Games / Command flash cards / SDC (Synopsys Design Constraints) / General purpose

General purpose

The cards come in more than one shape: a command to explain, and steps to put in order. 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 SDC (Synopsys Design Constraints) as implemented by OpenSTA, the open-source static timing analyzer used by OpenROAD, and each answer links to that command in the OpenSTA command reference. Commercial timers accept the same commands but can differ in details; where OpenSTA reads a command without acting on it, the card says so. Reference: OpenSTA command reference (OpenSTA on GitHub).
All 4 cards in this section, with answers
  • set_units -time ns -capacitance pF
    Declares the file's times are in ns and capacitances in pF.

    Hint: Numbers in a constraint file carry no units of their own.

    Here ns is nanoseconds and pF is picofarads. Numbers in SDC (Synopsys Design Constraints) carry no units of their own. Without this declaration they are read in the library's units, so a file written for one library can be misread with another. OpenSTA checks the declared units and warns if they differ from the ones it uses. OpenSTA reference

  • current_instance u_core
    Makes later object names resolve relative to the instance u_core.

    Hint: Ask what a relative name means afterwards.

    After it, get_pins alu/A means u_core/alu/A. This lets a block's own constraints be applied inside a larger design. With no argument, it returns to the top level. OpenSTA reference

  • set_hierarchy_separator .
    Makes names use . instead of / between hierarchy levels.

    Hint: The default separator is a slash. Ask what this changes.

    The default separator is /, as in u_core/alu/A. A file written with u_core.alu.A style names needs this command so its names resolve. OpenSTA reference

  • Put these in the order a timing session must run them.
    1. read_liberty NangateOpenCellLibrary_typical.lib
    2. read_verilog gcd.v
    3. link_design gcd
    4. read_sdc gcd.sdc

    Hint: Each line needs something the line before it created.

    The netlist names cells, so the library has to be read first. Linking resolves those names into a design, and only then do objects exist for the constraints to find. A constraints file read before linking matches nothing, and usually says so only as a warning. OpenSTA reference