Question 1

Digital one-shot implementation in HDL using a counter “The idea of using a counter to implement a digital one-shot (monostable) in HDL is not used.” Judge the correctness of this claim.

Accepted Answer

Introduction / Context: In hardware description languages (HDLs) such as VHDL or Verilog, monostable (one-shot) behavior is frequently required to stretch pulses or generate fixed-duration events. A standard digital method uses a counter or timer register to hold an output high for a specified number of clock cycles after a trigger. Given Data / Assumptions: Synchronous digital systems have an available clock. A trigger event can be detected as a rising or falling edge. A counter can measure a fixed number of cycles to set the pulse width. Concept / Approach: The HDL one-shot approach: upon detecting a trigger edge, load a counter with a preset value N (or start incrementing from 0) and assert an output. On each clock, decrement (or increment) the counter; when the terminal count is reached, deassert the output. This exactly realizes a monostable with pulse width = N * Tclk. Step-by-Step Solution: Detect trigger edge using a two-flip-flop synchronizer and XOR or a rising-edge detector.Start a counter/timer and assert pulse_out = 1.Update the counter each clock; maintain pulse_out = 1 while count > 0.When count reaches 0, set pulse_out = 0 to end the one-shot. Verification / Alternative check: Simulation waveforms show a fixed-width pulse irrespective of trigger pulse width (subject to retrigger rules you implement). Synthesis on FPGA/CPLD is straightforward and standard practice. Why Other Options Are Wrong: “Correct” contradicts prevalent digital design methods. The other distractors about analog monostables or FSM types are irrelevant to the basic counter-based technique. Common Pitfalls: Not synchronizing asynchronous triggers; choosing pulse width that violates downstream timing; ignoring re-trigger behavior (one-shot vs. retriggerable). Final Answer: Incorrect

Question 2

Understanding SISO shift registers: role relative to parallel buses “A serial-in/serial-out (SISO) shift register transfers data from one line of a parallel bus to another line, one bit at a time.” Determine whether this description is accurate.

Accepted Answer

Introduction / Context: Shift registers are categorized by how data enters and leaves: SISO, SIPO, PISO, and PIPO. Accurate terminology matters when specifying interfaces between serial links and parallel buses. This question tests understanding of what SISO actually does. Given Data / Assumptions: SISO = Serial-In/Serial-Out: one serial input, one serial output. Parallel buses carry multiple bits simultaneously across multiple lines. SISO lacks parallel load or parallel readback capability by definition. Concept / Approach: A SISO shift register simply shifts a serial bitstream through cascaded flip-flops; bits enter serially and emerge serially after a fixed number of clocks. It does not “transfer data from one line of a parallel bus to another line.” Devices that bridge between serial and parallel are SIPO (serial-in/parallel-out) or PISO (parallel-in/serial-out), not SISO. Step-by-Step Solution: Identify the I/O modes: SISO = serial-to-serial pipeline.Contrast with SIPO: serial input converted to multiple parallel outputs.Contrast with PISO: captured parallel data shifted out serially.Therefore, the statement describing SISO as moving between parallel “lines” is inaccurate. Verification / Alternative check: Look at timing diagrams of a SISO; only one data line in and one data line out are active per clock, with no parallel load/read ports. Why Other Options Are Wrong: “Correct” conflicts with the definition of SISO. References to “universal” shift registers or bidirectional pins do not change the fundamental SISO interface. Common Pitfalls: Mixing up SISO with SIPO/PISO; assuming all shift registers can interface to parallel buses without explicit parallel ports. Final Answer: Incorrect

Question 3

Expanding counter width: can basic counters be cascaded “in parallel” to handle more bits? “Basic counters can be cascaded in parallel to increase the number of data bits that the counter can handle.” Evaluate this statement for correctness and clar…

Accepted Answer

Introduction / Context: When designers need a counter with more bits than a single IC or module provides, they “cascade” multiple counters. However, the phrasing “cascaded in parallel” can be misleading because true width expansion uses series chaining of count stages while maintaining common clocking for synchronous designs. Given Data / Assumptions: Goal: increase modulus/bit width beyond a basic counter's capacity. Synchronous expansion uses common clock and cascaded carry/enable signals (not Q-to-CLK). Asynchronous expansion uses Q-to-CLK chaining (ripple), not parallel connections. Concept / Approach: To handle more bits, counters are chained in series so that overflow/carry from a lower-order stage advances the next stage. This is often called cascading, but it is not a parallel connection of independent counters. “Parallel” would imply independent counters operating side-by-side without carry linkage, which does not, by itself, increase a single count width. Step-by-Step Solution: Identify required operation: single counter with more bits.Choose chaining method: ripple (Q→CLK) or synchronous (carry/enable) with a shared clock.Understand that the modules are connected in series logic sense, not in parallel.Therefore, the statement as written is incorrect due to misuse of “in parallel.” Verification / Alternative check: Typical 74HC163/193 datasheets show “cascading counters” diagrams using ripple carry output (RCO) feeding the next stage’s enable; the blocks are drawn side-by-side physically but functionally chained in series. Why Other Options Are Wrong: “Correct” accepts an imprecise and misleading phrase. Limiting correctness to ripple or synchronous variants still does not justify the term “in parallel.” Common Pitfalls: Interpreting physical side-by-side placement on a schematic as “parallel” connectivity; forgetting to distribute a common clock in synchronous chains. Final Answer: Incorrect

Question 4

Serial-in/parallel-out (SIPO) shift register — does it move data “from one parallel bus to another” through a single serial line? Consider a classic SIPO register. It accepts data on a single serial input line under clocking and then presents the ac…

Accepted Answer

Introduction / Context: Shift registers are versatile building blocks for data movement and temporary storage in digital systems. Two common types are PISO (parallel-in/serial-out) and SIPO (serial-in/parallel-out). This question probes whether a SIPO by itself “moves data from one parallel bus to another” via a single wire, which is a subtle but important point for system designers. Given Data / Assumptions: SIPO has one serial input, a shift clock, and multiple parallel outputs. Transferring data between parallel buses typically implies parallel capture at the source and parallel presentation at the destination. No additional stages (like a PISO at the source) are explicitly included in the statement. Concept / Approach: A SIPO converts a serial stream into a parallel word. To move a word from a parallel source bus onto a single serial line first, you need a PISO at the source to serialize it. Then a SIPO at the destination deserializes it back to parallel. A SIPO alone does not perform parallel-to-parallel transfer; it only performs serial-to-parallel conversion. Step-by-Step Solution: Identify SIPO function: serial input → parallel outputs.Identify requirement for bus-to-bus: parallel source → serialize → send → deserialize → parallel destination.Notice missing PISO stage in the claim.Conclude the stated claim about SIPO alone is incorrect. Verification / Alternative check: Textbook signal-flow examples always pair PISO (source) with SIPO (destination) when bridging two parallel buses over a single line. Why Other Options Are Wrong: Correct: Overstates what a SIPO can do unaided.Applies only with an added PISO stage: This is a condition, not the requested true/false evaluation of the original claim.Valid for tri-state buses only: Bus electrical style does not change the logical requirement. Common Pitfalls: Conflating the paired PISO+SIPO link with SIPO alone.Assuming “parallel bus” implies any register with multiple outputs. Final Answer: Incorrect

Question 5

HDL counter behavior — meaning of “rollover” in a full-featured counter design In hardware description language (HDL) implementations, what does it mean when a counter “rolls over”?

Accepted Answer

Introduction / Context: Counters in HDL (such as VHDL or Verilog) are foundational to timers, dividers, and state machines. Designers frequently mention “rollover” (or wrap-around). Understanding the precise meaning helps avoid off-by-one errors and mis-specified terminal counts. Given Data / Assumptions: A full-featured counter has a defined modulus (e.g., modulus-10, modulus-256). Counting proceeds through a known sequence, then returns to the start value. No special saturation mode is assumed; normal modular arithmetic applies. Concept / Approach: “Rollover” means the counter reached its terminal count (maximum defined value for the sequence) and on the next active clock edge, it wraps to the initial value of the sequence. In HDL, this is typically coded via comparisons against the terminal count and resetting to the start value (often zero) or via arithmetic modulo operations. Step-by-Step Solution: Define the modulus: e.g., a 4-bit free-running counter has values 0–15.Detect terminal count: compare count to 15 (binary 1111) or parameter MAX.On the next update: set count back to 0 (or a programmed start).Hence, “rollover” = reached limit and started again at the beginning. Verification / Alternative check: Examine synthesis results or simulation waveforms: observe count sequence repeating after the terminal value. Why Other Options Are Wrong: Incorrect: Rollover most definitely is the wrap to the start value.Only when saturation arithmetic is enabled: Saturation is the opposite (stops at max, no wrap).It jumps to an undefined state: Well-designed counters do not use undefined states for rollover. Common Pitfalls: Confusing rollover with saturation behavior.Mishandling enable/reset so the rollover happens one count late or early. Final Answer: Correct

Question 6

Decoder output glitches — is “strobing” a reliable method to suppress spikes? Evaluate the technique of gating (strobing) a decoder’s outputs so they are only observed during stable input intervals to eliminate visible spikes.

Accepted Answer

Introduction / Context: Decoders and combinational logic can exhibit brief hazards or glitches during input transitions because different internal paths settle at different times. System designers often apply “strobing” to mask these transients so that downstream circuits see clean outputs. Given Data / Assumptions: Decoder outputs may glitch during input changes. A strobe (enable) signal is available, typically synchronous to the system clock. Outputs only need to be valid during defined sampling windows. Concept / Approach: Strobing means gating the decoder output with an enable signal that opens only when the inputs are guaranteed stable (for example, shortly after a clock edge when propagation delays have expired). This masks hazards without requiring internal redesign, making it a reliable practical solution alongside design-for-hazard-free logic or Gray coding. Step-by-Step Solution: Identify the unstable interval: input transitions to the decoder.Delay observation: hold the strobe low until inputs settle.Assert strobe: gate the output to downstream logic in the stable window.Result: visible spikes are suppressed at the observation point. Verification / Alternative check: Scope measurement shows hazards at the raw decoder output but not at the strobed output. Why Other Options Are Wrong: Incorrect: Strobing is a well-known mitigation technique.Only for asynchronous inputs: Even synchronous transitions can glitch due to unequal path delays.Only if Gray coding is used: Gray coding reduces multi-bit changes but is not required for strobing to help. Common Pitfalls: Failing to align the strobe with data-valid timing.Assuming strobing fixes timing violations; it only masks glitches at the observation point. Final Answer: Correct

Question 7

Digital design notation — is “dependency notation” truly obsolete and no longer used? Assess the claim that dependency notation is no longer used in modern timing/sequence documentation.

Accepted Answer

Introduction / Context: Engineering documentation evolves, but older notations rarely vanish completely. “Dependency notation” refers to a style of indicating how one signal or event depends on another within timing or sequence diagrams. This question asks whether such notation is truly no longer used. Given Data / Assumptions: Dependency notation encompasses marks/arrows/labels that express causal or conditional relationships. Modern EDA tools and standards (timing diagrams, waveform viewers, HDL comments) still require expressing dependencies. Organizations may use mixed documentation conventions. Concept / Approach: While standards have shifted (for example, toward more detailed timing constraints or formal properties), the need to express dependencies persists. Many teams still annotate timing diagrams with dependency cues, and training materials continue to show such notation. Therefore, stating it is “no longer used” overgeneralizes. Step-by-Step Reasoning: Identify purpose: show causal links between signals/events.Check current practice: datasheets, app notes, and lecture materials still depict dependencies.Conclude: Notation remains in use, even if styles vary by context and tool. Verification / Alternative check: Review contemporary vendor timing diagrams; you will find annotated dependencies or equivalent constructs (enables, valid windows, setup/hold callouts). Why Other Options Are Wrong: Correct: Overstated; usage persists.Only used in analog timing charts: Digital documentation also uses dependency cues.Replaced exclusively by UML diagrams: UML serves different abstraction levels and is not a one-to-one replacement for signal-level timing. Common Pitfalls: Assuming a local team’s style reflects an industry-wide standard.Confusing the obsolescence of a specific symbol set with the broader concept of depicting dependencies. Final Answer: Incorrect

Question 8

Structure of sequential logic — do most sequential circuits include both memory and combinational sections? Evaluate the statement that typical sequential systems combine a memory element bank (e.g., flip-flops/registers) with surrounding combinatio…

Accepted Answer

Introduction / Context: Sequential logic differs from combinational logic by incorporating memory of past inputs. Real designs typically interleave storage elements and combinational functions to create controllers, counters, and pipelines. This item checks recognition of the canonical structure. Given Data / Assumptions: Sequential circuits use flip-flops, latches, or RAM elements as memory. Combinational logic computes next-state and outputs from current state and inputs. Synchronous clocking is assumed for mainstream designs. Concept / Approach: The standard synchronous model is: next_state = f(current_state, inputs); outputs = g(current_state, inputs). Memory holds current_state; combinational logic implements functions f and g. Almost all practical sequential blocks, from simple counters to complex CPUs, follow this template. Step-by-Step Solution: Identify memory elements (flip-flops/registers) as state holders.Identify combinational logic as next-state/output computation.Recognize this pairing is ubiquitous in sequential systems.Therefore, the statement is correct. Verification / Alternative check: Examine a simple FSM diagram: state register + combinational next-state/output logic. Why Other Options Are Wrong: Incorrect: The presence of both sections is the essence of sequential logic.Only true for synchronous counters / True only in microprocessors: Too narrow; the structure is general. Common Pitfalls: Assuming memory is optional in “sequential” systems; by definition, it is not.Overlooking small registers embedded inside larger IP blocks. Final Answer: Correct

Question 9

Terminal count of a decade (modulus-10) binary counter — is it 1010 (decimal 10)? Assess the statement: “The terminal count of a typical modulus-10 binary counter is 1010.”

Accepted Answer

Introduction / Context: A decade (modulus-10) counter cycles through ten states, representing decimal 0 to 9 in binary-coded form. Confusion often arises between the highest valid state and the binary pattern representing decimal 10, which is not part of the decade cycle. Given Data / Assumptions: Modulus-10 counter counts 0,1,2,...,9 then rolls to 0. Binary for 9 is 1001; binary for 10 is 1010. Terminal count typically means the last valid state before rollover. Concept / Approach: In a typical BCD decade up-counter, the terminal valid state is 1001 (decimal 9). On the next clock, internal logic clears to 0000 (decimal 0). The pattern 1010 (decimal 10) is not a legal displayed state in BCD decade counting; it is either skipped by design or used transiently only for internal detect/reset. Step-by-Step Solution: List valid BCD states: 0000 through 1001.Identify terminal count: 1001 (9), not 1010 (10).Determine rollover action: next state after 1001 is 0000.Therefore the statement claiming 1010 as terminal count is incorrect. Verification / Alternative check: Consult truth tables for BCD counters: detection of 1010 triggers reset to 0000. Why Other Options Are Wrong: Correct / down-counters / with BCD correction: These do not change the definition of legal BCD states; 1010 is not a valid displayed state. Common Pitfalls: Equating “binary 10” with “decimal 10” inside a BCD decade sequence.Misreading terminal count as the value that causes reset rather than the last valid state. Final Answer: Incorrect

Question 10

74192 BCD up-/down-counter — are the terminal count outputs active-LOW? Evaluate the statement: “In a 74192 BCD decade up-/down-counter, the terminal count up and terminal count down outputs are active-LOW.”

Accepted Answer

Introduction / Context: Classic TTL counters such as the 74192 (BCD up-/down-counter) provide terminal count outputs used for cascading or generating control pulses. Designers must know the active polarity to wire counters reliably in multi-decade chains. Given Data / Assumptions: Device: 74192, BCD decade up-/down-counter with dedicated terminal count outputs (carry/borrow style). Terminal count outputs signal when the device is about to roll in the chosen direction. The outputs are used to trigger the next stage in a cascade. Concept / Approach: For the 74192, the terminal count outputs (often labeled CO and BO or similar) are active-LOW pulses indicating the counter has reached the limit in the up or down direction and is transitioning. Knowing the polarity ensures that cascade wiring correctly clocks or enables the adjacent stage without added inversion. Step-by-Step Solution: Identify function: terminal count provides a pulse at rollover/roll-under.Recall polarity: outputs go to the active level (LOW) at the terminal event.Implication: devices can be cascaded by feeding this low-active pulse appropriately to the next counter stage’s clock/enable line.Therefore, the statement about active-LOW behavior is correct. Verification / Alternative check: Check typical cascade examples in TTL counter datasheets where terminal lines interface directly without extra inverters due to active-LOW convention. Why Other Options Are Wrong: Incorrect / Only-up / Only-down: These contradict the standard active-LOW behavior reported for both terminal directions in BCD 74192 implementations. Common Pitfalls: Confusing 74192 with other counters (e.g., 74193 binary) that may label pins differently.Assuming active-HIGH based on generic “carry” naming; always verify polarity. Final Answer: Correct

Question 11

System integration with IC counters — should output devices (e.g., LEDs) be designed to operate from counter outputs? Judge the statement that, in a complete system built around counter ICs, output indicators must be interfaced to the counter output…

Accepted Answer

Introduction / Context: Turning a counter IC into a working application requires proper interfacing to displays, LEDs, or other loads. The statement emphasizes that outputs must be configured to drive the chosen indicators correctly—an obvious but vital systems point. Given Data / Assumptions: Counters present logic-level outputs representing count states. Indicators (LEDs, seven-segment modules, etc.) require appropriate current limiting and polarity. Power supply, fan-out, and timing constraints are respected. Concept / Approach: Correct operation requires matching logic output characteristics to load requirements: voltage levels, source/sink current capability, and whether the indicator is common-anode or common-cathode. Sometimes buffers or drivers (e.g., transistor arrays) are inserted to meet current needs or to translate voltage levels. Step-by-Step Solution: Determine output type: totem-pole, open-collector, or CMOS push-pull.Design LED interface: series resistor sizing, correct polarity, possible driver IC.Check timing: ensure indicators sample after stable transitions (or use latches/decoders).Integrate with power budget and thermal limits for reliability. Verification / Alternative check: Prototype on a bench: confirm brightness, logic correctness, and absence of ghosting or glitches. Why Other Options Are Wrong: Incorrect: Ignoring interfacing leads to under/over-drive and unreliable outputs.Only if synchronous / Only with open-collector: Interfacing is necessary regardless of counter style; specifics vary but the need persists. Common Pitfalls: Forgetting current-limiting resistors for LEDs.Assuming logic can source more current than the datasheet allows. Final Answer: Correct

Question 12

Resource comparison — do synchronous counters generally require more circuitry than asynchronous (ripple) counters? Evaluate the statement about implementation complexity versus performance for synchronous versus asynchronous counters.

Accepted Answer

Introduction / Context: Asynchronous (ripple) counters toggle stages sequentially, while synchronous counters clock all stages simultaneously with added combinational logic to determine state transitions. The trade-off is classic: less logic vs. better timing performance. Given Data / Assumptions: Asynchronous counters use minimal gating; propagation ripples through flip-flops. Synchronous counters add gating to derive each next state in parallel. Target frequency and timing closure influence architecture choice. Concept / Approach: Synchronous counters remove ripple delay, enabling higher speeds and predictable timing at the cost of additional gates (look-ahead or decoding logic). Therefore, they generally require more circuitry than ripple counters, which are simpler but slower due to cumulative propagation delay and potential glitches at intermediate counts. Step-by-Step Solution: Contrast architectures: ripple (serial toggling) vs. synchronous (parallel update).Identify added logic: synchronous uses gating to compute toggles for each stage.Conclude: more logic resources are typical in synchronous designs.Benefit: improved maximum clock frequency and timing predictability. Verification / Alternative check: Synthesis reports show higher LUT/gate count for synchronous implementations compared to ripple equivalents of the same width. Why Other Options Are Wrong: Incorrect: Undercuts the well-known trade-off.Frequency/Gray-specific options: The complexity difference holds broadly, not only under special conditions. Common Pitfalls: Ignoring that FPGA vendors may offer dedicated carry chains that blur resource differences but not the conceptual trade-off.Confusing “more circuitry” with “less efficient”; it is usually necessary for speed. Final Answer: Correct

Question 13

Must counters be strobed to eliminate glitches? Determine whether counters themselves typically require strobing, or whether strobing is more relevant to associated decoding/display logic.

Accepted Answer

Introduction / Context: Glitches commonly arise in combinational decoding of multi-bit changing signals, not in the storage elements themselves. This question separates the behavior of counters (sequential storage) from decoders/displays that interpret the count and may require strobing to avoid transient misreads. Given Data / Assumptions: Counters change multiple bits around a clock edge. Combinational logic observing those bits can produce hazards during transitions. Strobing masks these hazards by enabling observation only during stable intervals. Concept / Approach: The counter registers update synchronously at clock edges and, by themselves, do not require “strobing” to function correctly. It is the downstream combinational circuitry (e.g., a binary-to-7-segment decoder) that may need strobing or latching to prevent transient glitches from being displayed or acted upon. Step-by-Step Solution: Recognize that flip-flops produce clean logic-level transitions at the clock edge.Identify where glitches originate: unequal path delays in combinational decoders.Apply strobing/latching at the decoder/display interface to mask hazards.Conclude that saying counters “must be strobed” is an overgeneralization and thus incorrect. Verification / Alternative check: Observe on an oscilloscope: counter outputs switch cleanly; decoded outputs may momentarily glitch unless gated or latched. Why Other Options Are Wrong: Correct: Misattributes the need for strobing to the counter block rather than the decoder.Only for Gray-coded / Johnson counters: Glitch behavior is about decoding transitions, not about a particular counter code alone. Common Pitfalls: Assuming any multi-bit change implies the source (counter) is glitchy.Neglecting to synchronize the strobe window with stable data timing. Final Answer: Incorrect

Question 14

BCD-to-seven-segment driver 7447 features and I/O polarity: Evaluate the statement: “The 7447 IC accepts a 4-bit BCD input, provides seven individual active-LOW segment outputs for driving a common-anode display, and includes ripple blanking input (…

Accepted Answer

Introduction / Context: The 7447 is a classic BCD-to-seven-segment decoder/driver used in introductory digital electronics. Understanding its input coding, output polarity, and auxiliary control pins is essential when interfacing counters or microcontrollers to seven-segment LED displays, particularly common-anode modules that expect active-LOW segment drive signals. Given Data / Assumptions: Target device: 7447 BCD-to-seven-segment decoder/driver. Inputs: four-bit BCD representing decimal 0–9. Outputs: seven lines intended to drive segments a–g. Control pins: ripple blanking input (RBI) and ripple blanking output (RBO) for multi-digit leading-zero suppression. Concept / Approach: The 7447 translates a BCD nibble into the appropriate segment pattern. Its outputs are active-LOW, meaning a logic 0 turns a segment on when used with a common-anode display. RBI/RBO allow cascaded blanking to automatically suppress leading zeros across multiple digits, improving readability without extra logic. Step-by-Step Solution: Identify the I/O convention: BCD in (4 bits) → seven active-LOW outputs.Confirm display compatibility: active-LOW suits common-anode LED modules.Note auxiliary features: RBI accepts blanking from a more-significant digit; RBO forwards blanking to the next digit.Conclude the stated feature set matches the 7447 device. Verification / Alternative check: Examine a typical 7447 truth table and pinout: outputs labeled a–g are open-collector, active-LOW; RBI/RBO pins are present for ripple blanking chains. Laboratory tests with a counter feeding the 7447 confirm correct segment patterns and blanking behavior. Why Other Options Are Wrong: Incorrect: Conflicts with the standard 7447 functionality. Only true for the 7448 variant: 7448 is for common-cathode (active-HIGH), not the 7447. Depends on supply voltage: Polarity and features do not change with typical Vcc. Applies only to LCD drivers: 7447 is an LED display driver/decoder. Common Pitfalls: Confusing 7447 (active-LOW) with 7448 (active-HIGH); omitting required segment current limiting; forgetting that open-collector outputs may require pull-ups depending on wiring. Final Answer: Correct

Question 15

Counter taxonomy and terminology in digital electronics: Evaluate the statement: “Asynchronous counters are known as modulus counters.”

Accepted Answer

Introduction / Context: Digital counters are categorized by timing (asynchronous/ripple vs. synchronous) and by modulus (the number of unique states before repeating). Mixing these terms leads to confusion in design discussions and troubleshooting. This item clarifies the vocabulary so learners can read datasheets and schematics accurately. Given Data / Assumptions: Asynchronous (ripple) counters use the output of one flip-flop to clock the next. Synchronous counters clock all flip-flops simultaneously from a common clock. Modulus is the count length (number of distinct states) regardless of timing style. Concept / Approach: “Asynchronous” describes the way flip-flops are clocked, not how many states the counter has. “Modulus” describes how many unique counts occur before the sequence repeats and applies to any counter style. Therefore, calling asynchronous counters “modulus counters” is incorrect; all counters possess a modulus (e.g., mod-8, mod-10), whether asynchronous or synchronous. Step-by-Step Solution: Define timing style: asynchronous → ripple; synchronous → parallel clocking.Define modulus: number of states in the counting cycle.Observe independence: a counter can be asynchronous mod-16 or synchronous mod-10.Therefore, the statement that equates “asynchronous” with “modulus” is incorrect. Verification / Alternative check: Inspect textbook examples: ripple mod-8 counters and synchronous mod-10 counters exist side-by-side. Both have a modulus; only one is asynchronous. Vendor counter ICs (e.g., ripple vs synchronous decade counters) prove the distinction. Why Other Options Are Wrong: Correct: Mislabels a timing attribute as a modulus property. Only true for ring counters / Only when modulus ≥ 10 / Depends on trigger edge: These are irrelevant constraints that do not fix the misuse of terminology. Common Pitfalls: Assuming “decade” implies synchronous; forgetting that propagation delays accumulate in ripple counters; conflating count length with clock topology. Final Answer: Incorrect

Question 16

Overall modulus of cascaded decade counters: Evaluate the statement: “Three cascaded modulus-10 (decade) counters yield an overall modulus of 1000.”

Accepted Answer

Introduction / Context: Understanding how counter moduli combine is essential when building frequency dividers, timers, or decimal displays. When counters are cascaded, their overall modulus is the product of the individual moduli, provided they are chained conventionally (carry/ripple from the lower decade drives the next). Given Data / Assumptions: Three decade (mod-10) counters are cascaded. Standard ripple or synchronous cascading is used with proper carry chaining. No additional gating is introduced to alter the count length. Concept / Approach: The overall modulus M_total of cascaded counters equals the product of the individual moduli. For three mod-10 stages: M_total = 10 * 10 * 10 = 1000. This results in a divide-by-1000 frequency division and a 000 to 999 counting range for display applications. Step-by-Step Solution: Identify each stage modulus: 10.Multiply stage moduli: 10 * 10 * 10 = 1000.Interpretation: chain counts 000–999 before rolling over.Conclusion: the statement is correct. Verification / Alternative check: Observe a three-digit decimal display driven by cascaded decade counters; it cycles from 000 to 999. Frequency measurement confirms the input is divided by 1000 at the most significant stage output. Why Other Options Are Wrong: Incorrect: Contradicts the multiplicative property of cascaded moduli. Only if synchronous / Only if clock is 1 kHz / Depends on load capacitance: Timing style and frequency do not change the count length; loading affects speed, not modulus. Common Pitfalls: Miswiring carry outputs so the chain fails to propagate; adding gating that truncates counts (creating non-10 moduli) and then forgetting the overall modulus changes accordingly. Final Answer: Correct

Question 17

PISO (parallel-in/serial-out) shift register behavior: Evaluate the statement: “A parallel-in/serial-out shift register loads all data bits simultaneously and then shifts them out one bit per clock.”

Accepted Answer

Introduction / Context: Shift registers are fundamental serial/parallel converters used in digital systems for data serialization, buffering, and communication. A PISO (parallel-in/serial-out) variant is common when multiple data lines must be captured at once and transmitted over a single wire to save pins or bandwidth. Given Data / Assumptions: Device type: PISO shift register. Control: a load signal to capture parallel data, then clock pulses to shift out serially. No special features (e.g., tri-state serial outputs) are needed to define core behavior. Concept / Approach: In a PISO, asserting LOAD captures the N-bit parallel word into internal flip-flops simultaneously. Subsequent clock pulses shift data toward the serial output, emitting one bit per clock (MSB-first or LSB-first per design). This is the canonical operation and aligns with timing diagrams in datasheets for common devices. Step-by-Step Solution: Apply LOAD to sample the entire N-bit input bus in one action.Release LOAD and apply N clock pulses to shift out all bits.Confirm serial stream order as specified (e.g., MSB first).Conclude the statement accurately describes PISO operation. Verification / Alternative check: Laboratory test with a known pattern (e.g., 10110010): after parallel capture, observe the serial line emitting bits in sequence with each clock tick until all bits are transmitted. Why Other Options Are Wrong: Incorrect: Conflicts with the definition of PISO. Only true for ring counters: Ring counters are not PISO shift registers. Only true with asynchronous load: Many PISO devices provide synchronous or asynchronous load; the parallel capture principle remains. Depends on tri-state outputs: Tri-state affects bus sharing, not core PISO behavior. Common Pitfalls: Confusing PISO with SIPO (serial-in/parallel-out); misreading bit order; forgetting to latch inputs before shifting begins, which can corrupt the output stream. Final Answer: Correct

Question 18

Properties of ring counters and state utilization: Evaluate the statement: “In a ring counter, the modulus equals the number of flip-flops and, consequently, there are never any unused or invalid states.”

Accepted Answer

Introduction / Context: Ring counters and Johnson counters are specialized shift-register counters used for simple sequencing. While a properly initialized ring counter cycles through a fixed set of states, it is important to understand what happens if the counter powers up in an unintended state and whether unused or invalid states exist. Given Data / Assumptions: Ring counter: a single 1 (or 0) circulates through N flip-flops. Modulus under ideal initialization equals the number of flip-flops (N). Power-on state may be random without additional reset circuitry. Concept / Approach: Although the intended sequence has N valid states, the overall state space is 2^N. Any state that does not contain exactly one 1 (for a single-hot ring) is outside the intended sequence. Without a proper reset, the ring may lock into an invalid pattern (e.g., all zeros), which is unused and not part of the N-state loop. Therefore, the claim that there are “never” any unused or invalid states is false; design practice includes a reset to force entry into the valid cycle. Step-by-Step Solution: Define intended states: exactly one flip-flop high → N states.Compare to total possible states: 2^N overall combinations.Identify invalids: all-zero, multi-one, or other patterns are unused and can occur at power-up.Conclude that unused/invalid states exist unless reset logic constrains them. Verification / Alternative check: Simulate a 4-bit ring counter with random initialization; only 4 of the 16 states belong to the ring sequence. Others are unreachable from the valid loop without reset or corrective feedback. Why Other Options Are Wrong: Correct: Ignores the many unused states outside the single-hot set. Only true for Johnson counters: Johnson counters have 2N valid states but still have unused states overall. Only when initialized to all ones / Depends on synchronous reset: Initialization/reset affects entry into the valid loop but does not eliminate unused states from the state space. Common Pitfalls: Assuming power-on always yields a valid single-hot state; forgetting to include reset or self-correction logic; confusing ring and Johnson counter properties. Final Answer: Incorrect

Question 19

Oscilloscope triggering best practice for digital troubleshooting: In general bench work on synchronous digital systems, the oscilloscope should be triggered by which signal source to obtain a stable display?

Accepted Answer

Introduction / Context: Reliable oscilloscope triggering is key to visualizing repeatable waveforms in digital systems. Without a stable trigger source, the trace will drift, making timing relationships and logic levels difficult to evaluate. Selecting the correct trigger improves measurement quality and speeds up troubleshooting. Given Data / Assumptions: System under test is synchronous, governed by a master clock. Goal is to observe data lines, control strobes, or edges relative to the clock. Standard bench oscilloscope with external trigger capabilities is available. Concept / Approach: In synchronous systems, most signals are time-aligned to a common clock. Triggering on the system clock phase-locks the oscilloscope acquisition to that reference, producing a stable display and preserving edge relationships. This is superior to generic channel-triggering or line sync, which may not correlate with the logic timing and can obscure real behavior. Step-by-Step Solution: Connect the clock (or a derived clean clock) to the scope’s trigger input.Select edge trigger on the relevant polarity (rising or falling) with proper level.Probe signals of interest on other channels; their timing will now lock to the clock.Fine-tune holdoff if needed to stabilize bursty or multi-cycle patterns. Verification / Alternative check: Compare displays while triggering on random data vs. the clock. Only clock-based triggering consistently stabilizes periodic synchronous activity across captures, revealing setup/hold margins and timing skew. Why Other Options Are Wrong: A channel or channel 1: Arbitrary data lines may be pseudo-random and won’t lock the display. Vertical input mode: Not a trigger source; it is a front-end setting. Line sync: Aligns to mains frequency, unrelated to digital timing. Arbitrary free-running trigger: Produces unstable, drifting traces. Common Pitfalls: Triggering on noisy or jittery derived signals; neglecting proper termination; ignoring clock inversion or duty cycle that affects trigger level; forgetting to use probe bandwidth and ground lead practices to avoid ringing. Final Answer: the system clock

Question 20

Minimum flip-flop count for a modulus-5 (divide-by-5) counter: What is the least number of flip-flops required to implement a mod-5 counter?

Accepted Answer

Introduction / Context: Determining the minimum number of flip-flops helps size counters efficiently. Each flip-flop stores one bit, and the number of distinct states available is 2^n for n flip-flops. A mod-N counter must provide at least N distinct states, possibly discarding extras with gating when 2^n is not exactly N. Given Data / Assumptions: Target modulus N = 5. Flip-flops are binary storage elements; state capacity is 2^n. It is acceptable to reset or skip unused states using combinational feedback. Concept / Approach: Find the smallest n such that 2^n ≥ N. For N = 5: 2^2 = 4 (insufficient), 2^3 = 8 (sufficient). Therefore, at least three flip-flops are needed. The counter can be designed to count 0–4 and then reset to 0, effectively creating a mod-5 sequence within the 8-state space of three flip-flops. Step-by-Step Solution: Compute powers of two: 2^2 = 4 < 5; 2^3 = 8 ≥ 5.Choose n = 3 flip-flops to ensure enough states.Design gating so that upon reaching state 101 (decimal 5), the counter resets to 000.Result: a sequence of five states 000→001→010→011→100→000. Verification / Alternative check: Simulate a 3-bit synchronous counter with decode logic on state 101 to assert CLEAR. The output cycles through five states per period, confirming mod-5 behavior while using only three flip-flops. Why Other Options Are Wrong: 5, 8, 10 flip-flops: Far exceed the minimum; 2^5, 2^8, and 2^10 states are unnecessary for mod-5. 2 flip-flops: Only 4 states available, which cannot realize a mod-5 sequence. Common Pitfalls: Forgetting to add gating to truncate the 8-state space; confusing Johnson/ring counters with binary counters; neglecting propagation delays when forming synchronous reset logic (glitch avoidance may require synchronous clear or additional gating). Final Answer: 3

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