EE466: VLSI Design Power Dissipation

Outline Motivation to estimate power dissipation Sources of power dissipation Dynamic power dissipation Static power dissipation Metrics Conclusion

Need to estimate power dissipation Power dissipation affects Performance Reliability Packaging Cost Portability

Where Does Power Go in CMOS?

Node Transition Activity and Power Due to charging and discharging of capacitance

Activity factors of basic gates AND OR XOR

Dynamic Power dissipation Power reduced by reducing V dd, f, C and also activity A signal transition can be classified into two categories  a functional transition and  a glitch

Glitch Power Dissipation Glitches are temporary changes in the value of the output – unnecessary transitions They are caused due to the skew in the input signals to a gate Glitch power dissipation accounts for 15% – 20 % of the global power Basic contributes of hazards to power dissipation are –Hazard generation –Hazard propagation

Glitch Power Dissipation P = 1/2.CL.Vdd. (Vdd – Vmin) ; Vmin : min voltage swing at the output Glitch power dissipation is dependent on –Output load –Input pattern –Input slope

Glitch Power Dissipation Hazard generation can be reduced by gate sizing and path balancing techniques Hazard propagation can be reduced by using less number of inverters which tend to amplify and propagate glitches

Short Circuit Power Dissipation Short circuit current occurs during signal transitions when both the NMOS and PMOS are ON and there is a direct path between V dd and GND Also called crowbar current Accounts for more than 20% of total power dissipation As clock frequency increases transitions increase consequently short circuit power dissipation increases Can be reduced : –faster input and slower output –V dd <= V tn + |V tp | So both NMOS and PMOS are not on at the same time

Static Power Consumption Wasted energy … Should be avoided in almost all cases

Static Power Dissipation Power dissipation occurring when device is in standby mode As technology scales this becomes significant Leakage power dissipation Components: –Reverse biased p-n junction –Sub threshold leakage –DIBL leakage –Channel punch through –GIDL Leakage –Narrow width effect –Oxide leakage –Hot carrier tunneling effect

Principles for Power Reduction Prime choice: Reduce voltage! –Recent years have seen an acceleration in supply voltage reduction –Design at very low voltages still open question (0.6 … 0.9 V by 2010!) Reduce switching activity Reduce physical capacitance –Device Sizing

Factors affecting leakage power Temperature –Sub-threshold current increases exponentially Reduction in Vt Increase in thermal voltage –BTBT increases due to band gap narrowing –Gate leakage is insensitive to temperature change

Factors affecting leakage power Gate oxide thickness –Sub-threshold current decreases in long channel transistors and increases in short channel –BTBT is insensitive –Gate leakage increases as thickness reduces

Solutions MTCMOS Dual Vt Dual Vt domino logic Adaptive Body Bias Transistor stacking

Metrics Power Delay product Energy Delay Product –Average energy per instruction x average inter instruction delay C unit_area –Capacitance per unit area

Conclusion Power dissipation is unavoidable especially as technology scales down Techniques must be devised to reduce power dissipation Techniques must be devised to accurately estimate the power dissipation Estimation and modeling of the sources of power dissipation for simulation purposes