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

In the frequency counter, the pulse width of the enable signal is very critical for taking an accurate frequency measurement.

Accepted Answer

True

Question 12

A frequency counter is a circuit that can measure and display the frequency of a signal.

Accepted Answer

True

Question 13

In the digital clock project HDL, the 1 pps signal is used as a synchronous clock for all of the counters' stages, which are synchronously cascaded.

Accepted Answer

True

Question 14

In the digital clock project, the 60 Hz signal is sent through a Schmitt-trigger circuit to produce sine wave pulses at the rate of 60 pps.

Accepted Answer

False

Question 15

One CASE construct inside another CASE construct is called a do-loop.

Accepted Answer

False

Question 16

In HDL when a circuit is simulated on a computer, the designer must create all the different scenarios that will be experienced by the actual circuit and must also know the proper response to those inputs.

Accepted Answer

True

Question 17

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

False

Question 18

One of the first steps in any HDL project is to define its scope by naming each input and output.

Accepted Answer

True

Question 19

In HDL, one of the strategies used in strategic planning is to find a way to test each piece of the project.

Accepted Answer

True

Question 20

Top-down design means that we start at the highest level of the hierarchy, or that the entire project is considered to exist in a closed dark box with inputs and outputs.

Accepted Answer

True

Digital System Projects Using HDL Questions