1. Day 16: September 15, 2010 Energy and Power
ESE370: Circuit-Level Modeling, Design, and Optimization for Digital Systems
Day 16: September 15, 2010
Energy and Power
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2. Previously Where capacitance arises What drives delay
How to optimize
Power as a limiting constraint
Energy, Power Density
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3. Today Power Sources and Design Options Static Capacitive Switching
Short Circuit
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4. Power
P=I×V
Where should we look at I?
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5. Power P=IV What’s V? What is I? Steady-State (input fixed)?
When input switches
01
10
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6. Observe I changes over time Data dependent At least two components
Istatic – no switch
Iswitch – when switch
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7. Static Power Where does Istatic come from? Subthreshold leakage
Gate-Drain leakage
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8. Data Dependent? How does value of input impact Istatic?
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9. Reduce Leakage? P=VI How do we reduce leakage?
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10. Switching
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11. Switching Where does current go during switching?
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12. Switching Currents Charge (discharge) output If both transistor on:
Current path from Vdd to Gnd
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13. Switching Currents Iswitch(t) = Isc(t) + Idyn(t)
I(t) = Istatic(t)+Iswitch(t)
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14. Charging Idyn(t) – why changing? Ids = f(Vds,Vgs) andVgs, Vds changing
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15. Look at Energy
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16. Energy to Switch
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17. Integrating
Do we know what this is?
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18. Capacitor Charge Do we know what this is? What is Q?
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19. Capacitor Charge
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20. Capacitor Charging Energy
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21. Switching Power Every time switch 01 pay:
E = CV2
Pdyn = (# 01 trans) × CV2 / time
# 01 trans = ½ # of transitions
Pdyn = (# trans) × ½CV2 / time
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22. Reduce Dynamic Power? Pdyn = (# trans) × ½CV2 / time
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23. Charging Power Pdyn = (# trans) × ½CV2 / time
Often like to think about switching frequency
Ideally, switch per clock cycle
Frequency f = 1/clock-period
Pdyn = (#trans/clock) ½CV2 f
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24. Charging Power Pdyn = (#trans/clock) ½CV2 f Let a = activity factor
a = average #tran/clock
Pdyn = a½CV2 f
Get back to talking about a….
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25. Short Circuit Power
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26. Short Circuit Power Between VTN and Vdd-VTP Roughly:
Both N and P devices conducting
Roughly:
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27. Peak Current Ipeak around Vdd/2
If |VTN|=|VTP| and sized equal rise/fall
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28. Short-Circuit Energy
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29. Short-Circuit Energy
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30. Short Circuit Energy Looks like a capacitance Q=I×t Q=CV
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31. 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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32. Reduce Short-Circuit Power?
Psc = aCscV2 f
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33. Charging Power Pswitch = Pdyn + Psc = a(½Cload+Csc)V2f
What values can a take on?
a>1?
a<1?
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34. Glitches Inputs Transition from 0 1 0  1 1 1
What does output look like?
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35. Class ended here
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36. Data Dependent Activity
Consider an 8b counter
What is activity, a, for:
Low bit?
High bit?
Assuming random inputs (no glitching)
Activity at output of nand4?
Activity at output of xor4?
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37. Total Power
Ptot = Pdyn + Psc + Pdyn
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38. Slow Down
What happens to power contributions as reduce clock frequency?
What suggest about Vth?
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39. Reduce V What happens as reduce V? Delay? Energy? Static Switching
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40. Reduce V (no physical scale)
tgd=Q/I=(CV)/I
V S×V
Id=(mCOX/2)(W/L)(Vgs-VTH)2
Id  S2×Id
tgd  tgd /S
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41. Observe
Ignoring leakage
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42. Energy vs. Power? What do we care about? Battery operated devices?
Desktops?
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43. Admin Project Baseline done SPICE Power Measurement 5.5.4
List of ideas to accelerate done?
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44. Ideas Three components of power
Static
Short-circuit
Charging
aCV2f dependence for short-circuit, charging
Energy-Delay tradeoff: Et2
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