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12 low power techniques


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12 low power techniques

  1. 1. LOW POWER TECHNIQUES IN NANOMETER TECHNOLOGY Name : Rama Krishna P. Reg No : CGB0911012 Course : M.Sc. [Engg.] in VLSI System Design Module : Integrated Circuit Analysis and Design Module Leader : Prof. Cyril Prasanna Raj P. M.S.Ramaiah School of Advanced Studies, Bangalore 1
  2. 2. DiscussionWhy low power?Types of power consumption • Dynamic power • Static powerLow power techniques • Clock gating • Multi vdd • Multiple vth • Power gatingTrade offFuture scopeConclusionReferences M.S.Ramaiah School of Advanced Studies, Bangalore 2
  3. 3. Why low power...? Desirability of portable devices. Advent of hand held battery operated devices. Large power dissipation requires larger heat sinks hence increased area. Cost of providing power has resulted in significant interest in power reduction of non portable devices. Lowering transistor threshold voltage. Pavg=Pswitching+Pshort-circuit + Pleakage =α0 1CL Vdd2 fclk + Vdd Isc +Ileakage Vdd M.S.Ramaiah School of Advanced Studies, Bangalore 3
  4. 4. Types of Power ConsumptionDynamic powerDuring the switching of transistorsDepends on the clock frequency and switching activityConsists of switching power and internal power.Static PowerTransistor leakage current that flows whenever power is applied to the deviceIndependent of the clock frequency or switching activity. M.S.Ramaiah School of Advanced Studies, Bangalore 4
  5. 5. Dynamic power ( Pswitching =CL Vdd2 fclk ) capacitance Vdd 1) Output node capacitance of the logic gate: due to the drain diffusion region.A 2) Total interconnects capacitance: has higher effect as PMOS technology node shrinks. VoutB Cdrain+ 3) Input node capacitance of the driven gate: due to the gate NMOS CloadCinterconnect+ Cinput oxide capacitance. Input voltage Internal node voltage swing can be only Vi which can be smaller than Fig 1:cmos inverter the full voltage swing of Vdd leading to the partial voltage swing. Frequency F increases then power automatically increases. M.S.Ramaiah School of Advanced Studies, Bangalore 5
  6. 6. (Pshort-circuit =Vdd Isc ) More rise/fall timemore short circuitBoth PMOS and NMOS are Lower threshold voltagemore shortconducting for a short duration of time circuit Vdd- |Vthp|short between supply power and 2.5V PMOSground curve Vout Vthn<Vin<Vdd-|Vthp| NMOS curve 0V Vthn Vin 2.5V Fig 3:Trance analysis of cmos Fig 2:shorte circuit To get equal rise/fall  balance transistor sizing Vdd<Vthn+|Vthp| M.S.Ramaiah School of Advanced Studies, Bangalore 6
  7. 7. Static power (Pleakage =Ileakage Vdd)1). Diode reverse bias–I1 2). Sub threshold current – I2 Vgs <~ Vth carrier diffusion causes sub threshold leakage. Vgs <=0  accumulation mode. 0< Vgs << Vth depletion mode. Vgs ~ Vth  weak inversion. Vgs > Vth  Inversion. Fig 4:Diode reverse bias3). Gate induced drain leakage – I3 4).Gate oxide tunneling – I4Higher supply voltage. high electric field across a thinthinner oxide. gate oxide. Direct tunneling through theincrease in Vdb and Vdg. silicon oxide layer if it is less than 3–4 nm thick. Fig 5:leakage currents M.S.Ramaiah School of Advanced Studies, Bangalore 7
  8. 8. Low power design techniques Table 1:low power design techniquesM.S.Ramaiah School of Advanced Studies, Bangalore 8
  9. 9. Clock gating× Clock tree consume more than 50 % of dynamic power Used for synchronous circuits Clock gating works by taking the enable conditions attached to registers, and uses them to gate the clocks. Save significant die area as well as power. Clock gating logic is generally in the form of "integrated clock gating" (ICG) cells.Logic added into the design Coded into the RTL code as enable conditions--clock gating logic (fine grain clock gating).Inserted into the design manually by the RTL designers (typically as module level clock gating)Semi-automatically inserted into the RTL by (automated clock gating tools) M.S.Ramaiah School of Advanced Studies, Bangalore 9
  10. 10. Two types of clock gating stylesLatch-based clock gating Latch-free clock En D Q D Q CK Gated CK clock clk Fig 6:Latch-based clock gating Fig 6:Latch-free clock gatingUses a simple AND or OR gate Level-sensitive latchGlitches are inevitable Less glitchLess used Easy adoption by EDA tools M.S.Ramaiah School of Advanced Studies, Bangalore 10
  11. 11. Multi Vdd (Voltage) SVS DVFS AVSDifferent but fixed Voltage as well as When highvoltage is applied to frequency is speed of  Voltage areasdifferent blocks or dynamically varied operation is with variable VDD.subsystems of the SoC as per the different required Voltage isdesign. working modes of voltage is controlled using a the design increased control loop. M.S.Ramaiah School of Advanced Studies, Bangalore 11
  12. 12. Multiple thresholdUse Hvt and Lvt cells Vdd standby High Prevents leakage Called as “sleep transistor” Vt in standby mode Extensively used in CMOS Logic High speed Power gating Low/Nom Vt operation standby High Prevents leakage Vt in standby mode Fig 6:Sleep transistor M.S.Ramaiah School of Advanced Studies, Bangalore 12
  13. 13. Variable threshold Vdd Vdd Vbias1 General variable substrate design: bias voltage from a substrate is control circuitry to tied to power Vbias2 vary threshold or ground voltage Fig 7:substrate bias voltage in cmosProsConsiderable power reductionNegligible area overheadConsRequires either twin well or triple well technology to achieve different substrate biasvoltage levels at different parts of the IC M.S.Ramaiah School of Advanced Studies, Bangalore 13
  14. 14. Power gating Circuit blocks that are not in use are temporarily turned off Affects design architecture more compared to the clock gating It increases time delays as power gated modes have to be safely entered and exited CMOS High Vt PMOS logicPower switching Header switch control signal Power switching control signal High Vt NMOS Footer switch CMOS logic A power switch (header or footer) is added to supply rails to shut-down logic (MTCMOS switches) Fig 8:power gating by sleep transistor M.S.Ramaiah School of Advanced Studies, Bangalore 14
  15. 15. Fine-grain power gating Coarse-grain power gating Fig 9:Fine grain power gating Fig 10:Coarse grain power gating Add a sleep transistor to every cell  Less sensitive to PVT variation Switching transistor as a part of the  Introduces less IR-drop variationstandard cell logic  Imposes a smaller area overhead ~10X leakage reduction  Ring based methodology  Column based methodology M.S.Ramaiah School of Advanced Studies, Bangalore 15
  16. 16. Fig 11:switces for reducing the powerLow-power design requires new cells with multiple power pins Additional modeling information in “.lib” is required to automatically handle these cells M.S.Ramaiah School of Advanced Studies, Bangalore 16
  17. 17. Trade –off for low power techniques Table 2:trade-off analysis of power reducing techniques M.S.Ramaiah School of Advanced Studies, Bangalore 17
  18. 18. Future low power strategy…?Asynchronous Design - Solution to Dynamic Power? ›Let’s get rid of the clock ›Micro pipeline: A Simple Asynchronous Design MethodologyIs Hi-k sufficient for 22nm and 16nm? Whether this type of transistor structure (hi-k, metal gate) will continue to scale to the next two generations—22 nm and 16 nm—is a question for the future.Is there a simple, coherent power strategy that unifies the best of DVFS, power gating,asynchronous ?How do we represent and verify very complex power intent such as asynchronous ?Carbon nano tubes…….?Spintronics…..? M.S.Ramaiah School of Advanced Studies, Bangalore 18
  19. 19. conclusion Power is becoming the restraining factor in further miniaturization andscaling. Various methodologies available but still a lot of scope for improvement.Need for developing of infrastructure. Combining of discrete power savingtechniques into a single integrated system. M.S.Ramaiah School of Advanced Studies, Bangalore 19
  20. 20. References[1] Keshava Murali, “Low power techniques”, SNUG 2007 and 2008 presentations on low power, Retrieved on 17 oct 2011.[2] Jan.M.Rabey and Massoud Pedram Kluwer academic publishers, “low power design methodologies”, Retrieved on 17 oct 2011. M.S.Ramaiah School of Advanced Studies, Bangalore 20
  21. 21. Thank you……. M.S.Ramaiah School of Advanced Studies, Bangalore 21