What is the difference between reading an SDF file and reading a SPEF file?
From PDVerse PrimeTime STA Interview Guide · pdVerse Mentor Guide
Ten-second Interview Answer
SDF carries pre-computed cell and net delays that PrimeTime reads with read_sdf and applies directly; SPEF carries extracted parasitic RC that PrimeTime reads with read_parasitics and computes delays from itself.
Complete Technical Explanation
Both files back-annotate the design, but they hand the tool different things. An SDF file - Standard Delay Format - contains delay values that have already been calculated: the actual cell and net delays, and often the timing checks too. PrimeTime reads them via read_sdf and applies them as given; it does not recompute anything from parasitics. A SPEF file - Standard Parasitic Exchange Format - contains detailed parasitic RC data instead, the resistance and capacitance of each net as extraction produced it, read in with read_parasitics . From that, PrimeTime computes the net delays itself, and the load-dependent cell delays with them, using its own delay calculator. So the one-line difference is that SDF provides delays while SPEF provides the parasitics that delays are derived from. For signoff, SPEF is generally the preferred input, and the reason is control and self-consistency. When PrimeTime does the calculation, it does it with the actual libraries and the analysis settings in force for that run, including signal-integrity and crosstalk effects, applied uniformly to the extracted parasitics. An SDF's delays came from whatever tool wrote them, under whatever settings that tool had, and cannot pick up PrimeTime's crosstalk analysis afterwards - so they may simply not reflect the signoff conditions you are signing off under.
In Pd / Signoff
This decides what you ask the physical-implementation side for. Post-route signoff runs read per-corner SPEF with read_parasitics so PrimeTime's own delay calculation and SI analysis apply consistently. SDF via read_sdf shows up mainly for correlation against another tool or for gate-level simulation timing, and any slack from an SDF run should be treated as reflecting the writer's settings, not yours.
Common Trap
Signing off on an SDF-annotated run and assuming it includes PrimeTime's crosstalk and analysis settings. It cannot - the delays were fixed before PrimeTime saw them, so SI delta delays and the current operating and variation settings have no way to influence them.
Expect Next
Which of the two gives PrimeTime more control over the delay calculation, and why does that matter for signoff?
Topic And Primetime Commands
STA Fundamentals / STA in the Flow. read_sdf, read_parasitics
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