Timing Slack

Timing Slack is the margin between the required arrival time and the actual arrival time of a signal at a flip-flop input — positive slack means timing is met, negative slack means a violation exists that must be fixed before tapeout.

Slack Formula

• $T_{required}$: When the signal MUST arrive = clock period - setup time margin.
• $T_{actual}$: When the signal actually arrives = clock-to-Q delay + combinational path delay.
• Positive slack: Path meets timing — timing is satisfied.
• Negative slack: Timing violation — path is too slow.

Key Slack Metrics

• WNS (Worst Negative Slack): Most negative slack across all endpoints. Zero or positive = no setup violations. Target: WNS ≥ 0 ns.
• TNS (Total Negative Slack): Sum of all negative slacks. Measures total severity of timing problem. Target: TNS = 0 ps.
• Critical Path: Timing path with worst (most negative or least positive) slack — the path limiting clock frequency.

Timing Report Components

Startpoint: FF_A (rising edge clocked by CLK)
Endpoint: FF_B (rising edge clocked by CLK)
Path type: max (setup)

Data path delay: 0.843 ns
  (cell: 0.512 ns, net: 0.331 ns)
Clock period: 1.000 ns
Setup time: 0.087 ns

Slack (VIOLATED): -0.041 ns

Setup vs. Hold Slack

• Setup Slack: Data must arrive before rising clock edge. Violated by long paths (slow logic).
• Hold Slack: Data must not arrive too early after previous clock edge. Violated by short paths (buffers removed).
• Fixing setup: Speed up path (resize cells, reduce fanout, restructure).
• Fixing hold: Insert buffers to slow path down.

Timing Closure Workflow

1. Run STA (Synopsys PrimeTime, Cadence Tempus).
2. Identify top-N negative slack paths.
3. Fix: Upsize cells, remove logic stages, reroute critical nets, reduce clock uncertainty.
4. Iterate until WNS ≥ 0, TNS = 0 at all PVT corners.

Timing slack is the fundamental metric of chip performance and correctability — achieving zero-WNS, zero-TNS at all required timing corners is the definition of timing closure for any digital design.