Question 1

Constructing a D-type flip-flop: “A D flip-flop is built by connecting an inverter between the Set and Clock terminals.” Evaluate the accuracy of this statement.

Accepted Answer

Introduction / Context: Understanding how flip-flops are constructed from latches (and from logic gates) helps with conversions between SR, JK, T, and D types. This item checks a common misconception about D flip-flop internals. Given Data / Assumptions: We consider a standard D-type flip-flop (edge-triggered). Claim proposes an inverter between Set and Clock pins. Standard gate-level constructions apply. Concept / Approach: A D flip-flop captures the data present at D on a clock edge and presents it at Q. A common construction is a gated SR latch where S receives D and R receives the complement of D (via an inverter), with proper gating to ensure only one of S or R is asserted at a time during the sampling window. The clock interacts with gating logic, not directly by inverting Set with Clock. Therefore, “inverter between Set and Clock” is not the defining construction. Step-by-Step Solution: Start from an SR latch: two cross-coupled NOR (or NAND) gates.Drive S with D and R with not-D through clocked gating (transmission gates or edge logic).Edge-triggering uses master–slave or pulse techniques; no inverter is placed between Set and Clock terminals.Thus the statement is incorrect. Verification / Alternative check: Datasheet logic diagrams for 74HC74/74LS74 show the D path and its inversion feeding internal set/reset gating, not a direct Set–Clock inversion connection. Why Other Options Are Wrong: Correct: Misstates the structure. Valid only for dynamic D latches / JK-to-D conversion: Conversions still rely on routing D and not-D appropriately; the proposed Set–Clock inverter is not a canonical method. Common Pitfalls: Confusing asynchronous SET/RESET pins with the synchronous data path; mixing latch gating with SR control signals. Final Answer: Incorrect

Question 2

S-R NOR latch behavior: When both S and R inputs are driven HIGH, the output of an S–R NOR latch is “unpredictable.” Choose the best evaluation.

Accepted Answer

Introduction / Context: S–R latches are the foundation of many storage elements. Knowing the “forbidden” input condition and its impact on output validity is vital when designing debouncers and state machines. Given Data / Assumptions: Latch type: S–R built from NOR gates. Inputs S and R are simultaneously driven HIGH. We consider steady-state behavior and subsequent release. Concept / Approach: For an S–R NOR latch, S = 1 and R = 1 force both NOR outputs low (Q = 0 and Q̄ = 0), which violates the complementary relationship. When the inputs are released (pulled low), internal propagation and any skew can cause either side to dominate, so the subsequent state is indeterminate. Therefore, the condition is called “forbidden” or “invalid,” often summarized as yielding an unpredictable outcome. Step-by-Step Solution: Apply S = 1, R = 1 → each NOR sees at least one HIGH → both outputs forced low.Release inputs S, R back to 0 → cross-coupled feedback resolves to either set or reset randomly depending on delays.Because the next state cannot be guaranteed, the situation is deemed unpredictable/invalid.Hence, the evaluation “unpredictable” is accepted in practice. Verification / Alternative check: Textbooks note that S = R = 1 is forbidden for NOR latches (and S = R = 0 is forbidden for NAND latches), exactly because the next state after release is indeterminate. Why Other Options Are Wrong: Incorrect: Ignores the indeterminate resolution after release. Applies only to NAND latches / only at power-up: The forbidden-state concept is general; the specific input combination differs for NAND vs NOR realizations. Common Pitfalls: Assuming unpredictability means “random during the high-high interval.” During S = R = 1 both outputs are known low; the unpredictability refers to the final state after inputs are deasserted. Final Answer: Correct

Question 3

Switch debouncing practice: “J–K flip-flops are often used as switch debouncers.” Assess the validity of this claim based on common design approaches.

Accepted Answer

Introduction / Context: Mechanical switches bounce, producing multiple transitions. Debouncing circuits ensure a single clean transition is delivered to digital logic. This question asks whether J–K flip-flops are the commonly used solution. Given Data / Assumptions: Goal: remove contact bounce from a mechanical switch. Candidate device: J–K flip-flop. Industry practice favors simple, robust solutions. Concept / Approach: Common hardware debouncers include SR latches using cross-coupled NAND/NOR gates, dedicated debounce ICs, Schmitt-trigger inputs with RC filters, and D-type flip-flops clocked from a conditioned signal. J–K flip-flops, especially level-sensitive versions, can suffer race-around when both J and K are high under long clock pulses. While it is possible to craft a JK-based debounce, it is not the typical or recommended approach compared to D latches/latches with Schmitt-trigger conditioning. Step-by-Step Solution: List mainstream methods: SR latch, RC + Schmitt, D flip-flop synchronizers.Evaluate JK suitability: potential race-around with level-sensitive clocks; increased complexity.Practicality and prevalence favor SR or D solutions.Therefore, the claim “often used” for JK is not accurate. Verification / Alternative check: Application notes and textbooks typically show SR-latch or D-FF debouncers; JK appears rarely and usually in educational examples, not as the mainstream solution. Why Other Options Are Wrong: Correct: Overstates the role of JK in debouncing. Conditional options: Constraints do not turn JK into the common choice; other solutions are simpler and more robust. Common Pitfalls: Equating “flip-flops can debounce” with “JK is most used.” The type matters; D or SR is simpler and safer. Final Answer: Incorrect

Question 4

Race-around versus invalid-state: “The J–K flip-flop eliminates the RACE problem when both J and K inputs are HIGH.” Provide the most accurate evaluation.

Accepted Answer

Introduction / Context: The J–K flip-flop improves upon the S–R by allowing J = K = 1 without an “invalid” state; however, the separate issue of race-around can still occur in level-triggered designs. This question distinguishes those concepts. Given Data / Assumptions: Inputs J and K may both be HIGH. We consider both level-triggered and edge-triggered JK implementations. Clock pulse width may be long relative to internal propagation delay. Concept / Approach: With J = K = 1, a JK flip-flop toggles its state. In a level-triggered device, a long clock gate can allow multiple toggles within a single clock pulse (race-around). This is not “eliminated” by the JK function itself. Edge-triggered or master–slave JK designs constrain the sampling window to an edge or short internal pulse, thereby avoiding race-around. Thus, the blanket claim is incorrect. Step-by-Step Solution: Recognize JK benefit: replaces SR’s invalid J = K = 1 case with toggling.Identify race-around: repeated toggling during a long level-sensitive clock.Note solution: use edge-trigger or master–slave to limit effective pulse width.Therefore, the claim that JK “eliminates race” is false for level-triggered forms. Verification / Alternative check: Textbook timing diagrams show race-around in level-triggered JK. Device datasheets for edge-triggered JK explicitly advertise “no race-around.” Why Other Options Are Wrong: Correct: Confuses “no invalid state at J = K = 1” with “no race-around.” Only true at low frequencies: A long clock pulse can still exceed internal delays even at low frequency; the issue is pulse width, not clock rate alone. Common Pitfalls: Assuming JK inherently solves all timing hazards. Triggering method matters as much as logic function. Final Answer: Incorrect

Question 5

Edge-triggered flip-flop requirements: An edge-triggered flip-flop must have which of the following characteristics?

Accepted Answer

Introduction / Context: Edge-triggered flip-flops capture data precisely at a transition of the clock. Understanding what they fundamentally require helps avoid misinterpretations of symbol details and clocking schemes. Given Data / Assumptions: The device responds at an edge (rising or falling). We compare architectural requirements versus optional implementation details. Options include claims about special generators or extra inputs. Concept / Approach: An edge-triggered flip-flop needs internal circuitry that reacts quickly to the clock’s transition and briefly opens a sampling window. This implies fast internal response relative to the edge duration so metastability windows are minimized and multiple sampling within one pulse is avoided. It does not inherently require two separate inputs for rising and falling edges, nor does it mandate an external positive- or negative-transition pulse generator; those edges are provided by the system clock, and selection of edge polarity is a device attribute. Step-by-Step Solution: Define edge-triggered: state changes only on a specific transition.Relate to internal design: short, well-controlled sampling aperture → fast response path.Eliminate non-requirements: no need for extra “edge inputs” or external transition generators as a rule.Thus, the essential requirement among the choices is fast response. Verification / Alternative check: Datasheets specify setup/hold and clock pulse characteristics, which reflect the device’s internal speed and edge-capture capability. No requirement exists for two separate inputs to handle both edge polarities. Why Other Options Are Wrong: At least two inputs: Edge polarity is inherent; devices select rising or falling edge via design, not dual inputs. Positive-/negative-transition pulse generator: External generators are not mandatory; a clean clock edge suffices. Common Pitfalls: Confusing optional external pulse-shaping circuits with inherent device requirements. Misreading symbol bubbles and triangles as extra inputs. Final Answer: Very fast response times

Question 6

Flip-flop classification: The S–R, D-type, and J–K flip-flops are all examples of which class of multivibrator?

Accepted Answer

Introduction / Context: Multivibrators are categorized by the number of stable states they possess. Recognizing where flip-flops like S–R, D, and J–K fit aids in understanding storage elements and timing circuits. Given Data / Assumptions: S–R, D, and J–K flip-flops are considered. We compare against astable, monostable, and bistable classes. Standard digital logic behavior is assumed. Concept / Approach: A bistable multivibrator has two stable states and remains in either until an input command causes a transition. This is exactly what flip-flops do: they store one bit of information (Q = 0 or Q = 1) until clocked or commanded to change. Astable multivibrators have no stable state and oscillate freely (e.g., RC oscillators); monostable multivibrators have one stable state and require a trigger to produce a single pulse returning to the stable state (one-shots). Step-by-Step Solution: Identify the number of stable states of flip-flops: two (0 and 1).Match to class definitions: two stable states → bistable.Exclude others: astable (0 stable states), monostable (1 stable state).Therefore, S–R, D, and J–K flip-flops are bistable multivibrators. Verification / Alternative check: Any flip-flop datasheet or text defines them as bistable storage elements. Timing diagrams show persistence in the current state absent a triggering event. Why Other Options Are Wrong: Astable: oscillators, not memory elements. Monostable: one-shot pulse generators, not two-state storage. Tristable: not a standard multivibrator class; “tri-state” refers to outputs with a high-impedance third state, not device stability. Common Pitfalls: Confusing “tri-state outputs” (bus drivers) with “multivibrator stability classes.” Flip-flops can have tri-stated outputs yet remain bistable devices. Final Answer: Bistable multivibrators

Question 7

Retriggerable one-shot timing in digital electronics: A monostable multivibrator (retriggerable one-shot) has nominal pulse width T = 10 ms. It is triggered at t = 0, and then a second valid trigger arrives 3 ms later (t = 3 ms) while the output is …

Accepted Answer

Introduction / Context: Retriggerable one-shots (monostables) are timing circuits that extend their output pulse whenever a new valid trigger occurs before the existing timing interval has expired. This question checks your understanding of how the output width accumulates when a second trigger occurs during the active pulse window. Given Data / Assumptions: Pulse width setting T = 10 ms. First trigger occurs at t = 0 ms. Second valid trigger occurs at t = 3 ms while output is still high. Monostable is retriggerable, meaning each new trigger restarts the timing window from that instant. Concept / Approach: In a retriggerable monostable, any trigger that arrives while the output is high resets the internal timing capacitor/threshold process so that the output remains high for a full additional interval T starting from the new trigger time. The output therefore stays high until (time of last trigger) + T. Step-by-Step Solution: Initial pulse: from t = 0 to t = 0 + 10 ms = 10 ms.A second trigger arrives at t = 3 ms while output is still high.Retriggering restarts the timing interval at t = 3 ms for another T = 10 ms.New end time = 3 ms + 10 ms = 13 ms after the original start. Verification / Alternative check: Draw a timeline and shade the high-level region. The first window would end at 10 ms, but the second trigger shifts the end to 13 ms. This matches standard timing diagrams from retriggerable one-shot IC datasheets (e.g., 74HC123 behavior). Why Other Options Are Wrong: 7 or 3: These ignore retriggering or miscalculate duration. 10: True only for non-retriggerable (one-shot) or if no second trigger occurs. 9: Arithmetic error; not aligned with retriggered timing from 3 ms. Common Pitfalls: Confusing retriggerable with non-retriggerable devices; assuming the second trigger “overlaps” without extending; or measuring from the second trigger only and forgetting the total duration from t = 0 when asked for overall pulse length. Final Answer: 13

Question 8

S–R NAND latch behavior with both inputs asserted LOW: For an S–R latch implemented with cross-coupled NAND gates, when both S and R inputs are LOW simultaneously, what is the output state?

Accepted Answer

Introduction / Context: S–R latches can be built from cross-coupled NAND or NOR gates, and the “active” level of S and R depends on the gate type. Understanding the illegal/forbidden input combination and the immediate output levels prevents design mistakes when using basic memory elements or debouncing circuits. Given Data / Assumptions: S–R latch constructed from two NAND gates (active-LOW inputs). Both inputs are driven LOW at the same time (S = 0, R = 0). Outputs are the usual Q and Q̅. Concept / Approach: For a NAND gate, any input at 0 forces the output to 1 regardless of the other input. With both S and R LOW simultaneously, each NAND gate sees a 0 at one input, so both outputs are forced HIGH. This is the “forbidden” (invalid) state because Q and Q̅ become 1 simultaneously, violating the requirement that they be complements. Upon releasing S and R back to 1, the latch may settle unpredictably. However, while S = 0 and R = 0 are applied, each output level is HIGH. Step-by-Step Solution: Set S = 0 → top NAND output goes HIGH regardless of feedback.Set R = 0 → bottom NAND output also goes HIGH.Thus Q = 1 and Q̅ = 1 concurrently (invalid but both HIGH).Therefore, the immediate output level under both inputs LOW is HIGH at both nodes. Verification / Alternative check: Write the NAND truth rule: Y = NOT(A * B). With A = 0, Y = NOT(0) = 1 regardless of B. Symmetry on both gates confirms both outputs are 1 when S = R = 0. Why Other Options Are Wrong: LOW: Contradicts NAND forcing behavior with a 0 input. Unpredictable/Floating/Metastable: These may describe the state after releasing inputs, but the instantaneous enforced level with both inputs LOW is HIGH at both outputs. Common Pitfalls: Mixing NAND and NOR latch conventions; forgetting that the “forbidden” term refers to the logical validity and subsequent uncertainty, not the immediate gate-level output when both inputs are asserted. Final Answer: HIGH

Question 9

Terminology in flip-flop triggering: If an input responds specifically to a signal transition (rising or falling), that input is described as __________.

Accepted Answer

Introduction / Context: In sequential logic, the way a storage element responds to its control input critically affects timing analyses and race avoidance. Designers must distinguish between edge-triggered behavior, which samples on transitions, and level-sensitive behavior, which responds while an enable level is present. Given Data / Assumptions: We consider standard digital storage elements such as D, T, J–K, and S–R flip-flops/latches. “Signal transition” refers to a change in logic level, e.g., low-to-high (rising edge) or high-to-low (falling edge). Concept / Approach: An input that causes action only at the moment of a transition is termed edge-triggered. Typical notations include a triangle on the clock pin for edge-triggered flip-flops and an added bubble for negative-edge triggering. In contrast, “level-triggered” or pulse-triggered devices respond for the duration of an enable level or pulse width, not just at a boundary instant. Step-by-Step Solution: Identify the defining clue: response to a transition.Map “transition” to “edge,” i.e., rising or falling edge.Therefore, the appropriate term is “edge-triggered.” Verification / Alternative check: Datasheets and timing diagrams show edge-triggered devices sampling inputs at clock edges (tSU/tH constraints), while level-sensitive devices have transparent intervals; this confirms the terminology. Why Other Options Are Wrong: Clock-triggered: Too broad—could be level- or edge-sensitive. Toggle-triggered: Refers to T flip-flop behavior when T=1, not generalized transition sensitivity. Noise-triggered: Undesirable and not a design feature. Level-enabled: Opposite of edge-triggered; responds to level duration. Common Pitfalls: Confusing “pulse-triggered” (responds while pulse is active) with “edge-triggered” (responds only at the boundary). Always inspect the clock symbol and timing specs in the datasheet. Final Answer: edge-triggered

Question 10

Master–slave J–K flip-flop toggle condition: When J = K = 1, Q and Q̅ will switch to their __________ state(s) at the _____________________. Choose the most accurate completion.

Accepted Answer

Introduction / Context: The J–K flip-flop generalizes the S–R flip-flop and avoids its forbidden state. One hallmark is the “toggle” condition when both J and K are asserted. Accurately stating when Q toggles is essential to prevent race-around issues and to interpret timing diagrams correctly, especially in master–slave organizations. Given Data / Assumptions: Master–slave J–K flip-flop. J = K = 1 (toggle condition). “Active clock edge” refers to whichever edge the device is specified to accept (rising or falling). Concept / Approach: With J = K = 1, the J–K flip-flop complements its present state at each active clock edge. In a master–slave configuration, the master captures on one phase and the slave updates on the opposite phase, but from a black-box perspective the external output Q toggles at the specified active edge. Thus, Q and Q̅ move to their opposite states synchronously with that edge. Step-by-Step Solution: Set J = K = 1 → toggle mode enabled.At the active clock edge, internal transfer causes Q := Q̅ (complement).Therefore, Q and Q̅ switch to opposite states at the active edge. Verification / Alternative check: Examine vendor timing diagrams for classic master–slave J–K devices; they show Q updating to the complement at each valid edge when J=K=1, confirming the statement. Why Other Options Are Wrong: Inverted, positive clock edge: Overly specific; also “inverted” is ambiguous versus “opposite.” Quiescent, negative clock edge: “Quiescent” is incorrect under toggling; edge polarity is device dependent. Reset, synchronous clock edge: Reset is not implied by J = K = 1. Unchanged, trailing edge only: Contradicts toggle behavior. Common Pitfalls: Assuming toggle occurs throughout a high level (race-around). True edge-triggered or master–slave designs confine the update to the clock edge, preventing race-around when designed correctly. Final Answer: opposite, active clock edge

Question 11

Ways to actuate an S–R flip-flop: An S–R flip-flop can be designed to respond to which general types of input specifications?

Accepted Answer

Introduction / Context: S–R flip-flops (and their latch counterparts) are implemented across families with differing control conventions. Some are level-sensitive, some edge-triggered, and some are specified by required pulse widths. Recognizing these broad categories helps when reading datasheets and integrating devices into synchronous systems. Given Data / Assumptions: We consider the generic S–R function independent of a specific logic family. Vendors may provide devices that react on levels (latches), edges (flip-flops), or pulses (pulse-triggered variants). Concept / Approach: The S–R mechanism is fundamentally a set/reset storage action. Depending on implementation, it can be activated by level inputs (transparent latch behavior), by clock edges (edge-triggered flip-flops), or by meeting a minimum pulse width (pulse-triggered devices). Therefore, the most comprehensive and accurate generalization is that S–R devices can be actuated by edges, levels, or pulses according to design type and timing specification. Step-by-Step Solution: Identify common actuation modes: level-sensitive latches, edge-triggered FFs, and pulse-triggered FFs.Map these modes to the wording: edges, levels, pulses.Confirm this covers mainstream device behaviors found in datasheets. Verification / Alternative check: Examine timing sections of vendor documentation: level-sensitive “transparent” latches, edge-triggered S–R flip-flops, and minimum pulse-width (tW) specs for pulse-triggered devices are all commonplace, verifying the inclusive answer. Why Other Options Are Wrong: Combinations with PRESET/CLEAR/HIGH/LOW: Mix control pin names with logic levels; they do not categorize the triggering mode itself. Tri-state/wired-OR/open-drain: These describe output structures, not triggering categories. Common Pitfalls: Confusing “PRESET/CLEAR” (asynchronous controls) with primary set/reset inputs; or assuming a single universal triggering style across all S–R devices. Final Answer: edges, levels, pulses

Question 12

Flip-flop triggering style nomenclature: Pulse-triggered flip-flops are also commonly referred to as __________ flip-flops in standard digital design texts.

Accepted Answer

Introduction / Context: Digital storage elements may be categorized by how their control inputs cause state changes. Pulse-triggered devices respond over the duration of an enable (or clock) pulse, which corresponds to level sensitivity during that interval. This contrasts with edge-triggered devices that respond only at the instant of a transition. Given Data / Assumptions: “Pulse-triggered” refers to devices that are transparent or responsive while a specified control level is present. We are using common textbook terminology. Concept / Approach: Pulse-triggered flip-flops are functionally equivalent to level-sensitive devices because the effective action occurs while the control signal is at an active level (high-level or low-level, depending on design). Master–slave organization is one technique to mitigate race-around by using two level-sensitive stages in sequence; however, the term “pulse-triggered” itself maps to “level-sensitive,” not “edge-triggered.” Step-by-Step Solution: Identify the behavior: responds during a time window (the pulse).A window response equals level sensitivity during the active level.Therefore, the conventional label is “level” flip-flop (level-sensitive). Verification / Alternative check: Timing diagrams show that state may change for any input variation occurring while the enable level is active. Edge-triggered devices, by contrast, sample at a single instant, confirming the distinction and the naming convention. Why Other Options Are Wrong: Edge: Opposite concept; acts at an instant. Master-slave: An implementation technique, not a trigger style name. Postponed/metastable: Not standard trigger-type nomenclature. Common Pitfalls: Equating “pulse” with “edge”; forgetting that a pulse has duration that allows transparency/race without proper design precautions. Final Answer: level

Question 13

Typical application of an S–R flip-flop: Select the most appropriate use-case that directly leverages the S–R flip-flop’s set/reset storage capability.

Accepted Answer

Introduction / Context: The S–R flip-flop is a fundamental one-bit memory element. It stores a binary state until explicitly changed by set or reset inputs. Understanding its canonical use helps distinguish it from oscillators and pulse generators that require feedback networks and timing components rather than simple set/reset control. Given Data / Assumptions: S–R flip-flop offers two stable states: Q=1 (set) and Q=0 (reset). No requirement for continuous toggling or timing components. We seek a direct, primary application. Concept / Approach: A binary storage register is built by combining multiple one-bit storage elements (flip-flops). Each S–R element can hold one bit reliably, making it a natural building block for registers, flags, and simple control latches. While S–R elements can be part of more complex timing circuits, their core role is storage, not oscillation or pulse shaping. Step-by-Step Solution: Identify S–R capability: explicit set and reset result in stable, latched states.Map this to an application: storing a single bit within a register.Extend to n-bit registers by ganging multiple flip-flops. Verification / Alternative check: Textbook block diagrams for registers depict banks of flip-flops; asynchronous set/reset controls are commonly used for initialization, validating the use-case. Why Other Options Are Wrong: Astable oscillator: Requires continuous feedback and timing components; not a simple S–R latch function. Transition pulse generator: Typically uses differentiating networks/monostables. Racer: Not a standard application; “race” usually refers to an undesirable timing condition. Frequency-to-voltage converter: Analog function unrelated to simple set/reset storage. Common Pitfalls: Assuming any two-state device is an oscillator; oscillation needs periodic feedback conditions, while a flip-flop stores state until commanded to change. Final Answer: binary storage register

Question 14

Input levels required to control an S–R flip-flop: For a conventional S–R flip-flop (active-HIGH control inputs), what input level is required on the respective S or R pin to SET or RESET the device?

Accepted Answer

Introduction / Context: Flip-flop control inputs can be active-HIGH or active-LOW depending on family and symbol notation. Being able to read symbols and determine which logic level asserts SET or RESET is essential when wiring control lines for initialization and control in synchronous systems. Given Data / Assumptions: We consider the common active-HIGH S–R flip-flop interface (e.g., edge-triggered or level-sensitive with S and R bubbles absent). Active-HIGH means a logic HIGH at S asserts SET; a logic HIGH at R asserts RESET. Concept / Approach: For active-HIGH S–R flip-flops, the functional inputs S and R respond to a HIGH assertion. An asserted S drives Q to 1; an asserted R drives Q to 0. This is distinct from S–R latches made from NAND gates, which use active-LOW inputs, but the question explicitly targets the conventional active-HIGH case, as is typical for symbol sets without inversion bubbles. Step-by-Step Solution: Check symbol: no inversion bubbles → active-HIGH control.To SET: drive S to logic HIGH.To RESET: drive R to logic HIGH.Hence, the required asserting level is HIGH. Verification / Alternative check: Review datasheets: pins labeled /S or /R (with slashes/bubbles) indicate active-LOW; plain S and R (no bubbles) indicate active-HIGH. This symbol convention confirms the answer when “conventional active-HIGH” is assumed. Why Other Options Are Wrong: LOW: Would be true only for active-LOW (bubbled) S–R inputs. Steering diodes / limiting resistor / tri-stated: These are wiring details or output conditions, not the necessary logic levels to assert the function. Common Pitfalls: Forgetting to read inversion bubbles on logic symbols; mixing NAND-latch conventions (active-LOW) with generalized flip-flop pin behavior (often active-HIGH). Final Answer: HIGH

Flip-Flops and Timers Questions