Day 18: October 17, 2012 Energy and Power Optimization ESE370: Circuit-Level Modeling, Design, and Optimization for Digital Systems Day 18: October 17, 2012 Energy and Power Optimization Penn ESE370 Fall2012 -- DeHon

Previously Three components of power Ptot = Pstatic + Psc + Pdyn Short circuit Capacitive switching Ptot = Pstatic + Psc + Pdyn Penn ESE370 Fall2012 -- DeHon

Today Power Sources Reducing Switching Energy Energy-Delay tradeoffs Static Short Circuit Capacitive Switching Reducing Switching Energy Energy-Delay tradeoffs Penn ESE370 Fall2012 -- DeHon

Short Circuit Power Penn ESE370 Fall2012 -- DeHon

Preclass 1 Vin vs. Ipwr,gnd? 140mV 400mV 500mV 600mV 840mV Penn ESE370 Fall2012 -- DeHon

Short Circuit Power Between VTN and Vdd-VTP Roughly: Both N and P devices conducting Roughly: Penn ESE370 Fall2012 -- DeHon

Peak Current Ipeak around Vdd/2 If |VTN|=|VTP| and sized equal rise/fall Penn ESE370 Fall2012 -- DeHon

Short-Circuit Energy Penn ESE370 Fall2012 -- DeHon

Short-Circuit Energy Penn ESE370 Fall2012 -- DeHon

Short Circuit Energy Looks like a capacitance Q=I×t Q=CV Penn ESE370 Fall2012 -- DeHon

Short Circuit Energy and Power Every time switch Also dissipate short-circuit energy: E = CV2 Different C = Csc Ccs “fake” capacitance (for accounting) Largely same dependence as charging Psc = aCscV2 f Penn ESE370 Fall2012 -- DeHon

Reduce Short-Circuit Power? Psc = aCscV2 f Penn ESE370 Fall2012 -- DeHon

Preclass 2 Vin vs. Ipwr,gnd vs. Vin @ Vdd=500mV? 140mV 250mV 360mV Penn ESE370 Fall2012 -- DeHon

Total Power Ptot = Pstatic + Psc + Pdyn Pdyn + Psc = a(½Cload+Csc)V2f Ptot ≈ a(½Cload+Csc)V2f+VI’s(W/L)e-Vt/(nkT/q) Penn ESE370 Fall2012 -- DeHon

Dynamic Power Penn ESE370 Fall2012 -- DeHon

Reduce Dynamic Power? Pdyn = a × ½CV2 f How do we reduce dynamic power? Penn ESE370 Fall2012 -- DeHon

Slow Down What happens to power contributions as reduce clock frequency? What suggest about Vth? Penn ESE370 Fall2012 -- DeHon

Reduce V What happens as reduce V? Delay? Energy? Static Switching Penn ESE370 Fall2012 -- DeHon

Old Reduce V (no vsat) tgd=Q/I=(CV)/I tgd impact? tgd α 1/V Id=(mCOX/2)(W/L)(Vgs-VTH)2 tgd impact? tgd α 1/V Penn ESE370 Fall2012 -- DeHon

Saturation Observe Ignoring leakage Penn ESE370 Fall2012 -- DeHon

Reduce V (velocity saturation) tgd=Q/I=(CV)/I Ids=(nsatCOX)(W)(Vgs-VTH-VDSAT/2) Preclass 3 Penn ESE370 Fall2012 -- DeHon

Energy vs. Power? Which do we care about? Battery operated devices? Desktops? Pay for energy by kW-Hr? Penn ESE370 Fall2012 -- DeHon

Increase Vth? Recall increasing threshold voltage decreased leakage (9W vs. 4.5mW for 4BT chip) What is impact of increasing threshold on delay? Penn ESE370 Fall2012 -- DeHon

Increase Vth tgd=Q/I=(CV)/I Preclass 4 Ids=(nsatCOX)(W)(Vgs-VTH-VDSAT/2) Preclass 4 Penn ESE370 Fall2012 -- DeHon

Admin HW5 Due tomorrow Project 1 out Milestone piece due in one week Full Report in two weeks That means you need to be starting on it now…and working on it all next week Read assignment today Penn ESE370 Fall2012 -- DeHon

Idea Short circuit energy looks like more capacitance for switching energy Tradeoff Speed Switching energy Leakage energy Energy-Delay tradeoff: Et2 ? Et Penn ESE370 Fall2012 -- DeHon