100+ Timing Analysis Multiple Choice Questions with Answers

Timing analysis is one of the most critical aspects of VLSI design, directly determining whether a digital circuit can operate reliably at its intended clock frequency across all manufacturing and operating conditions. As modern semiconductor technologies scale to deep-submicron and nanometer nodes, timing behavior is increasingly influenced by process variations, voltage fluctuations, temperature gradients, interconnect parasitics, and clock uncertainty.

Static Timing Analysis (STA) has therefore become a cornerstone of digital design verification, enabling designers to evaluate timing correctness without relying on exhaustive simulation. Concepts such as setup and hold time, clock skew and jitter, PVT corners, multi-cycle and false paths, on-chip variation (OCV), clock tree synthesis (CTS), and post-layout signoff form the foundation of reliable high-performance chip design.

This collection of 100 carefully curated multiple-choice questions (MCQs) focuses exclusively on Timing Analysis, covering topics across:

Fundamentals of STA and timing paths

Set up and hold checks under worst-case conditions
Clocking concepts, skew, uncertainty, and gating
Multi-mode multi-corner (MMMC) analysis
Parasitic extraction, SI/PI-aware timing, and IR-drop effects
Advanced timing closure and signoff practices

Each question is designed to be unique and conceptually tricky, encouraging deep understanding rather than rote memorization. Hints and detailed explanations accompany every MCQ to help learners grasp not only what the correct answer is, but why it is correct—bridging the gap between theoretical knowledge and real-world VLSI design challenges.

This MCQ set is ideal for VLSI students, digital design engineers, ASIC/SoC professionals, and semiconductor interview preparation, serving both as a self-assessment tool and a structured revision resource for mastering timing analysis concepts.

1). Which of the following best describes the “critical path”?

Shortest combinational path in design

Path with maximum slack

Longest timing path determining max frequency

Path with minimal fanout

2). In STA, a negative slack indicates?

Setup margin is satisfied

Hold margin violation

Timing violation (path too slow)

No paths exist

3). Which corner is typically used to check setup timing worst-case?

Fast-Fast (FF)

Slow-Slow (SS)

Typical-Typical (TT)

Any random corner

4). Which corner is most critical for hold checks?

Slow-Slow (SS)

Fast-Fast (FF)

Typical (TT)

SPICE only

5). What does STA mean by “arrival time”?

Time a clock edge occurs

Time a data signal reaches flop input

Time to synthesize a netlist

Time for fabrication

6). “Required time” for a path equals?

Launch clock edge + clock period - setup margin

Arrival time + hold margin

Flip-flop propagation delay only

Zero always

7). Which SDC command defines a clock to STA tools?

create_clock

set_false_path

set_multicycle_path

add_clock

8). What is a “multicycle path”?

Path measured across multiple clock domains

Path allowed to complete over multiple clock cycles

Path that toggles multiple times per cycle

Path created by clock gating

9). Which of these is a reason to declare a false path?

To increase power

Path never sensitizes in real operation

To slow down the clock intentionally

To fix hold violations

10). If two sequential elements are in different clock domains, STA should treat paths between them as?

Setup paths

Hold paths

Clock domain crossing (CDC) — avoid normal STA assumptions

Non-existent

11). What does “slack” equal in timing analysis?

Required - Arrival

Arrival - Required

Clock period - Setup

Propagation delay - Clock skew

12). Clock skew is defined as?

Difference in data arrival times

Difference in clock arrival times between launch and capture elements

Delay in combinational logic

Variation in process corner

13). Which technique helps reduce clock skew?

Up-sizing all gates

Balanced clock tree (CTS)

Adding more flop stages

Increasing metal layers

14). What is “clock uncertainty” in STA?

Unknown clock existence

Variation due to jitter, insertion delay, and synthesis ambiguity

A wire short

Incorrect netlist

15). In presence of positive clock skew (capture clock arrives later than launch), setup timing?

Is harder to meet

Is easier to meet

Is unaffected

Converts to hold violation

16). Which of these is not usually modeled in STA?

RC parasitics (wire delays)

Logical X propagation in RTL

Look-up table delays from library (.lib)

Clock uncertainties

17). What does “on-chip variation (OCV)” model?

Variations in manufacturing across different chips only

Variation of delay across a single chip due to location-dependent effects

Tool differences across EDA vendors

Die temperature only

18). Which is the likely effect of adding buffers in a data path?

Always improves setup and hold simultaneously

Can fix hold but may worsen setup

Eliminates clock skew entirely

Converts combinational to sequential logic

19). What is meant by “pessimism removal” in STA?

Introducing more worst-case margins

Removing overly conservative assumptions that cause false violations

Deleting false paths

Increasing clock speed automatically

20). Which file contains the parasitic back-annotation for gate-level timing?

.lib

.sdf

.vcd

.gds

21). What does SPEF contain?

Logical netlist

Parasitic RC details for nets (post-layout)

Power intent data

Test vectors

22). Which of the following improves setup timing with minimal area change?

Upsizing cells on critical paths

Adding new pipeline stages indiscriminately

Inserting scan chains

Increasing via count

23). What is “late mode” in STA contexts?

A simulation time slot at midnight

Analysis mode modeling slow library delays (for setup)

A debug mode only available in place-and-route

A testbench feature

24). Why are multi-mode multi-corner (MMMC) analyses needed?

To reduce run-time

To capture interactions across multiple operating modes and PVT corners

For layout scaling only

For DRC checks

25). How does process variation affect timing?

It has no effect on timing

It causes device parameter shifts (Vth, mobility), altering delays across chips and locations

Only changes power consumption

Only impacts package-level tests

Timing Analysis MCQs for Exams

26). Which of the following is true about “statistical STA” vs deterministic STA?

Statistical STA treats variations probabilistically for more realistic margins

Deterministic STA uses distribution-based approaches only

Both are identical in approach

Statistical STA is always faster

27). What does hold time depend on?

Clock period only

Data launch timing, path delay, and clock skew

Only register setup time

Power grid impedance

28). Which is an effective hold-fix that doesn’t increase setup slack?

Upsizing cells on path

Adding delay in data path (small buffer) placed close to source

Adding pipeline stage

Increasing clock frequency

29). What is “false path” declaration effect on timing closure?

It helps by removing irrelevant paths so focus is on realistic violations

It increases pessimism

It increases hold violations

It rewires nets at layout level

30). Which of the following can create new timing paths during ECO?

Metal-only changes without logic changes

Adding/removing buffers or re-routing nets

Changing package only

Modifying SDC only

31). What is worst negative slack (WNS)?

Maximum positive slack across design

Magnitude of the largest timing violation (most negative slack)

Average slack across paths

Number of violated paths

32). What is total negative slack (TNS)?

Sum of all negative slacks across failing paths

Number of paths failing

Slack normalized by clock period

Same as WNS

33). What does setup time of a flop represent?

Time for flop to be manufactured

Minimum time data must be stable before clock edge for correct capture

Time for clock tree synthesis

Time for scan shifting

34). Which factor reduces slack in a path?

Increasing drive strength

Adding capacitive load (fanout)

Reducing wire length

Lowering operating frequency

35). What is “path sensitization” in timing?

Making gates sensitive to power noise

Determining if a specific logical path can be activated by input patterns

Enabling clock gating always

Physical routing of nets

36). Which STA check would you use to ensure crossing edges meet timing when clocks are generated from the same PLL but routed differently?

CDC check only

Intra-domain STA with specified clock skew/uncertainty

DRC check

Check power grids only

37). What is “multi-mode” timing analysis?

Single-clock checks only

Analyzing timing across different operational modes (e.g., low power vs normal)

Checking for metastability only

Runtime simulation technique

38). How is clock gating likely to affect STA?

Always simplifies timing closure

Introduces enable signals and requires gating-aware constraints and proper isolation

Eliminates hold issues only

Not relevant to STA

39). Which STA metric helps prioritize fixes across many failing paths?

Wire width

TNS and WNS combined with path count and path slack distribution

Number of buffers only

RTL lines of code

40). What is “subpath” partitioning used for in STA optimization?

To reassign library files only

To focus optimization on a portion of long paths to reduce runtime and target fixes

To split memory blocks

To increase overall wirelength

41). Why are hold checks often more challenging after physical optimization?

Tools ignore hold constraints

Placement and routing reduce delays unpredictably, making min paths faster

Hold checks are not affected by layout

Hold checks happen only in RTL

42). If the path is declared multi-cycle with 2 cycles, how is required time computed?

Launch + 1×period - setup

Launch + 2×period - setup

Launch + 0.5×period - setup

Launch + period/2 - setup

43). Which is true about “re-timing” (algorithmic register relocation)?

Only moves combinational logic

Shifts registers across combinational logic to balance path delays without changing functionality

Changes logical behavior of the design

Requires layout changes always

44). Which is a practical method to reduce CTS insertion delay variability?

Use a skewed clock intentionally for all flops

Use balanced H-tree or buffered tree with matched delay cells and minimized branches

Remove clock buffers entirely

Use only one clock buffer globally

45. What is the main limitation of gate-level timing simulation compared to STA?

Gate-level sims are faster

Gate-level sims are vector-based and can miss worst-case patterns; they're slower and not exhaustive like STA

STA provides functional checks only

Gate-level sims never use SDF

46). Which statement about “setup recovery checks” when dealing with clock enable signals is correct?

Enables never affect setup timing

Care must be taken to model clock enables as they can change launch or capture timing and require gating-aware constraints

Always mark clock enable paths as false

They are only for DFT

47). Which is true about parasitic extraction accuracy trade-offs?

Higher accuracy always faster

More accurate extraction (RC) increases runtime but gives better post-layout STA results

Extraction accuracy doesn't matter for timing

Low accuracy always preferred for signoff

48). What is the role of “slew” in timing analysis?

Measure of voltage noise only

Signal edge rate (rise/fall) affecting cell delay and downstream loading

Number of buffers on a path

Frequency of clock jitter

49). Which is a typical cause of “multi-fanout net” delay issues?

Low capacitance

Excessive branching causing high capacitive load and skew between branches

Single driver only

Perfect balances always exist

50). What is “timing derating” used for?

To make timing requirements stricter always

Safety margins applied to cell/library timing to account for uncertainties or variations

Remove timing checks for test mode

Derating is unrelated to timing

Timing Analysis MCQs for Quiz

51). What’s the effect of increased supply voltage (VDD) on timing?

Slower logic elements

Faster transistor switching (reduced delay), but may increase leakage and stress

No change in speed

Only affects hold checks

52). What is a “timing path collapse” during optimization?

Aggregating multiple non-related paths into a single failing path due to merging logic or constraints

Physical collapse of routing layers

SDF corruption

Removing all buffers

53). How does temperature affect STA?

Higher temperature typically slows devices increasing delay (worse setup)

No effect on timing

Only affects leakage current

Improves transistor mobility always

54). What is the possible downside of aggressive buffering to fix setup?

It reduces delay always without cost

It can create more hold issues and increase power & area

It eliminates clock skew completely

It improves leakage power only

55). Which check ensures that scan chains can capture and shift test data correctly under timing constraints?

Functional STA only

Scan timing (scan shift and capture timing) considering shift-clocks and captureclocks

DRC only

CDC only

56). Which is true about cross-talk impact on timing?

Cross-talk always reduces delay

Cross-talk can both increase/suppress delay or cause logic flips due to coupled switching, requiring SI-aware timing checks

Cross-talk only matters in power grids

Cross-talk is the same as IR drop

57). What is “min-delay path” important for?

Setup checking only

Hold checking (since very short paths can violate hold)

Power grid design

Package design only

58). Why is “clock gating” sometimes considered in STA as introducing false paths?

Because gating logic makes certain launch-capture combinations impossible during valid operation

Clock gating always creates new physical paths only

Clocks are unaffected by gating logic

Clock gating removes the need for SDC

59). How is “waveform” parameter in create_clock used in STA?

Not used at all

To define high and low time offsets for non-50% duty clocks and compute arrival times correctly

Only for FFT analysis

For DFT generation

60). Which of these is an example of “exception” constraint used in SDC?

create_clock

set_false_path

set_input_delay

report_timing

61). What is meant by “arrival path reconstruction” in STA debug?

Rebuilding library models

Tracing actual predecessors and gate delays to understand why arrival times are computed as such

Running formal verification

Removing clocks from SDC

62). What effect can clock tree insertion buffers have on hold?

No effect on hold

They can change skew and insertion delay causing hold violations or relief depending on placement

Always fix hold issues

Always create setup violations only

63). Which is an outcome of inaccurate library characterization (liberty)?

No impact on timing

STA may produce incorrect slack values leading to false signoff or over-conservative fixes

Only impacts DRC rules

Changes clock tree algorithm automatically

64). What is “windowing” in timing signoff?

A graphical UI tool only

Dividing design into manageable windows/regions for localized timing optimization and reduce runtime

A method of packaging chips

Design-for-test technique

65). Why is “cross-corner correlation” important?

To ensure variability across corners is independent

To evaluate realistic combinations of PVT variations that may not be worst-case simultaneously

It is not important

Only for packaging tests

66). What is “common path pessimism removal (CPPR)”?

Adding more pessimism to common path segments

Removing double-counted pessimism in shared path segments between launch and capture to reduce false violations

A routing process

A packaging rule

67). What is multi-voltage timing analysis concern?

Only one voltage used throughout design

Timing across power-domain crossings with different operating VDDs and potential level shifters or retention states

Only for off-chip IO

Power grid irrelevant

68). Which is true about timing closure for high-frequency designs (>1GHz)?

Easy compared to low-frequency designs

Demands precise modeling (parasitics, IR, OCV), aggressive optimizations, and tighter margins

No special steps required

Only increases area, not timing complexity

69). What’s the role of “post-layout STA” in signoff?

Optional for small designs only

Mandatory check with extracted parasitics to ensure timing remains met after routing and placement

Only for pre-layout enforcement

Only for checking DRC rules

70). Which SDC command is used to define the path as multi-cycle?

set_false_path

set_multicycle_path

set_input_delay

create_clock

71). How does physical optimization like buffer insertion affect crosstalk?

Buffers reduce wire length and can decrease coupling, but additional switching nodes may increase local switching noise — net effect depends on placement

Buffers always increase crosstalk

Buffers are irrelevant to crosstalk

Buffers only affect power grids

72). What is “signal integrity aware STA”?

STA ignoring cross-talk

STA that includes SI effects (crosstalk-induced delay/do not cares) and checks for SIrelated timing failures

Same as DRC

Timing only for analog blocks

73). Which scenario most likely requires “false path” marking?

A path through a reset generator that cannot be enabled during functional operation

The clock path itself

I/O pads always toggling

Power grid nets

74). What is the meaning of “late mode slack” being negative while early mode slack is positive?

Hold violations only

Setup violation at slow corner, hold okay at fast corner; implies worst-case setup fails but hold is safe

Corrupted SDC file

Unrelated metrics

75). Why are clocks with variable duty cycles tricky in STA?

They’re never used in silicon

Non-50% duty affects launch/capture spacing and both rising/falling edge definitions must be clear for correct required/arrival calculation

Duty cycle only impacts power

Duty cycle ignored by STA always

Timing Analysis MCQs for Interviews

76). Why is “min/max delay” modeling important for bi-directional IO buffers?

IO buffers don’t affect timing internally

Bi-directionality causes different propagation delays in each direction; min and max must be considered for both setup and hold constraints

IOs only impact static power

Only TTL compatible designs need this

77). What is “path-based STA” vs “block-based STA”?

Synonyms

Path-based STA analyzes specific start-to-end paths for deeper, more precise timing; block-based uses aggregate timing constraints and is faster but less detailed C) Path-based is only for logic synthesis D) Block-based is for packaging only

Path-based is only for logic synthesis

Block-based is for packaging only

78). Which factor can invalidate STA results if not modeled?

Variation in process corners only

Power supply noise (IR drop) since VDD reduction slows transistors and changes timing across die

DRC violations only

Number of test vectors

79). What is the advantage of incremental STA?

Re-run full tool every time

Faster timing updates on localized changes rather than full re-run, enabling quicker iterations during ECO or placement fixes

Only for clock trees

No practical advantage

80). Which is true about multi-threaded STA?

Always less accurate

Uses parallel threads to speed up computation while maintaining accuracy if algorithms are thread-safe

Replaces need for SPEF

Only used in simulation not analysis

81). What is “path group” used for in timing reports?

Grouping logically related paths to reduce noise in reports and prioritize fix families

Random grouping only

Packaging group for wafer-level tests

Grouping RTL statements

82). Which is a common STA-derived metric used to justify silicon signoff?

Clock tree buffer count only

WNS, TNS, and number of timing violations across MMMC supported by guardbands and signoff signoffs like SI/IR checks

Amount of routing used

Number of DRC errors

83). What is “min pulse width” checking related to timing?

Only for analog circuits

Ensuring that gated clocks or pulses meet minimum width requirements of flip-flops to reliably capture data

DRC only

Not relevant to STA

84). Which technique helps handle long interconnect delays in deep-submicron processes?

Ignore wiring delays

Introduce repeaters/buffers, better routing, and pipelining (register insertion)

Move everything to analog domain

Decrease library size

85). Why must STA consider both rise and fall transitions separately?

They are always identical

Rise and fall have different drive strengths and loads leading to different delays; worst-case per path may be rise or fall

Only rise matters

Only fall matters

86). What is ‘timing pessimism’ and why is it used?

Random number added to slack

Conservative margin added to account for unknowns; avoids false success in timing but can increase unnecessary fixes if excessive

Tool failure mode

A technique to ensure everything fails

87). Why might a timing path be “non-sensitizable” although it exists in netlist?

Wrong library file

Logical conditions never make the path active in operation (e.g., mutually exclusive enables)

Always sensitizable by definition

Because SDF omitted it

88). Which metric helps identify the number of unique timing failures?

Clock period only

Number of unique violated endpoints or unique startpoint–endpoint pairs (violating path count)

LIB file size

Number of metal layers

89). What is the purpose of “path-based optimization” as opposed to generic netlist optimizations?

Only used in DFT

Target particular critical paths with transformations (gate sizing, restructuring) to meet timing while minimizing impact elsewhere

Replace entire design with pipelined version always

Only used for analog blocks

90). What happens if SDC constraints are incorrect or incomplete?

STA always fails gracefully

STA results may be meaningless — potential false failures or missed issues leading to silicon re-spins

Tools ignore SDC and proceed correctly

Only affects DRC checks

91). Why is clock domain crossing (CDC) analysis complementary to STA?

CDC is only about timing within one clock

CDC checks asynchronous interactions, metastability and synchronization whereas STA checks timing requirements — both needed for reliable cross-domain communication

DC replaces STA completely

CDC irrelevant for synchronous systems

92). What is “time borrowing” in multi-phase or pulsed-clock designs?

Borrowing money for faster clocks

Technique allowing latch transparency where data can be captured across phase boundaries giving extra time to paths

Illegal design practice

Same as multi-cycle path

93). In timing signoff, why is it important to model temperature gradients rather than single value?

Single temp always worst-case

Different die regions can run hotter or cooler due to activity causing local variation in delay — gradients ensure local worst-cases are caught

Temperature only affects leakage

Gradients only for packaging

94). Which is true about “interface timing” in multi-block chip designs?

Interfaces are purely logical and don't need timing checks

Timing across block interfaces requires agreed constraints, matched clocking, and consideration of latency/handshake for integrated STA across hierarchy

Interfaces only affect power

Only place-and-route stage handles interfaces

95). What is “time-based correlation” between STA and silicon measurement?

Not required

Comparing measured path delays on silicon with STA predictions to calibrate models and ensure signoff margins were adequate

Only about voltage thresholds

A manufacturing test only

96). When is “setup time margin” reduced unintentionally?

When frequency lowered

When pessimism is compounded (double-counted) or clock uncertainty increases due to PLL jitter or routing variations

When additional test vectors are added

When SDF used in gate-level sim

97). Why is balancing between setup and hold fixes important?

Setup only matters for performance

Fixing setup aggressively (e.g., removing buffers) may create hold violations; solutions must consider both simultaneously to avoid toggling issues

Hold fixes never affect setup

Hold is only for DFT

98). What is the effect of clock gating insertion on skew and uncertainty?

Always reduces skew

If gating logic creates extra insertion delay and variability, it increases clock uncertainty and must be compensated in STA

Clock gating removes duty cycle issues

Clock gating has no timing impact

99). What is a typical sign-off flow order regarding timing?

STA pre-synthesis only

RTL simulation → synthesis → pre-route STA → placement → CTS → routing → PEX → post-route STA signoff (MMMC, SI, IR)

Only gate-level sim required

SDC generation after signoff

100). Which description best reflects the role of “timing engineers” in chip projects?

Only write clocks in SDC

Analyze and close timing across the flow (front-to-back), coordinate constraints, identify root-causes, and propose fixes balancing area/power/performance

Only simulate analog blocks

Only do place-and-route tasks