Games / Command flash cards / PD fundamentals / Power and current

Power and current

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 cover the reasoning behind physical design rather than any tool's interface: how a timing check is formed, what skew and variation do to it, and what the physical effects cost. Where an idea can be observed in the open flow, the explanation names the command that shows it, and the answer links to its documentation. Reference: the OpenSTA command reference (the open flow on GitHub).
All 9 cards in this section, with answers
  • where the power goes
    Switching the load, the cell's internal switching cost, and static leakage.

    Hint: Three components. Ask which one exists even when nothing switches.

    Switching power is the load capacitance times the supply squared times the activity; internal power is the cell's own cost per transition, from its library; leakage flows whether or not anything switches. Clock networks dominate the first two because they switch every cycle. the open flow reference

  • why the clock network dominates dynamic power
    It switches every cycle and reaches every register, unlike data.

    Hint: Ask how often a clock switches compared with a data net.

    A data net switches on a fraction of cycles; the clock switches twice per cycle everywhere. This is why clock gating, multi-bit flops, and fewer clock buffers all matter so much, and why clock power is reported as its own group. the open flow reference

  • why IR drop slows a design
    A cell at a lower supply than characterized is slower than the analysis assumed.

    Hint: Ask at what voltage the library was characterized, and what the cell actually gets.

    Timing is computed at the library's voltage, so a local droop makes real silicon slower than the report. Designs either keep drop under a budget (a few percent of the supply) or analyse timing at the reduced voltage. analyze_power_grid finds the drop. the open flow reference

  • electromigration, and why it is not about heat
    Current gradually displaces the metal's atoms, thinning the wire over years.

    Hint: Ask what physically moves, and over what timescale.

    Momentum from the electron flow moves atoms along the wire, opening it eventually. The limit is a current density, not a current, which is why widening a strap fixes it. It is a wear-out mechanism: the part works when new and fails in the field. the open flow reference

  • what a decoupling capacitor actually does
    It holds charge locally for the spike the grid cannot supply.

    Hint: Ask what the grid cannot deliver quickly enough on its own.

    The charge is held beside the cells that need it. The grid has inductance and resistance, so a sudden demand outruns it. A capacitor beside the switching cells supplies that moment and recharges between. It costs leakage and area, so decaps are placed where switching is heaviest. the open flow reference

  • power gating a block
    The supply is switched off, so outputs need isolating and state needs retaining.

    Hint: Ask what has to happen to the block's outputs and its state.

    Gating the clock stops switching power; gating the supply also stops leakage, which dominates when a block is idle for long. The price is isolation cells on the outputs, retention flops for the state worth keeping, and a wake-up sequence that costs time and a current surge. the open flow reference

  • the activity factor in a power calculation
    How often a net switches, taken from simulation or assumed by default.

    Hint: Ask where a tool gets it when no simulation has been run.

    Switching power is proportional to it, so the number matters as much as the capacitance. Simulation gives real activity through a value-change dump or a switching-activity file; without one, a default is assumed at the inputs and propagated, which is an estimate only. the open flow reference

  • glitch power
    Power spent on transitions that settle to the same value they started at.

    Hint: Ask what a net does when its inputs arrive at different times.

    An unbalanced logic cone makes a gate switch several times before settling, and each transition costs a full charge and discharge. It can be a noticeable fraction of dynamic power in arithmetic logic, and it does not appear in a report that uses a single activity factor per net. the open flow reference

  • the leakage and temperature loop
    Leakage heats the die, and a hotter die leaks more.

    Hint: Leakage rises with temperature. Ask what dissipating it does next.

    The feedback is real and, if the package cannot remove the heat, it runs away. It is one reason sign-off includes a hot corner and why leakage is controlled with threshold-voltage choice and power gating rather than treated as a fixed cost. the open flow reference