Walk through how you'd debug one unexpected timing violation.
From PDVerse MMMC Interview Masterclass, part of the pdVerse Mentor Series
Direct answer
To debug an unexpected timing violation, start by validating constraint integrity [PrimeTime: check_timing] to rule out false paths or clock definition errors, then inspect a full-clock expanded timing report [PrimeTime: report_timing -path_type full_clock_expanded -input_pins -nets -capacitance -transition_time] to isolate whether the root cause is clock skew, high cell delay, or wire RC/crosstalk.
Mentor explanation
Systematic timing debug isolates constraint errors, clock issues, and datapath bottlenecks.
Key terms
check_timing— audits unconstrained endpoints, generated clock errors, and loops before analyzing slack.report_timing -path_type full_clock_expanded— displays full launch and capture clock trees alongside the datapath.- Arrival vs Required Time breakdown — identifies whether the violation was caused by late data arrival or an early required clock edge.
- Slew and load audit — checks if an individual cell delay was inflated by high fanout or degraded input transition.
- CRPR check [PrimeTime:
report_crpr] — verifies that common clock tree pessimism removal was correctly computed.
Always verify constraints first: fixing a path with buffer ECOs before checking if it was an unconstrained CDC or false path wastes silicon area and design time.
Practical example
Reading a Failing Timing Report:
Startpoint: u_core/alu/reg_a_reg[3] (rising edge-triggered flip-flop clocked by CLK_1G)
Endpoint: u_core/alu/res_reg[3] (rising edge-triggered flip-flop clocked by CLK_1G)
View: func_ss_125c_rcmax (Setup Check)
--------------------------------------------------------------------------------
Pin Type Incr Delay Transition Edge
--------------------------------------------------------------------------------
clock CLK_1G (launch edge) 0.000 0.040 r
u_core/clk_buf_3/Y CLKBUF 0.150 0.150 0.035 r
u_core/alu/reg_a_reg[3]/CK DFF 0.000 0.150 0.035 r
u_core/alu/reg_a_reg[3]/Q DFF 0.085 0.235 0.042 r
u_core/alu/u_add_1/Y ADD_X1 0.320 0.555 0.185 * r <-- Slew blowout!
u_core/alu/u_mux_4/Y MUX_X1 0.410 0.965 0.210 * r
u_core/alu/res_reg[3]/D DFF 0.000 0.965 0.210 r
data arrival time 0.965
clock CLK_1G (capture edge) 1.000 0.040 r
clock network delay (prop) 0.120 1.120 r
clock uncertainty -0.080 1.040
u_core/alu/res_reg[3]/CK setup -0.045 0.995
data required time 0.995
--------------------------------------------------------------------------------
slack (VIOLATED) -0.030 psInterview trap
Jumping directly to sizing or buffering cells without checking constraints. Many severe violations stem from incorrect clock latency, missing false paths, or unconstrained asynchronous crossings.
Key takeaways
- Follow a structured 5-step triage: View → Clock Path → Data Path → Slews → Constraints.
- Check for slew blowouts, high logic depth, and missing timing exceptions.
- Always verify if the violation is structural, physical, or a false constraint issue before applying ECOs.
Self-check: can you answer this aloud?
Try a 45-second answer using this structure:
- State the direct answer.
- Explain the timing or physical reason.
- Name one caveat.
- Say how you would verify it in a real flow.
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