How is an RC technology file different from SPEF?
From PDVerse PnR Interview Handbook · pdVerse Mentor Guide
Short Answer
An RC technology file is a reusable extraction model for a process; SPEF records extracted parasitic networks for a particular design. One describes how to calculate parasitics, and the other carries design-specific results.
Technical Explanation
The extractor uses technology information together with layout geometry to determine resistance and capacitance. A resulting SPEF can identify design nets, connections, capacitances, and resistances, with naming and unit conventions that the receiving timing flow must interpret correctly. When the route changes, the extracted parasitics for affected nets can change even though the process model has not changed. This explains the normal stage distinction. At the start of a new place-and-route run, completed signal-route geometry does not yet exist, so the flow estimates parasitics and later extracts them. A final SPEF is usually not a mandatory initial input. It can be a relevant input for a restart, hierarchical timing model, or a downstream timing-analysis flow, depending on the methodology. When parasitics are exchanged, verify the design revision, extraction corner, units, hierarchy and bus-name mapping, and annotation coverage. A report that reads a file without fatal syntax errors does not prove every intended net was annotated. Also check whether the consuming flow expects coupling or reduced parasitic representations.
Common Mistake
Calling TLUPlus a chip-specific wire-delay report, or requiring final SPEF before initial placement can begin.
Follow-up Question & Model Response
What is the first question when SPEF timing differs unexpectedly? Confirm that the netlist, layout revision, corner, and annotation coverage correspond to the same design state.
Practical Example
Routing a net around a newly placed SRAM can lengthen it. Its extracted SPEF network should then change, while the approved RC technology model may remain identical. Reusing an old SPEF can silently analyze the old route.
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