What is the difference between reading an SDF file and reading a SPEF file?

Short Answer

An SDF (standard delay format) file carries pre-computed cell and net delays that PrimeTime reads with read_sdf (PT) and applies directly. A SPEF (standard parasitic exchange format) file carries extracted parasitic RC data instead, read with read_parasitics (PT), from which PrimeTime computes the delays itself.

Technical Explanation

Both file types back-annotate the design with real, extracted timing information, but they hand the tool different things.

• What SDF contains: an SDF file holds delay values that have already been calculated elsewhere — the actual cell and net delays, and often timing checks too.
• How PrimeTime handles SDF: read_sdf (PT) reads those delays and applies them as given; the tool does not recompute anything from parasitics for that data.
• What SPEF contains: a SPEF file holds detailed parasitic RC data instead — the resistance and capacitance of each net, as produced by extraction.
• How PrimeTime handles SPEF: read_parasitics (PT) reads that RC data in, and from it PrimeTime computes the net delays itself, along with the load-dependent cell delays, using its own delay calculator.
• The one-line distinction: SDF provides finished delays; SPEF provides the raw parasitics that delays get derived from.
• Why SPEF is generally preferred for signoff: when PrimeTime computes the delay itself, it does so with the actual libraries and analysis settings in force for that specific run, including signal-integrity and crosstalk effects, applied consistently across the design.
• Why SDF can be a signoff risk: an SDF file's delays came from whatever tool wrote them, under whatever settings that tool had at the time — they can't retroactively pick up PrimeTime's own crosstalk analysis, so they may simply not reflect the exact conditions being signed off under.

Follow-up Question & Model Response

If SPEF is preferred for signoff, is there ever a legitimate reason to read an SDF file at all in a PrimeTime flow?

Candidate Model Response: Yes, SDF is useful for cross-checking or for flows where the delay calculation genuinely needs to match another tool exactly, for instance comparing PrimeTime's numbers against a gate-level simulation that was itself annotated with the same SDF. It's also common early in a flow, before parasitics exist, as a placeholder. But for final timing signoff, where PrimeTime's own crosstalk-aware delay calculation on real, extracted RC data is the point of running PrimeTime at all, SPEF is the input that lets the tool actually do that job rather than trust someone else's numbers.