Power Analysis and Power-Aware Design - Part 1

Importance of Power and Energy:
  1. Laptops, PDA, cell-phones, etc —obvious!
  2. For microprocessors in personal computers, every watt above 40W adds $1 to manufacturing cost.
  3. Approx 25% of operating expense of server farm goes to energy bills.
  4. Sandia Labs had to build a special sub-station when they took delivery of Teraflops massively parallel supercomputer (over 9000 Pentium Pros)
  5. High-speed microprocessors today can run so hot that they will damage themselves—Athlon reliability problems, Pentium 4 processor thermal throttling
  6. Future power viruses: cell phone viruses cause cell phone to run in full power mode
Most people talk about "power" reduction, but sometimes they mean "power" and sometimes "energy."
  1. Power minimization is usually about heat removal.
  2. Energy minimization is usually about battery life or energy costs.
Power Equations:
= Switching Power+Shortcircuit Power+Leakage Power
Dynamic power = Switching Power+Shortcircuit Power
Static Power = Leakage Power
  1. Dynamic Power dependent upon clock speed
  2. Switching Power useful —charges/discharges transistors
  3. Short Circuit Power not useful —both N and P transistors are on
  4. Static Power independent of clock speed
  5. Leakage Power not useful —leaks around transistor
Dynamic power is proportional to how often signals change their value (switching)
  • Roughly 20% of signals switch during a clock cycle.
  • Need to take glitches into account when calculating activity factor. Glitches increase the activity factor.
  • Equations for dynamic power contain clock speed and activity factor.
Some power reduction techniques:
  • Analog
    • Parameters to work with:
      • capacitance for example, Silicon on Insulator (SOI)
      • resistance for example, copper wires
      • voltage low-voltage circuits
    • Techniques:
      • dual-VDD Two different supply voltages: high voltage for performance-critical portions of design, low voltage for remainder of circuit. Alternatively, can vary voltage over time: high voltage when running performance-critical software and low voltage when running software that is less sensitive to performance.
      • dual-Vt Two different threshold voltages: transistors with low threshold voltage for performance-critical portions of design (can switch more quickly, but more leakage power), transistors with high threshold voltage for remainder of circuit (switches more slowly, but reduces leakage power).
      • exotic circuits Special flops, latches, and combinational circuitry that run at a high frequency while minimizing power
      • adiabatic circuits Special circuitry that consumes power on 0 - 1 transitions, but not 1 - 0 transitions. These sacrifice performance for reduced power.
      • clock trees Up to 30% of total power can be consumed in clock generation and clock tree
  • Digital
    • Parameters to work with:
      • capacitance (number of gates)
      • activity factor
      • clock frequency
    • Techniques:
      • multiple clocks Put a high speed clock in performance-critical parts of design and a low speed clock for remainder of circuit
      • clock gating Turn off clock to portions of a chip when it's not being used
      • data encoding Gray coding vs one-hot vs fully encoded vs ...
      • glitch reduction Adjust circuit delays or add redundant circuitry to reduce or eliminate glitches.
      • asynchronous circuits Get rid of clocks altogether....

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