Preparing for a Static Timing Analysis interview? STA questions appear in every physical design and timing signoff interview. Here are the most-asked STA interview questions in 2026 with clear, interviewer-ready answers.
Basic STA Interview Questions
1. What is Static Timing Analysis?
STA verifies that a design meets timing requirements by analyzing all timing paths mathematically — without simulation vectors. It checks every path exhaustively against setup and hold constraints, making it the standard signoff method for digital chips.
2. What are setup and hold violations, and how do you fix them?
A setup violation means data arrives too late before the capture clock edge — fixed by upsizing cells, reducing logic depth, adjusting clock skew, or relaxing the clock. A hold violation means data changes too soon after the edge — fixed by adding delay buffers on the fast path. Hold violations are more dangerous: they cannot be fixed by slowing the clock.
3. What is slack?
Slack = required time − arrival time. Positive slack means timing is met with margin; negative slack is a violation. The worst negative slack (WNS) and total negative slack (TNS) are the key health metrics of a design.
4. What are false paths and multicycle paths?
A false path is a topologically existing path that can never be functionally exercised — excluded via set_false_path. A multicycle path intentionally takes more than one clock cycle — constrained with set_multicycle_path so STA doesn’t flag it incorrectly.
Intermediate Questions
5. What is clock skew, jitter and uncertainty?
Skew is the arrival-time difference of the clock at different flops; jitter is cycle-to-cycle variation of the clock edge; clock uncertainty models both (plus margin) in constraints. Positive skew can help setup but hurt hold, and vice versa.
6. What is OCV, AOCV and POCV?
On-Chip Variation models process variation across the die by derating delays. AOCV makes derates depth- and distance-aware; POCV/SOCV uses statistical sigma-based delays per cell — progressively less pessimistic and standard at advanced nodes.
7. What is CRPR (clock reconvergence pessimism removal)?
When launch and capture clocks share a common path segment, applying different derates to the shared portion is pessimistic. CRPR removes that artificial pessimism from the common path.
8. What corners and modes do you sign off?
Multi-corner multi-mode (MCMM) analysis: process corners (SS/FF/TT), voltage and temperature extremes, RC corners (Cmax/Cmin), across functional, test and low-power modes. Setup is typically worst at slow corners; hold at fast corners.
Advanced Questions
9. What is timing ECO and how do you approach it?
Engineering Change Orders fix residual violations post-route: cell sizing/swapping (footprint-compatible), buffer insertion/removal, and metal-only fixes late in the flow. Tools generate ECO lists; the skill is fixing timing without disturbing closed paths.
10. Explain latency, insertion delay and useful skew.
Clock latency is source-to-flop clock delay (insertion delay is its network portion). Useful skew deliberately advances/delays specific flops’ clocks to steal margin from paths with slack and give it to failing paths.
How to Prepare
- Master the timing report: read arrival/required paths line by line
- Practice SDC constraints — most questions trace back to them
- Know your numbers: typical skew budgets, derate values, margin conventions
STA is core to the ChipXpert Physical Design Course, taught on real signoff tools. Also see physical design interview questions and VLSI internships.
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