Question 1

Schmitt-trigger conditioning of a 60 Hz reference: In a digital clock front-end, a 60 Hz mains-derived waveform is passed through a Schmitt-trigger buffer to create clean logic transitions at the rate of ________.

Accepted Answer

Introduction / Context: Digital clocks often derive timing from either a crystal oscillator or the AC mains frequency (50/60 Hz). When using the mains, the sinusoidal voltage must be isolated, scaled, and squared up into clean logic pulses. A Schmitt trigger provides hysteresis to convert the slowly changing waveform into sharp digital edges. Given Data / Assumptions: Input frequency is 60 Hz. A Schmitt trigger outputs rail-to-rail logic-level pulses on threshold crossings. No frequency division or multiplication is performed in the Schmitt stage itself. Concept / Approach: The Schmitt trigger does not change the fundamental frequency; it improves edge quality and noise immunity. Therefore, its output toggles at the same rate as the input fundamental, producing a pulse train at 60 pulses per second (pps), suitable for gating counters or for further division to 1 pps. Step-by-Step Solution: Feed the isolated 60 Hz waveform into the Schmitt buffer.As the input crosses the upper threshold, the output toggles HIGH; crossing the lower threshold toggles LOW.This produces square pulses synchronized to 60 Hz.Choose 60 pps as the output rate. Verification / Alternative check: Oscilloscope captures show one complete output cycle per 60 Hz input cycle after the Schmitt stage. Why Other Options Are Wrong: 1 pps: Requires a divider chain.100 pps / 600 pps: Not produced by a simple Schmitt trigger from a 60 Hz input. Common Pitfalls: Mislabeling Schmitt outputs as sine pulses; ignoring isolation and safety when sampling the mains waveform. Final Answer: 60 pps

Question 2

Keypad encoder freeze detection: In a typical keypad encoder design with active-low column lines, the logic that activates the “freeze” bit only when exactly one column is low is implemented using ________.

Accepted Answer

Introduction / Context: Matrix keypads are scanned by driving or reading rows and columns. In active-low schemes, a pressed key pulls one column line low while others remain high. To capture a valid keypress and prevent chatter or ghosting, many encoders assert a “freeze” (or latch) signal only when exactly one column indicates activation. Given Data / Assumptions: Columns are active-low: pressed key yields a logic 0 on one column line. Only a single key should be recognized at a time (no rollover). Freeze asserts when the detection logic confirms one-hot behavior. Concept / Approach: NAND gates are convenient for detecting combinations and ensuring that only one low is present. For example, by appropriately combining signals (possibly after inversion), a NAND network can produce a HIGH (asserted) freeze only when one column is low and all others are high. Additional gating can reject two-or-more simultaneous lows. Step-by-Step Solution: Define the one-hot condition for columns C3..C0 (active-low): exactly one Ci = 0 and the rest = 1.Construct logic using NANDs (and inverters where needed) to detect this condition.Drive the freeze latch with the NAND network output to hold the detected key code until release.Debounce and time-qualify as necessary. Verification / Alternative check: Truth-table simulation confirms freeze = 1 for one-low cases and 0 for none or multiple-low cases. Gate-level schematics from synthesis will show NAND-rich implementations for compactness. Why Other Options Are Wrong: CASE structure: A behavioral construct; implementation still maps to gates like NAND/NOR.freeze function: Descriptive term, not a gate-level implementation.BCD counter: Counts digits; unrelated to one-hot detection. Common Pitfalls: Allowing ghost keys due to shared row/column paths; failing to debounce; not masking multiple simultaneous keypresses. Final Answer: NAND columns

Question 3

Project management focus for digital-system builds: Among the classic constraints (scope, cost, quality, resources), a very critical dimension that directly drives schedules, milestones, and delivery risk is ________.

Accepted Answer

Introduction / Context: Engineering projects—especially digital-system designs—operate under competing constraints: we must achieve a defined scope with acceptable quality, within a budget, and by a deadline. Of these, the factor that most visibly governs milestones and risk is time, the project schedule dimension. Given Data / Assumptions: Digital projects include design, simulation, synthesis, verification, and prototyping. Schedules are planned around dependencies and critical paths. Delays increase costs and compress testing time, elevating risk. Concept / Approach: “Time” sits at the center of planning: the critical path method identifies tasks that determine the minimum completion time. Slack on noncritical tasks does not alter the delivery date, but any slip on critical-path items immediately threatens deadlines. Contingency buffers and iterative milestones revolve around time management. Step-by-Step Solution (Reasoning): List constraints: scope, cost, quality, resources, time.Identify what directly dictates milestone dates and risk exposure: time.Conclude that “time” is the most critical dimension for on-time delivery.Select “time.” Verification / Alternative check: Project frameworks (PMI, Agile) center on time-boxing, sprints, and milestone-based gates; risk registers often track schedule impact first. Why Other Options Are Wrong: cost: Important, but budgets can sometimes be adjusted; deadlines cannot always shift.skill / personnel: Affect productivity, yet schedules remain the primary external commitment. Common Pitfalls: Underestimating verification time; inadequate buffers; ignoring vendor lead times; parallelizing tasks that are actually dependent. Final Answer: time

Question 4

Stepper-motor interface operating modes: A practical stepper-motor interface typically supports one of ________ operating modes (for example, wave drive, full-step, half-step, or microstepping).

Accepted Answer

Introduction / Context: Stepper motors can be driven in several distinct modes, each trading torque, smoothness, and resolution. A well-designed interface or controller often allows selecting a mode that best suits the application, from simple coarse steps to fine, smooth motion. Given Data / Assumptions: Common modes: wave drive, full-step, half-step, and microstepping. Controller exposes configuration pins or registers to select the mode. Power stage and current control are compatible with the chosen mode. Concept / Approach: Wave drive energizes one phase at a time (lower torque). Full-step energizes two phases for higher torque. Half-step alternates between one and two phases to double position resolution. Microstepping modulates currents sinusoidally for very fine resolution and smoother motion. Summarizing these gives four canonical modes. Step-by-Step Solution: List standard operating modes for stepper control.Count distinct modes: four.Map interface selection to these modes via control bits.Choose “four.” Verification / Alternative check: Commercial drivers (e.g., popular stepper ICs) document these modes and their selection tables, reinforcing the four-mode taxonomy. Why Other Options Are Wrong: one / two / three: Understate the range of standard options provided by most controllers. Common Pitfalls: Assuming microstepping is a single fixed resolution; in practice, microstep resolution can be selectable (e.g., 1/4, 1/8, 1/16). Final Answer: four

Question 5

Key release behavior in keypad simulation: In a keypad encoder testbench, just after the 4 ms mark the test releases the key by setting the 4-bit column value to ________, which represents no column pulled low and places the data output in Hi-Z.

Accepted Answer

Introduction / Context: Matrix keypad interfaces typically use active-low columns. When a key is pressed, one column is driven or read as LOW; when no key is pressed, all columns read HIGH. In HDL simulations, we model this behavior with a 4-bit bus for columns, using hexadecimal shorthand. Releasing a key restores the bus to the “no key” condition: all ones. Given Data / Assumptions: Columns are 4 bits, active-low. Pressed key forces one bit to 0; others remain 1 (one-hot low). Released state is all bits = 1 (1111b). Concept / Approach: If columns are represented as a 4-bit vector, no key pressed corresponds to 1111 in binary, which is F in hexadecimal. Many encoders use this state to tri-state (Hi-Z) or idle the data outputs until a valid keypress is detected, often with a freeze or debounce mechanism. Step-by-Step Solution: Identify active-low logic: 0 indicates pressed; 1 indicates released.Released state for 4 columns: 1111b.Binary 1111 equals F hex.Therefore, set columns to F hex to model key release and Hi-Z output. Verification / Alternative check: Waveform inspection: After changing columns to F, the encoder should remove the valid code and drive its output bus to Hi-Z or idle, consistent with the design specification. Why Other Options Are Wrong: 0 hex: 0000 implies all columns low (multiple keys or fault).4 hex / 8 hex: Only one bit high among otherwise low bits, inconsistent with no-key state in active-low logic. Common Pitfalls: Forgetting active-low convention; mixing up row/column polarity; not modeling pull-ups in the testbench, which can leave buses at undefined levels. Final Answer: F hex

Question 6

Digital clock counter behavior (hours vs. minutes/seconds): In common clock designs, the hours section is implemented so that, unlike the seconds and minutes sections, it never goes to ________.

Accepted Answer

Introduction / Context: Digital clocks can be realized in 12-hour or 24-hour formats. The seconds and minutes sections are typically modulo-60 counters that cycle from 00 through 59 and then back to 00. The hours section, however, is often constrained differently depending on the format: 1–12 for 12-hour clocks (with AM/PM) and 00–23 for 24-hour clocks. This question focuses on the common 12-hour display behavior. Given Data / Assumptions: We consider a 12-hour clock display (typical in consumer devices). Seconds/minutes roll from 59 to 00. Hours should present human-friendly values 1 to 12, never “00.” AM/PM toggles at the 11→12 rollover. Concept / Approach: In a 12-hour implementation, the hours counter is not a simple BCD decade pair that reaches 00. Instead, it implements a modulo-12-with-offset sequence: 12, 1, 2, ... 11, 12, 1, ... This avoids the “00” hour on the display. That difference is what makes the hours section distinct from seconds/minutes, which do display 00 at rollover. Step-by-Step Solution (Reasoning): Identify minutes/seconds behavior: modulo-60 with visible 00.Identify 12-hour hours behavior: cycle 1..12 without 00.Note display mapping may require special gating when counter hits 13, mapping it to 1.Conclude the hours section never goes to the 0 state in 12-hour format. Verification / Alternative check: Schematic/HDL implementations commonly detect 12→1 and 11→12 boundaries and prevent 00 from appearing on the hour display; testbenches show that at midnight/noon, the hour shows 12, not 00. Why Other Options Are Wrong: 13: In a correct 12-hour design, “13” will not be presented; internal states may transiently encode it but are remapped.the ring counter / the BCD counter: Implementation choices; not the behavioral difference intended here. Common Pitfalls: Implementing hours as a plain BCD pair resulting in 00 on rollover; forgetting to handle 12→1 and AM/PM toggling correctly. Final Answer: the 0 state

Question 7

Project planning for HDL-based digital systems One critical aspect of early project planning and management is the selection of the hardware platform (for example, FPGA family or target ASIC process) that will best fit the application's performance,…

Accepted Answer

Introduction / Context: Choosing the correct implementation target is foundational for any HDL-based digital system project. The hardware platform—such as a particular FPGA family, SoC with programmable logic, or an ASIC technology node—directly influences achievable speed, resource utilization, I/O standards, power consumption, development cost, and time to market. Given Data / Assumptions: The design will be written in HDL and must meet specified throughput, latency, and power goals. Potential targets include multiple FPGA families or an ASIC process. Board-level constraints (I/O count, memory interfaces, transceivers) must match the target device features. Concept / Approach: Platform selection aligns application needs with device capabilities. Key criteria include logic resources (LUTs, flip-flops), embedded memory, DSP slices, high-speed serial transceivers, clocking resources, and toolchain maturity. Total cost of ownership includes silicon cost, development tools, IP licensing, and NRE for ASICs. Step-by-Step Solution: List application requirements: bandwidth, latency, I/O, memory, power budget.Map requirements to candidate platforms' resource and feature sets.Evaluate development risk and schedule: tool support, IP availability, team experience.Select the hardware platform that best balances performance, cost, power, and risk. Verification / Alternative check: Build a quick feasibility prototype or timing/resource estimate in the vendor tools to confirm the fit before committing fully. Why Other Options Are Wrong: “software” is important, but HDL tools follow from platform choice. “personnel” planning matters, yet it does not by itself fit the application’s technical constraints. “time” is a schedule constraint, not a platform. Common Pitfalls: Underestimating I/O requirements; ignoring transceiver availability; overlooking power/thermal limits; choosing a device with insufficient on-chip memory. Final Answer: hardware platform

Question 8

Digital clock design using synchronous counters In the digital clock project, the 1 pulse-per-second (1 pps) signal is used as the common synchronous clock for all counter stages, which are designed to be synchronous cascaded stages.

Accepted Answer

Introduction / Context: A digital clock typically counts seconds, minutes, and hours using BCD counters. Using a single, clean time base ensures that all digits change coherently and avoids ripple delays or glitches associated with asynchronous designs. Given Data / Assumptions: The clock provides a 1 pps reference (already debounced and stable). All counting stages (seconds, minutes, hours) share this same clock edge. Cascading between digits is synchronous using enable/carry signals. Concept / Approach: Synchronous cascading means every flip-flop in all stages is clocked by the same 1 pps signal. Inter-stage connections use enable or ripple-carry-out signals to control when higher-order digits increment, maintaining precise timing and eliminating ripple skew. Step-by-Step Solution: Distribute 1 pps to all counter stages as the common CLK.Use terminal count of the ones stage to assert an enable for the tens stage on the same edge.Constrain minute tens to MOD-6 and second tens to MOD-6 behavior, with BCD encoding.Verify that updates occur only at 1 pps edges and without intermediate glitches. Verification / Alternative check: Simulate 0–59 seconds and 0–59 minutes to confirm orderly rollover at 59 → 00 with an increment of the next stage. Why Other Options Are Wrong: “advanced BCD counters” describes a type, not the cascade method. “MOD-6 counters” applies only to some digits, not all. “1 pulse per second” restates the clock source, not the cascade property. Common Pitfalls: Mixing asynchronous ripple counters; forgetting synchronous enables; mishandling the 59→00 rollover. Final Answer: synchronous cascaded

Question 9

HDL development granularity and hierarchy Each block, starting at the simplest level of the design hierarchy, should be implemented in HDL so it can be simulated, verified, and reused cleanly.

Accepted Answer

Introduction / Context: Modular HDL design improves scalability and verification efficiency. By defining and coding self-contained blocks, teams can simulate and validate functionality early, enabling parallel development and consistent integration as the project grows. Given Data / Assumptions: The design is hierarchical, composed of blocks and subsystems. Each block has a clear interface and purpose. Reusable IP is desirable for future projects. Concept / Approach: “Block” is the fundamental unit of HDL organization. A block maps to a module/entity with ports, generics/parameters, and a well-defined behavior. Coding from the simplest blocks upward allows unit testing, code review, and resource/timing estimation at each layer. Step-by-Step Solution: Identify the smallest meaningful functions (e.g., adders, counters, FIFOs) as blocks.Write HDL for each block with a self-checking testbench.Compose larger subsystems by instantiating validated blocks.Integrate subsystems into the top level, preserving clean interfaces. Verification / Alternative check: Use regression tests per block so that later changes do not break earlier functionality; track coverage and assertions. Why Other Options Are Wrong: “subsystem” is a higher aggregation of blocks. “circuit” is too generic and informal for HDL hierarchy. “function” in HDL is a coding construct, not necessarily a compiled verification unit with ports. Common Pitfalls: Overly large blocks that hinder reuse; ambiguous interfaces; lack of independent testbenches for leaf blocks. Final Answer: block

Question 10

Frequency counter timing architecture In the frequency counter project, the control clock is derived from the system clock signal by frequency dividers governed by the control and timing block.

Accepted Answer

Introduction / Context: Frequency counters require precisely timed gates and sample windows to measure input frequency accurately. Internally, these timing signals are created by dividing a stable system clock to produce the control clocks that orchestrate counting, latching, and display updates. Given Data / Assumptions: A stable system clock is available (e.g., crystal-derived). Dividers create 1-second or other reference intervals. A control/timing block manages when the main counter runs and when results are latched. Concept / Approach: Using synchronous dividers on the system clock yields deterministic timing. The control and timing block enables counting during a gate interval, then disables it and latches the result into registers for display. Accuracy depends on the stability and precision of the system clock. Step-by-Step Solution: Feed the system clock into a divider chain to generate gate intervals.Assert control signals (enable, latch) according to the divided timing.Count input pulses only during the enabled window.Latch and display the count after the window ends; repeat continuously. Verification / Alternative check: Simulate timing diagrams ensuring no overlap between counting and latching; measure cumulative error versus the system clock accuracy. Why Other Options Are Wrong: “BCD counters,” “display register,” and “decoder/display” are consumers of timing, not the original precise timing source. Common Pitfalls: Jittery or inaccurate system clock; asynchronous control causing metastability; insufficient divider width. Final Answer: system clock signal

Digital System Projects Using HDL Questions