1. Day 18: October 21, 2011 Energy and Power Optimization
ESE370: Circuit-Level Modeling, Design, and Optimization for Digital Systems
Day 18: October 21, 2011
Energy and Power Optimization
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2. Previously Three components of power Ptot = Pstatic + Psc + Pdyn
Short circuit
Capacitive switching
Ptot = Pstatic + Psc + Pdyn
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3. Today Power Sources Reducing Switching Energy Energy-Delay tradeoffs
Static
Short Circuit
Capacitive Switching
Reducing Switching Energy
Energy-Delay tradeoffs
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4. Short Circuit Power
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5. Preclass 1 Vin vs. Ipwr,gnd? 140mV 400mV 500mV 600mV 840mV
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6. Short Circuit Power Between VTN and Vdd-VTP Roughly:
Both N and P devices conducting
Roughly:
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7. Peak Current Ipeak around Vdd/2
If |VTN|=|VTP| and sized equal rise/fall
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8. Short-Circuit Energy
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9. Short-Circuit Energy
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10. Short Circuit Energy Looks like a capacitance Q=I×t Q=CV
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11. 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
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12. Reduce Short-Circuit Power?
Psc = aCscV2 f
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13. Preclass 2 Vin vs. Ipwr,gnd vs. Vin @ Vdd=500mV? 140mV 400mV 500mV
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14. 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)
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15. Dynamic Power
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16. Reduce Dynamic Power? Pdyn = a × ½CV2 f
How do we reduce dynamic power?
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17. Slow Down
What happens to power contributions as reduce clock frequency?
What suggest about Vth?
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18. Reduce V What happens as reduce V? Delay? Energy? Static Switching
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19. 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
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20. Saturation Observe
Ignoring leakage
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21. Reduce V (velocity saturation)
tgd=Q/I=(CV)/I
Ids=(nsatCOX)(W)(Vgs-VTH-VDSAT/2)
Preclass 3
Vdd
Tau/Tau(Vdd=1V)
Esw/Esw(1V) 1V 1
700mV
1.75
0.5
500mV
3.5
0.25
350mV 14
0.13
250mV
1200
0.06
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22. Energy vs. Power? Which do we care about? Battery operated devices?
Desktops?
Pay for energy by kW-Hr?
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23. Increase Vth? Recall increasing threshold voltage decreased leakage
(9W vs. 4.5mW for 4BT chip)
What is impact of increasing threshold on delay?
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24. Increase Vth tgd=Q/I=(CV)/I Preclass 4
Ids=(nsatCOX)(W)(Vgs-VTH-VDSAT/2)
Preclass 4
Vth=-Vthp
Tau/tau(|Vth|=300mV)
300mV 1
500mV
1.4
700mV
2.3
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25. Admin Project 1 out Two weeks
That means you need to be starting on it now…and working on it all next week
Read assignment today
Baseline design no later than Wed (10/26)
Set of opt. ideas no later than Fri (10/28)
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26. Idea
Short circuit energy looks like more capacitance for switching energy
Tradeoff
Speed
Switching energy
Leakage energy
Energy-Delay tradeoff: Et2 ? Et
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