Question 1

Fixed regulators by number For a standard 78xx series linear regulator, what regulated DC output does a 7814 provide under proper operating conditions?

Accepted Answer

Introduction / Context: The 78xx family are positive fixed-output linear regulators used in countless power supplies. The “xx” suffix indicates the nominal output voltage. Knowing these at a glance speeds up design and troubleshooting when scanning a board or a parts bin. Given Data / Assumptions: Device is a 7814 from the 78xx family. Proper input voltage headroom and heat sinking are assumed. Standard pinout and positive regulation behavior apply. Concept / Approach: For 78xx regulators, “xx” corresponds to the output voltage in volts. Examples: 7805 → +5 V, 7812 → +12 V, 7815 → +15 V. Therefore, 7814 provides a regulated +14 V DC output. The companion negative series are 79xx (e.g., 7912 for –12 V), not to be confused with 78xx. Step-by-Step Solution: Identify series: 78xx → positive regulator.Read suffix: “14” → 14 volts nominal.Conclude output: +14 V DC (with tolerance per datasheet). Verification / Alternative check: Datasheets from multiple vendors list VO = +14 V typical for 7814, with dropout and line/load regulation specs ensuring stable +14 V when input exceeds VO + dropout. Why Other Options Are Wrong: –14 V dc: negative outputs belong to 79xx series. ±8 V options: wrong suffix and wrong polarity for 78xx family. Common Pitfalls: Confusing 7815 (15 V) with 7814 (14 V). Ignoring dropout; if input is too low, regulation will be lost. Final Answer: +14 V dc

Question 2

Identifying optocouplers (opto-isolators) True or False: An optocoupler is an IC package that contains an LED and a zener diode.

Accepted Answer

Introduction / Context: Optocouplers (also called opto-isolators) provide galvanic isolation between an input circuit and an output circuit using light, improving safety and noise immunity. Knowing their internal composition helps you select the correct device and avoid confusing them with unrelated protection components like zener diodes. Given Data / Assumptions: Typical optocoupler packages include a light emitter and a light sensor. Common pairings: LED + phototransistor, LED + photodiode, LED + phototriac, or LED + photodarlington. No zener breakdown element is part of the essential optocoupler function. Concept / Approach: The essence of optical isolation is that an LED on the input side converts electrical current into light, which traverses an internal gap and is detected on the output side by a photosensitive device. This optical path transfers information while maintaining electrical isolation (high isolation voltage). A zener diode is a voltage-clamping device; it is not needed for optical coupling and would not perform the required photodetection. Step-by-Step Solution: Recall standard optocoupler internals: LED + photo device.Recognize that a zener diode is not light sensitive.Therefore, the statement “LED and a zener diode” is false. Verification / Alternative check: Part examples: 4N25 (LED + phototransistor), PC817 (LED + phototransistor), MOC3021 (LED + phototriac). Datasheets confirm no zener is used for the coupling mechanism. Why Other Options Are Wrong: “True” would misleadingly equate zener devices with photodetectors. While some ICs may add protection diodes, the optocoupling action is not achieved with a zener. Common Pitfalls: Confusing input protection (which might use zeners elsewhere on a board) with opto-isolation internals. Assuming any diode paired with an LED forms an optocoupler; it must be a photo-responsive element. Final Answer: False

Question 3

Flip-flop timing requirement — setup time What does the setup time specification of a flip-flop define in synchronous digital design?

Accepted Answer

Introduction / Context: Setup time is a fundamental timing parameter for edge-triggered flip-flops and registers in synchronous systems. Violating setup time can cause metastability or incorrect data capture, leading to logic malfunction that is often intermittent and difficult to debug. Given Data / Assumptions: We consider a clocked flip-flop capturing input D at a specified clock edge. Other timing parameters (hold time, clock-to-Q delay) exist but are separate. “Control levels” or “data levels” refers to the logic inputs to be captured. Concept / Approach: Setup time (tsu) is the minimum interval before the active clock edge during which the input data must remain stable to ensure the internal latching nodes settle properly. If data transitions too close to the clock edge (less than tsu), the flip-flop may capture the wrong value or enter metastability. Step-by-Step Solution: Identify edge: positive or negative clock edge that triggers capture.Apply rule: input must be stable for at least tsu before that edge.Ensure timing paths meet tsu by budgeting clock period ≥ propagation + skew + tsu. Verification / Alternative check: Static timing analysis tools report worst-case arrival times; meeting tsu across all paths validates reliable operation. Lab testing may reveal that reducing clock period below the STA limit introduces sporadic errors, a hallmark of setup violations. Why Other Options Are Wrong: (b) Describes neither a standard parameter nor the essence of setup; it is muddled with output behavior. (c) Irrelevant to timing; “operator” and “power level” are unrelated. (d) This is clock-to-Q propagation delay, not setup time. Common Pitfalls: Confusing setup with hold time; hold is the stability after the edge, setup is before. Ignoring clock skew and jitter, which effectively increase timing requirements. Final Answer: the minimum time the control levels need to be maintained on the inputs prior to the triggering edge of the clock in order to be reliably clocked into the flip-flop

Question 4

Power-on conditioning of flip-flops Can an automatic RC (power-on) network be arranged to set a flip-flop at power-up instead of resetting it?

Accepted Answer

Introduction / Context: Automatic RC power-on networks are frequently used to force a known state in flip-flops during system start-up. Designers often default to generating an asynchronous reset pulse, but in some applications the required initial state is a logical 1 (set). Understanding how to wire the RC network to achieve set instead of reset is practical for reliable initialization. Given Data / Assumptions: Flip-flop has asynchronous SET and RESET inputs (often active-LOW). Power supply rises from 0 V to its nominal value. An RC differentiator/integrator creates a momentary pulse at power-up. Concept / Approach: The RC network can be connected to whichever asynchronous input establishes the desired state. If the flip-flop provides both preset (SET) and clear (RESET) inputs, connecting the RC pulse to the active-input that forces Q = 1 will “set” the device on power-up. In devices with only a reset, a small logic inverter or transistor stage can invert the pulse polarity and route it to the set input of an equivalent latch/register stage. Step-by-Step Solution: Determine which asynchronous input forces Q = 1 (SET or PRE).Design RC so that, upon Vcc ramp, the active level appears briefly on that pin.Include series resistors and clamp diodes if necessary to meet input current/voltage limits.Verify pulse width exceeds device minimum pulse-width requirements. Verification / Alternative check: Scope the asynchronous input at power-up to confirm a clean pulse of correct polarity and duration. Confirm via observation that the output Q starts at 1 consistently across temperature and supply ramp rates. Why Other Options Are Wrong: “No” would imply RC start-up networks always reset; in reality, they can be wired to either asynchronous input, or inverted, to achieve a set condition. Common Pitfalls: Pulse too narrow for the datasheet minimum asynchronous set time. Forgetting pull-down/up to define a DC state after the pulse ends. Driving beyond absolute maximum ratings during the supply ramp. Final Answer: yes

Question 5

TTL JK flip-flop (74LS76) datasheet practice: What is the required setup time (ts) for valid J/K data relative to the active clock edge on a 74LS76 device?

Accepted Answer

Introduction / Context: Setup time (ts) is a fundamental timing parameter for flip-flops. It specifies how long the input data must be stable before the active clock edge to ensure the internal latches sample correctly. For TTL JK devices such as the 74LS76, honoring ts prevents metastability and incorrect toggling. Given Data / Assumptions: Target device: 74LS76 (JK flip-flop, TTL, Low-Power Schottky family). We assume standard TTL supply (5 V) and typical datasheet conditions. We need the recommended setup time ts for J or K relative to the active clock edge. Concept / Approach: Setup time must be met so that the input logic levels are reliably captured by the clocked storage elements. Violating ts can lead to unpredictable outputs or oscillations at Q. Datasheets provide a minimum ts that designers should meet or exceed in timing closure analysis. Step-by-Step Solution: Identify timing parameter: ts (data stable before clock edge).Consult typical LS-family values for JK flip-flops like 74LS76.Adopt the commonly specified requirement: ts ≈ 20 ns for reliable operation. Verification / Alternative check: Designers often include margin beyond ts to account for temperature, voltage, and process variations, ensuring robust timing even under worst-case conditions. Why Other Options Are Wrong: 5 ns, 10 ns: unrealistically short for LS TTL JK timing at 5 V.40 ns: overly conservative compared to typical datasheet requirements.15 ns: closer, but still below the common 20 ns figure used for safe design. Common Pitfalls: Confusing setup with hold time; both must be met. Also, assuming the same ts across all logic families can cause violations; always check the actual part’s datasheet. Final Answer: 20 ns

Question 6

Board-level best practice: A small 0.01 µF capacitor is typically recommended by TTL manufacturers for what purpose on the power rails near each IC?

Accepted Answer

Introduction / Context: Every digital IC switches current in short bursts, creating supply transients. Local bypass capacitors (for example 0.01 µF or 0.1 µF ceramics) provide a nearby charge reservoir to keep VCC stable at the IC pins. This practice is especially common in TTL-era designs and remains standard in modern digital boards. Given Data / Assumptions: TTL logic, 5 V supply. Capacitor value: approximately 0.01 µF per IC (sometimes used in combination with 0.1 µF). Goal: suppress local supply noise and prevent ground bounce. Concept / Approach: Decoupling (also called bypassing) places a small capacitor between VCC and GND as close as possible to the IC power pins. The capacitor provides a low-impedance path for high-frequency transients, mitigating voltage dips and ringing caused by fast edges and package lead inductance. Step-by-Step Solution: Recognize the role: stabilize the local supply by sourcing transient current.Choose correct term: “decoupling” is the standard terminology.Implement near each IC, short traces to minimize ESL/ESR. Verification / Alternative check: Oscilloscope measurements of VCC near the IC with and without bypassing clearly show reduced noise and fewer spurious resets or timing errors when decoupling is present. Why Other Options Are Wrong: Filtering is a broader term; here the specific function is local supply decoupling.Rectifying and grounding are unrelated to small ceramic capacitors across VCC-GND.Timing integration refers to RC time constants, not supply stabilization. Common Pitfalls: Placing capacitors far from the IC or using long vias undermines the decoupling effect due to added inductance. Final Answer: decoupling

Question 7

Digital timing integrity: Why might a designer insert a dedicated delay gate (buffer chain) in a synchronous digital circuit?

Accepted Answer

Introduction / Context: In synchronous logic, data must satisfy both setup and hold constraints relative to the clock at flip-flop inputs. When propagation along a path is too fast or too slow, designers sometimes insert intentional delay (e.g., buffers) to satisfy these constraints without changing functionality. Given Data / Assumptions: Synchronous registers driven by a common clock. Paths with either excessive or insufficient delay exist. Goal: meet both ts (before edge) and th (after edge). Concept / Approach: Setup time requires sufficient path delay so data arrives and settles before the capturing clock edge. Hold time requires a minimum delay so data does not change too soon after the clock edge. A delay gate increases propagation time, helping meet hold. Buffering can also help align skews to meet setup on marginal paths. Step-by-Step Solution: Evaluate the failing path: identify whether setup or hold is violated.Add delay where needed to satisfy th (common use) and adjust path balancing to meet ts.Reverify timing across PVT corners. Verification / Alternative check: Static timing analysis reports before/after buffer insertion demonstrate positive slack for both ts and th post-fix. Why Other Options Are Wrong: “Never needed” is incorrect; hold violations routinely require added delay.Only ts or only th is incomplete; practical fixes often must satisfy both constraints. Common Pitfalls: Fixing setup at the expense of creating new hold violations, or ignoring clock skew that shifts apparent margins. Final Answer: to provide for setup times and hold times

Question 8

60 Hz TTL-level clock conditioning: Why include a Schmitt trigger stage when building a 60 Hz clock pulse generator from a noisy or slow-edged source?

Accepted Answer

Introduction / Context: Schmitt triggers are input conditioning circuits with hysteresis. They transform slow, noisy, or sinusoidal inputs into clean digital pulses. For mains-derived 60 Hz timing or RC-derived clocks, edge shaping ensures reliable triggering of subsequent TTL logic. Given Data / Assumptions: Input may have slow rise/fall or noise around switching thresholds. TTL devices require fast, monotonic edges to avoid multiple transitions. Goal: stable, crisp square pulses. Concept / Approach: Hysteresis creates distinct Vt+ and Vt− thresholds. The Schmitt trigger ignores small excursions around the threshold and outputs rail-to-rail transitions with steep edges. This minimizes false triggers and reduces jitter introduced by input noise. Step-by-Step Solution: Feed the slow/noisy 60 Hz signal into a Schmitt trigger input.The device outputs a clean digital pulse with fast edges.Drive TTL counters or flip-flops reliably from the conditioned waveform. Verification / Alternative check: Oscilloscope comparison shows ragged, threshold-chattering edges without the Schmitt stage versus square, clean transitions with it. Why Other Options Are Wrong: Triangle, sine, and sawtooth shapes are analog waveforms not provided by Schmitt triggers.“Rounded pulse” contradicts the goal of sharp edges. Common Pitfalls: Driving TTL directly from a slow edge can cause multiple toggles and excessive power due to cross-conduction in downstream gates. Final Answer: to provide a sharp pulse waveform

Question 9

JK flip-flop 7476 timing comparison: Are the LOW-to-HIGH and HIGH-to-LOW propagation delays (clock to output) the same, and if not, what typical values apply?

Accepted Answer

Introduction / Context: Propagation delay in TTL devices often differs for rising and falling transitions due to internal transistor structures and output stages (e.g., totem-pole behavior). The 7476 JK flip-flop typically has different delays for LOW-to-HIGH (tPLH) and HIGH-to-LOW (tPHL) transitions. Given Data / Assumptions: Device: 7476 JK flip-flop. We compare clock-to-Q delays for both edges. Representative typical values are sought. Concept / Approach: Datasheets list separate tPLH and tPHL because device paths and transistor saturation differ per transition. Designers must consider the larger of the two when budgeting timing to avoid overestimating maximum system frequency. Step-by-Step Solution: Identify both delays: tPLH (Q rising), tPHL (Q falling).Typical 7476 values show tPHL > tPLH.Adopt tPLH ≈ 25 ns and tPHL ≈ 40 ns as representative. Verification / Alternative check: Cross-check multiple vendor datasheets; while absolute numbers vary, the asymmetry (falling often slower) remains consistent. Why Other Options Are Wrong: “Yes” and “identical” contradict typical TTL behavior.Swapped numbers or alternative pairings don’t reflect common 7476 figures. Common Pitfalls: Using only a single “tpd” value in timing closure; always use the worst case between the two directions for safe margins. Final Answer: no, tPLH = 25 ns, tPHL = 40 ns

Question 10

Power-on reset (RC network): In an automatic reset circuit for a flip-flop, approximately how long does it take an RC network to charge essentially to its final value?

Accepted Answer

Introduction / Context: Power-on reset circuits commonly use an RC network to generate a brief reset pulse as the supply ramps. Understanding exponential charging helps estimate how long the reset signal will take to deassert when the capacitor reaches near its final voltage. Given Data / Assumptions: Simple first-order RC charging. Capacitor voltage follows V(t) = Vfinal * (1 − e^(−t/RC)). “Essentially charged” means within about 1% of final value in practical design. Concept / Approach: Rule of thumb: after 5*RC, the capacitor reaches about 99.3% of Vfinal. This is sufficient for most digital thresholds to declare the reset expired and allow normal operation. Step-by-Step Solution: At t = 1 RC → ~63% charged; too low for many thresholds.At t = 2 RC → ~86.5% charged; still not close to final.At t = 5 RC → ~99.3% charged; generally considered “fully charged.” Verification / Alternative check: Compute Verror = e^(−t/RC). For t = 5 RC, Verror ≈ 0.0067; small enough to treat as fully charged in most digital contexts. Why Other Options Are Wrong: 1 or 2 RC leave significant residual error; 10 RC is more than needed in typical designs. Common Pitfalls: Ignoring tolerance of R and C and temperature drift; engineers should design reset margins accordingly. Final Answer: 5 time constants (RC)

Question 11

Flip-flop power-up behavior: What is the normal starting state of a settable flip-flop when power is first applied to a circuit?

Accepted Answer

Introduction / Context: Digital systems require a known state at power-up. Although raw flip-flops can power up unpredictably, practical designs add a power-on reset network so that key registers begin in a defined condition. The question references a “settable” flip-flop commonly integrated into systems with a reset path. Given Data / Assumptions: A reset signal is provided via power-on reset (POR) or supervisory IC. Goal: ensure deterministic initial conditions. Conventional practice prefers starting from reset (Q = 0) unless specified otherwise. Concept / Approach: Because raw silicon may come up in either state, designers impose a reset to force the “reset” state as the normal starting point. This ensures counters, state machines, and control logic do not begin in an unknown or illegal state. Step-by-Step Solution: Add RC or dedicated POR IC to assert reset on power rise.Hold reset long enough (e.g., several RC time constants) to stabilize supplies and clocks.Release reset, guaranteeing a defined initial state. Verification / Alternative check: Board bring-up confirms stable initial conditions when reset is enforced, versus random states if reset is absent. Why Other Options Are Wrong: Set/toggle/dual are not standard default power-up choices; without reset, the state is indeterminate.Indeterminate is true for a bare flip-flop, but standard design practice uses reset, so “reset” is the normal starting state. Common Pitfalls: Relying on uninitialized FF power-up; behavior can vary with process and temperature. Final Answer: reset

Question 12

Timing definitions: What is the correct distinction between setup time and hold time for clocked storage elements?

Accepted Answer

Introduction / Context: Setup and hold times are the two primary timing constraints for reliable sampling in synchronous circuits. Violating either can cause metastability or incorrect data capture. Understanding which side of the clock edge each applies to is essential for timing closure. Given Data / Assumptions: Flip-flops sample data on a specified active edge (rising or falling). Data must be stable before and after this edge for defined durations. Clock and data skew may erode these margins. Concept / Approach: Setup time (ts) is the interval data must be valid before the clock edge. Hold time (th) is the interval after the clock edge during which data must remain stable. The combination ensures the internal latching window sees valid logic levels during the sensitive period. Step-by-Step Solution: Identify the active edge.Ensure data is valid ts before the edge.Keep data unchanged for at least th after the edge. Verification / Alternative check: Static timing analysis computes slack for both constraints; oscilloscopes can validate on hardware by measuring timing between data transitions and clock edges. Why Other Options Are Wrong: Options A and C reverse or conflate definitions.Interchangeable or asynchronous claims are incorrect; both are synchronous input requirements. Common Pitfalls: Fixing setup issues but introducing hold violations by over-optimizing path delays or clock skews. Final Answer: Setup time occurs before the active clock edge, hold time occurs after the active clock edge.

Question 13

Flip-flop speed limiting factor: Which circuit parameter most commonly limits the maximum operating frequency (toggle rate) of a flip-flop?

Accepted Answer

Introduction / Context: Although multiple timing parameters influence synchronous performance, the intrinsic propagation delay through the flip-flop and its output stage is usually the dominant term that bounds maximum clock frequency. Designers often estimate fmax ≈ 1 / (tPLH + tPHL) for a simple toggle path, subject to additional path and setup/hold constraints. Given Data / Assumptions: Clock-to-Q delays tPLH and tPHL are known. Setup/hold are met by design (positive slack). Clock duty cycle meets minimum high/low times. Concept / Approach: Propagation delay time directly affects how quickly the output can reflect a new state and be recaptured in the next cycle. While setup/hold, duty cycle, and edge rate matter, the core ceiling for toggling is typically determined by the sum of internal propagation delays over a cycle. Step-by-Step Solution: Identify worst-case tpd among tPLH and tPHL.Calculate approximate fmax bound using total delay per cycle.Verify auxiliary constraints (setup/hold, pulse widths) are satisfied. Verification / Alternative check: Datasheets often specify maximum toggle frequency or minimum clock period directly; this value aligns with propagation delays and additional internal constraints. Why Other Options Are Wrong: Setup/hold and pulse-width limits matter but are typically designed to meet margins; tpd remains the core limiting factor.Clock transition time influences jitter and metastability risk but is not usually the top limit. Common Pitfalls: Ignoring asymmetry (tPHL ≠ tPLH) and using only one delay number can overestimate fmax. Final Answer: propagation delay time

Question 14

TTL JK flip-flop 7476 timing: Is the clock-to-output propagation delay the same as the asynchronous set/reset to output delay?

Accepted Answer

Introduction / Context: Flip-flops often provide both clocked inputs and asynchronous set/reset inputs. Internal signal paths for these controls differ, so their propagation delays to Q are not the same. This is true in classic TTL parts like the 7476 JK flip-flop. Given Data / Assumptions: Clocked path: master–slave stages with defined tPLH/tPHL. Async path: direct set/reset transistors that bypass normal clocked latching. Different internal paths imply different timing. Concept / Approach: Asynchronous controls are designed for immediate override of state, often with shorter or otherwise different delays than clocked transitions. Designers must consult separate datasheet specs for clock-to-Q and set/reset-to-Q to ensure timing is honored for both control methods. Step-by-Step Solution: Identify two independent timing specs: tpd(clock→Q) and tpd(S/R→Q).Note that the async path bypasses the clocked datapath.Conclude they are not equal and must be checked separately. Verification / Alternative check: Datasheets list separate numeric ranges for these delays; bench tests confirm different response times to clock versus set/reset. Why Other Options Are Wrong: “Yes” and conditional equality statements overlook distinct internal transistor networks. Common Pitfalls: Treating async set/reset like synchronous inputs in timing analysis; they can create race conditions if not properly synchronized to the clock domain. Final Answer: no

Question 15

Decoupling capacitors: In digital electronics PCB design, should each IC's decoupling capacitor be connected between that same device's VCC (power pin) and its own local ground near the pins, or should it be tied from the VCC of one device to the gr…

Accepted Answer

Introduction / Context: Decoupling (bypass) capacitors are critical in digital electronics to provide a low-impedance local energy reservoir and to control high-frequency noise near an integrated circuit. This question checks correct placement of decoupling capacitors relative to each device's power and ground pins. Given Data / Assumptions: ICs draw fast transient currents when internal gates switch. Power distribution networks have nonzero inductance and resistance. A small ceramic capacitor (for example, 0.1 µF) is typically used per IC. Concept / Approach: For effective high-frequency decoupling, the loop area formed by the capacitor, the IC's VCC pin, and the IC's ground return must be minimized. The capacitor must be placed physically close to the same device's pins so that current flows in a very short loop, reducing inductive impedance and voltage droop (delta V = L * di/dt). Step-by-Step Solution: 1) Each IC needs its own local bypass path to ground to handle its switching current spikes.2) Connecting a capacitor from one device's VCC to a different device's ground increases the loop length and inserts shared return paths, which raises inductance and noise coupling.3) The correct practice is to connect the capacitor directly between the same IC's VCC and local ground pins, with the shortest traces possible.4) On multilayer boards, vias to solid planes are fine, but the local loop should remain tight around the IC. Verification / Alternative check: Examine supply ringing on an oscilloscope: with proper local decoupling, high-frequency droop and ground bounce reduce markedly compared to cross-device connections. Why Other Options Are Wrong: True: Incorrect—cross-connecting decouplers increases loop area and noise. Only on multilayer boards: Layer count does not justify cross-device decoupling. Only if star ground is far: Star points are system-level; local HF loops still must be minimized. Depends on capacitor value: Placement and loop inductance dominate HF behavior more than value alone. Common Pitfalls: Placing capacitors far from pins, sharing one capacitor across several ICs, and routing long serpentine traces to the capacitor reduce effectiveness. Final Answer: False — the decoupling capacitor should be local to the same IC, between its VCC and its ground.

Question 16

Pull-up resistor purpose: In digital logic, a pull-up keeps a normally floating (high-impedance) input at a defined logic level. Which levels correctly describe this behavior?

Accepted Answer

Introduction / Context: Floating inputs can wander unpredictably due to leakage and noise. A pull-up resistor biases a high-impedance input to a known logic state when no active driver is present. Understanding this is foundational for reliable digital design. Given Data / Assumptions: Input can be undriven (tri-stated, open-collector/drain, or disconnected). Pull-up connects the node to VCC through a resistor. Active devices may still pull the line LOW when needed. Concept / Approach: With a pull-up, the node tends to HIGH by default. When an open-collector/open-drain output sinks current, the same node transitions to LOW. The resistor sets a bias current and limits short-circuit power when pulled LOW. Step-by-Step Solution: 1) Without a driver, an input is floating and undefined.2) Adding a resistor to VCC establishes a default logic HIGH.3) An active device can override by pulling the line to ground.4) Proper value selection balances noise immunity and current draw. Verification / Alternative check: Measure the node with no driver: it reads near VCC and is stable. When a sink turns on, it reads near 0 V. Why Other Options Are Wrong: LOW, float: Describes pull-down behavior, not pull-up. clock, float: A clock is a signal type, not a level. pulsed, float: Pulse describes timing, not default logic state. Common Pitfalls: Using too strong (low-ohm) pull-ups causing high power dissipation, or too weak (high-ohm) values causing slow edges and susceptibility to noise. Final Answer: HIGH, float

Question 17

7476 J-K flip-flop datasheet timing: Why is only a minimum clock pulse width (HIGH and LOW) specified rather than nominal or maximum values?

Accepted Answer

Introduction / Context: Datasheets for edge-triggered flip-flops such as the 7476 specify minimum clock HIGH and LOW pulse widths. This question probes understanding of how timing specifications guarantee correct operation over process, voltage, and temperature variations. Given Data / Assumptions: Edge-triggered J-K flip-flop (7476 family behavior representative). Clock pulse width specs appear as minimums for tW(H) and tW(L). Designs must meet or exceed these minimums under worst-case conditions. Concept / Approach: Minimum pulse width ensures internal latches and gating have sufficient time to propagate signals and reset/set internal nodes, even in the slowest, highest-temperature, lowest-voltage silicon corners (the worst case). Step-by-Step Solution: 1) Identify what the spec protects: reliable state capture without runt pulses.2) Recognize PVT variations: devices can be slower at high temperature/low voltage.3) A minimum width guarantees enough time for internal propagation and setup/hold satisfaction.4) Nominal/maximum widths are less relevant for correctness; too-short pulses are the risk. Verification / Alternative check: Simulate or measure across temperature and supply extremes; pulses shorter than the minimum lead to missed or metastable captures, while longer pulses work safely. Why Other Options Are Wrong: Nominal value: Nominals do not guarantee operation across corners. Best-case condition: Guarantees must cover the slow corner, not the fast corner. Typical operating condition: Typicals are informative but not sufficient for design guarantees. Common Pitfalls: Designing to typical datasheet numbers, ignoring temperature and voltage derating, and neglecting clock duty-cycle requirements. Final Answer: worst-case condition

Question 18

Schmitt trigger fundamentals: Identify the correct characteristics of a Schmitt trigger comparator stage used for noise-immune switching.

Accepted Answer

Introduction / Context: A Schmitt trigger is a comparator circuit with hysteresis used for cleaning noisy or slow-ramping input signals. This question checks recognition of its defining traits. Given Data / Assumptions: Noninverting or inverting Schmitt topologies exist. Two distinct thresholds: VT+ (rising) and VT− (falling). Feedback network sets hysteresis width. Concept / Approach: Positive feedback shifts the effective switching threshold depending on output state, creating two trip points. This hysteresis rejects small noise around the threshold and eliminates chatter. Step-by-Step Solution: 1) With output LOW, the input threshold is VT+ for rising transitions.2) After switching HIGH, the feedback moves the effective threshold down to VT− for falling transitions.3) The difference VT+ − VT− is the hysteresis voltage, giving noise immunity. Verification / Alternative check: Plot the transfer with arrowed path; observe different thresholds on up and down sweeps. Why Other Options Are Wrong: Has two trip points: Incomplete—does not mention the cause (positive feedback). Is a zero crossing detector: A basic comparator without hysteresis fits zero-crossing; Schmitt does not switch exactly at zero. Has positive feedback: True but incomplete—must also mention two trip points. Common Pitfalls: Confusing Schmitt triggers with simple comparators and assuming a single threshold. Final Answer: has two trip points and positive feedback

Question 19

Schmitt trigger hysteresis calculation: Given VT+ = 2.0 V and VT− = 1.2 V, compute the hysteresis voltage (width) of the Schmitt trigger.

Accepted Answer

Introduction / Context: Hysteresis in a Schmitt trigger defines the voltage band that prevents chatter by requiring a different threshold for rising versus falling transitions. This problem is a straightforward computation. Given Data / Assumptions: Rising threshold VT+ = 2.0 V. Falling threshold VT− = 1.2 V. Hysteresis is the absolute difference between thresholds. Concept / Approach: The hysteresis width is VT+ − VT−. Step-by-Step Solution: Compute: VH = VT+ − VT− = 2.0 − 1.2 = 0.8 V.Interpretation: Input must move 0.8 V across the band before the output can toggle back. Verification / Alternative check: Check signs: thresholds are ordered VT+ > VT−, so difference is positive. Why Other Options Are Wrong: 0.4 volt / 0.6 volt: Underestimates the difference. 1.2 volts: Confuses with the lower threshold magnitude. Common Pitfalls: Adding thresholds instead of subtracting, or using an average rather than the difference. Final Answer: 0.8 volt

Question 20

Driving LEDs from logic gates: Why is it usually recommended to sink LED current (pull LOW) from a logic output rather than source current (pull HIGH)?

Accepted Answer

Introduction / Context: Many logic families (especially TTL and some 5 V CMOS variants driving TTL-compatible loads) can sink more current at a valid LOW than they can source at a valid HIGH. Correctly orienting the LED and resistor takes advantage of this capability for brighter and more reliable operation. Given Data / Assumptions: Logic output provides defined VOL and VOH limits. Sinking current generally has a higher capability (mA) than sourcing. LED with series resistor can be placed to VCC (for sinking) or to ground (for sourcing). Concept / Approach: Configure the LED to VCC with its resistor, and let the gate pull the cathode LOW to turn it on. This uses the stronger sink path, ensuring the LED current meets brightness targets while keeping output within valid logic levels. Step-by-Step Solution: 1) Review datasheet: IOH (source) is often lower than IOL (sink).2) Choose sink configuration: VCC → resistor → LED → output pin.3) When output = LOW, current flows, LED turns ON; when output = HIGH, LED is OFF.4) Verify VOL stays under maximum at desired current. Verification / Alternative check: Compare brightness/current achievable when sourcing versus sinking using the same output device limits. Why Other Options Are Wrong: LOW-level current is smaller.: Opposite of typical capability. HIGH-level current is larger.: Not generally true for TTL-style outputs. LOW smaller & HIGH larger: Self-contradictory to common datasheets. Common Pitfalls: Ignoring VOL rise with excessive sink current, not recalculating resistor value, and forgetting per-pin and per-package current limits. Final Answer: LOW-level current is larger.

Digital Design Questions