step-by-Step Solution: Connect the controller to the driver and motor with rated power supply.Measure phase currents during idle (hold), low-speed stepping, and ramping.Verify current limit setpoints and temperature rise remain within specs.Adjust PWM/chopper parameters or sequences if measured current is too high. Verification / Alternative check: Cross-check current measurements with thermal behavior and torque output; confirm no missed steps or acoustic resonance that might indicate drive issues. Why Other Options Are Wrong: Software speed: HDL timing in hardware is deterministic; the concern is electromechanical current, not software throughput. Voltage levels of real outputs: Important, but current is the critical safety/performance metric for motors. Provide a fully operational system: A broad goal, not the specific reason most unique to steppers. Common Pitfalls: Relying only on logic simulation; ignoring inductive kick and driver decay modes; not validating hold current reduction to reduce heating. Final Answer: To check the current levels in the motor

Question 18

What determines a frequency counter’s accuracy? The overall accuracy of a digital frequency counter is primarily determined by which factor?

Accepted Answer

Introduction / Context: Frequency counters measure cycles against a reference timebase. Two quality aspects matter: accuracy (how close to the true value) and resolution/precision (how finely it can read). This question asks specifically about accuracy. Given Data / Assumptions: Counters use a crystal-controlled or disciplined oscillator as the system clock/timebase. Gate time (sampling interval) can be adjusted for resolution. Display formatting does not change the underlying measurement truth. Concept / Approach: Accuracy depends on the timebase frequency and its stability (tolerance, aging, temperature coefficient). If the oscillator runs 10 ppm high, every measurement will read about 10 ppm high, regardless of how long you count. Longer sampling intervals improve resolution but do not fix timebase error. Step-by-Step Solution: Identify that frequency f_meas ≈ N / Tbase, where Tbase derives from the timebase frequency.If the timebase is off by ε, then measured frequency carries the same fractional error ε.Increasing the gate time reduces quantization noise but not ε.Thus, accuracy is governed by the system clock/timebase quality. Verification / Alternative check: High-accuracy counters use OCXO/TCXO or GPS-disciplined references; switching to a better reference tightens absolute accuracy without changing sampling rate. Why Other Options Are Wrong: Number of digits: Cosmetic; shows resolution, not accuracy. Sampling rate: Affects resolution/precision, not absolute accuracy. Display update rate: User interface choice with no impact on correctness. Common Pitfalls: Confusing precise (many digits, stable reading) with accurate (correct). A long, stable reading with a poor timebase is precisely wrong. Final Answer: system clock frequency.

Question 19

HDL-based frequency counter design: identify which of the following is not a major functional block of a typical frequency counter (as implemented in hardware description languages such as VHDL/Verilog). Consider the standard blocks used: input cond…

Accepted Answer

Introduction / Context: A digital frequency counter, whether built discretely or in HDL (VHDL/Verilog), follows a well-known architecture. The key blocks include input signal conditioning, a timing and control unit to define the sample window, a counter/accumulator to tally pulses, and a display path (register plus decoder) to present the result. This question checks recognition of an element that is not core to that architecture. Given Data / Assumptions: We are discussing a generic frequency counter implementation. Essential blocks typically include: Schmitt trigger or comparator (conditioning), timing/control, counters, display register, and display decoder/driver. Bit-shifting is not inherently required to count edges within a timed window. Concept / Approach: The architecture counts input edges within a precisely known gate time (sample interval) and latches the result. Decoding and display logic then show a human-readable value (often BCD to 7-segment). A “bit shifter” is useful in serial I/O or arithmetic operations, but it is not an essential block for frequency counting. Step-by-Step Solution: List common blocks: input conditioner, timing gate, counter, latch/display register, decoder/display.Compare each option to this list.Display register → required to hold the count for stable display.Decoder/display → required to translate BCD/binary to segments.Timing and control unit → required to create the measurement window.Bit shifter → not required for core frequency counting. Verification / Alternative check: Block diagrams in textbooks consistently omit a shifter in basic frequency counters unless specific serial interfaces are added. Why Other Options Are Wrong: Display register: needed to freeze the measured value while the next interval counts. Decoder/display: needed to drive 7-seg or similar modules. Timing and control: essential to define accurate gate time. Common Pitfalls: Confusing optional serial-output features (which could use shifters) with core counter architecture. Final Answer: Bit shifter

Question 20

Digital clock design (HDL): purpose of the frequency prescaler In a 12-hour digital clock project, identify the main purpose of the prescaler (frequency divider) used ahead of the seconds counter.

Accepted Answer

Introduction / Context: Digital clocks require an accurate 1 pulse per second (1 pps) tick to advance seconds. Real-world reference sources (mains 60 Hz or crystals like 32.768 kHz) must be divided down to 1 Hz. The prescaler (frequency divider) performs this task, delivering a clean timing reference to the seconds counter. Given Data / Assumptions: Clock reference might be 60 Hz (or 50 Hz, or a crystal frequency). Seconds counters advance on a 1 Hz enable pulse. We are focusing on the prescaler function, not on display range-limiting logic. Concept / Approach: The prescaler divides a higher-frequency reference to the required base timing. For a 60 Hz mains reference, dividing by 60 produces 1 Hz (1 pps). This pulse then clocks the seconds unit counter reliably. Step-by-Step Solution: Identify reference: 60 pulses per second.Required tick: 1 pulse per second.Apply division: 60 / 60 = 1 pps.Feed 1 pps to seconds counter; other logic handles minutes/hours rollover. Verification / Alternative check: Designs using crystals (e.g., 32.768 kHz) similarly divide by powers of 2 to reach 1 Hz; concept is identical. Why Other Options Are Wrong: A: The “basic frequency” is known; prescaler divides it, not finds it. C: Rollover logic limits 12:59:59, not the prescaler. D: BCD value ranges are enforced by counter modulus and decoding, not the prescaler. Common Pitfalls: Mixing up prescaler duties with display decoding or hour/minute rollover logic. Final Answer: transform a 60 pps input to a 1 pps timing signal.

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