Games / Command flash cards / Liberty timing libraries / Timing arcs

Timing arcs

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SourceLiberty is the format that carries a cell library's timing and power. These cards are checked against the Nangate45 library that OpenROAD-flow-scripts ships, and the excerpts are quoted from it. Liberty is a Synopsys format whose specification is licensed; the behaviour described here is what the open timer OpenSTA implements, and answers link to its command reference. Reference: the OpenSTA command reference (Liberty on GitHub).
All 7 cards in this section, with answers
  • timing () { related_pin : "CK"; timing_type : hold_rising;
    An arc from CK to this pin, holding a hold check on the rising edge.

    Hint: Two attributes: one names the other end of the arc, one names its kind.

    A timing group lives inside the pin it ends at, and related_pin says where it starts. timing_type says what kind of arc it is: a combinational delay, an edge-triggered delay, or a check such as setup_rising or hold_rising. Liberty reference

  • 'timing_sense : negative_unate;'
    A rising input makes the output fall, as in an inverter.

    Hint: Ask what happens to the output when the input rises.

    Unateness tells the timer which output edge to propagate for a given input edge. positive_unate preserves direction, negative_unate inverts it, and non_unate, as in an exclusive-OR gate, means both must be considered. It is also why a clock through such a gate needs set_sense. Liberty reference

  • timing () { related_pin : "A1"; when : "!A2"; timing_sense : negative_unate;
    This arc applies only while A2 is low, as a NAND's other input must be.

    Hint: The condition names another pin. Ask when this arc is the one that applies.

    A gate's delay from one input depends on what the others are doing, so a library can hold several arcs for the same pin pair with different conditions. The timer picks the one whose condition can hold, and case analysis can make that choice definite. Liberty reference

  • sdf_cond : "A2 == 1'b0";
    The same condition written for an SDF (Standard Delay Format) file.

    Hint: Compare it with `when`: same idea, different consumer.

    when is the condition the timer uses; sdf_cond is the same condition in the syntax an SDF (Standard Delay Format) writer needs, so that annotated delays land on the right arc. They should agree, and a mismatch shows up as unannotated arcs. Liberty reference

  • pin (Q) { timing () { related_pin : "CK"; } }
    An arc from CK to Q, since the arc belongs to the pin holding it.

    Hint: The group sits inside one pin and names another.

    Every arc is read this way: the enclosing pin is the endpoint, and related_pin is the start. The same shape carries both delays and checks, so a timing group inside a data pin naming a clock is a setup or hold check, while one inside an output is a delay. Liberty reference

  • timing_type : combinational; timing_type : rising_edge; timing_type : preset; timing_type : three_state_enable;
    Four kinds of arc, from plain logic to a clocked or enabling one.

    Hint: Ask what each one tells the timer about when the arc is active.

    combinational is a plain propagation, rising_edge fires only on a rising clock, preset is an asynchronous path to the output, and the tristate types turn a driver on or off. The type decides which edges the timer pairs and which checks it looks for. Liberty reference

  • timing () { related_pin : "CK"; timing_type : min_pulse_width; fall_constraint(Pulse_width_3) { index_1 ("0.00117378,0.0449324,0.198535"); values ("0.075346,0.087068,0.198733"); } }
    The shortest the clock may stay low, given how sharp its edges are.

    Hint: The values are times, and the axis is a transition. Ask what has to last that long.

    A pulse shorter than this may not be captured at all. The constraint grows with a slower clock transition, from 0.075 nanoseconds at a sharp edge to 0.199 at the slowest, which is one reason the clock network is held to a tight transition limit. set_min_pulse_width can tighten it. Liberty reference