Games / Command flash cards / Timing reports / Corners, modes, and scenes

Corners, modes, and scenes

Each card shows a command to explain. A missed card returns later in the round and starts the schedule over. A card answered correctly moves up the schedule and next appears after 1, 3, 7, then 21 days. Keys 1 to 4 choose an option, h shows a hint, and Enter moves on. A hint keeps the card where it is on the schedule. An ordering card is answered by clicking its items in order, and one wrong click ends it. Time values are in nanoseconds. Progress is stored in this browser.

SourceThese cards follow the reporting commands of OpenSTA, the open-source static timing analyzer used by OpenROAD, and each answer links to that command in the OpenSTA command reference. Report commands are not a standard: other timers name and format their reports differently. The report excerpts are real OpenSTA output from the gcd example design. Reference: OpenSTA command reference (OpenSTA on GitHub).
All 6 cards in this section, with answers
  • define_scene slow_func -mode func -liberty ss_0p72v_125c -spef gcd_max.spef
    Defines one analysis combination of mode, library, and parasitics.

    Hint: Three things are named. Ask what each one contributes to an analysis.

    The mode brings the constraints, the library the cell timing, and the SPEF (Standard Parasitic Exchange Format) file the wires. A design is signed off at several such combinations: each pairs an operating condition (the library and parasitics) with a mode (the constraints that apply in it). A scene names one such combination, so reports can cover several at once. Scenes are defined after the libraries and parasitics are read. OpenSTA reference

  • set_mode func create_clock -name clk -period 2.0 [get_ports clk] set_mode test create_clock -name clk -period 20.0 [get_ports clk]
    Puts each set of constraints in its own mode, so both can exist at once.

    Hint: The same clock is created twice with different periods. Ask why that is not a conflict.

    A chip is constrained differently in different modes: functional, scan shift, at-speed test. Modes keep those constraint sets apart, and a scene then says which mode is analysed with which library. Creating the first mode removes the default one. OpenSTA reference

  • set_scene slow_func
    Makes that scene the one later commands report on.

    Hint: Ask what the commands typed after it now refer to.

    Reporting commands work on the current scene unless -scenes names others. Switching scenes is how one session compares the same path at the slow and the fast condition. get_scenes lists what is defined. OpenSTA reference

  • report_checks -scenes {slow_func fast_func} -format end
    Reports the paths across both scenes rather than only the current one.

    Hint: Ask what the report covers that the current scene alone would not.

    Most reporting commands take -scenes, and the default is every scene. Sign-off timing is the worst result across all of them, since setup usually fails slow and hold fails fast. OpenSTA reference

  • define_corners slow fast
    The older way of declaring corners, now deprecated in favour of scenes.

    Hint: Check whether this is the current way to do it.

    It is kept so older scripts still run, and it is used before read_liberty, which is what distinguishes it. New flows use define_scene, which ties a mode, a library, and parasitics together rather than naming a corner alone. OpenSTA reference

  • get_scenes -modes func
    Returns the scenes that use the functional mode's constraints.

    Hint: Ask what a mode can have several of.

    One mode is usually paired with several operating conditions, so a script iterates the scenes of a mode to report each. Like the other object-access commands, it accepts -filter. OpenSTA reference