What is SPEF, and what does it actually represent?
From PDVerse MMMC Interview Masterclass, part of the pdVerse Mentor Series
Direct answer
SPEF (Standard Parasitic Exchange Format, IEEE 1481) is the standard ASCII file format that transfers extracted wire resistance, ground capacitance, and cross-coupling capacitance from physical extraction tools (StarRC, Quantus) into STA engines (PrimeTime, Tempus).
Mentor explanation
SPEF provides the complete physical RC interconnect representation needed for signoff timing analysis.
Key terms
*D_NET— identifies a net name and its total lumped capacitance.*CONN— lists the driver pin and receiver load pins connected to the net.*CAP— specifies ground capacitance at internal nodes and cross-coupling capacitance to adjacent nets.*RES— defines segment-by-segment resistance connecting internal RC nodes.- Distributed vs. reduced SPEF — distributed SPEF contains the full node-by-node RC network (required for signoff); reduced SPEF uses a Pi-model or Elmore approximation for faster iteration.
Each SPEF file represents one specific RC extraction corner (e.g., design_rcmax.spef, design_rcmin.spef).
Practical example
SPEF Code Snippet:
*D_NET core_clk_div2 0.04825
*CONN
*I U12/Y O *L 12.4 45.2
*I U45/CK I *L 88.6 45.2
*CAP
1 core_clk_div2:1 0.01240
2 core_clk_div2:2 net_77:1 0.02150 // Coupling Cap to net_77
3 core_clk_div2:3 0.01435
*RES
1 core_clk_div2:1 core_clk_div2:2 14.85
2 core_clk_div2:2 core_clk_div2:3 18.20
*ENDInterview trap
Assuming that a SPEF loading without errors means annotation is 100% complete. Always run report_annotated_parasitics to verify that net names matched and no nets were silently unannotated.
Key takeaways
- SPEF is the IEEE 1481 standard format for transferring extracted wire R and C into STA tools.
- It represents distributed RC networks including grounded and inter-wire coupling capacitance.
- A complete signoff flow requires distinct SPEF files generated for each RC extraction corner.
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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