Simultaneous Supply, Threshold and Width Optimization for Low-Power CMOS Circuits

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# Simultaneous Supply, Threshold and Width Optimization for Low-Power CMOS Circuits - PowerPoint PPT Presentation

Simultaneous Supply, Threshold and Width Optimization for Low-Power CMOS Circuits. With an aside on System based shutdown. Gord Allan PhD Candidate ASIC Design. Factors vs Power and Delay. Changing VDD Dyn Stat Delay Supply.

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### Simultaneous Supply, Threshold and Width Optimization for Low-Power CMOS Circuits

With an aside on System based shutdown.

Gord AllanPhD CandidateASIC Design

Factors vs Power and Delay
• Changing VDD Dyn Stat DelaySupply
• Changing Vt Dyn Stat DelayThreshold
• Changing Width W Dyn Stat Delay

Delay = A *C/(V*K) Pload = C*V2 Psc = f(tr/f)*f(V3)*f(K)Pstat = Ileak*V

Where: A = 1/(1-n) * [2n/(1-n) + ln(3-4n)], n=Vt/VDD

Reducing Vt and VDD
• Lower Vt so that we can reduce VDD further
• Ideal when Power Dynamic = Power Static

Example: - 600 gates - 18 gates deep - Critical Delay = 3.3 ns

Vt VDD Static Dyn Total0.7 2.5V .03aW 260fW 260fW

0.13 0.4V 9 fW 6 fW 15fW

StaticPower

Vt

An Aside: System Shutdown
• If you’re not using something - TURN it OFF!
• If it dosen’t need speed, TURN it DOWN!Aka. “Predictive System Shutdown and Other Architectural Techniques for Energy Efficient Programmable Computation,” Srivastava, et al.
• But When and How?
• When - Using user history to predict best time to sleep - There are costs (time and power) to go to sleep/awake - Voltage regulation problems.
• How - Hard Enough in normal circuits (clock gating, supply red) - What to do when dynamic power is an issue? (cutting off VDD is tough and poses other problems)

4*2/3 = 2.7 nS

1.3 nS

2 ns

E(2)

E(2)

F(1)

F(1)

G(1)

G(1)

2 nS

4 nS

C(1)

C(1)

D(1)

D(1)

A(2)

A(2)

1 nS

1 nS

B(4)

B(4)

2 ns

H(1)

H(1)

2 ns

I(1)

I(1)

Back to Optimization
• How do we pick VDD, Vt, and Ws given a clk freq?
• Draw Circuit
• Assign Delay Estimates (based on fan-out)
• Find Critical Path
• Assign Maximum Delays (weighted to estimates) along most critical path

Eg. Tclk = 8 ns

• Assign Maximum Delays to other gates on next most critical path, etc...
Pseudo-Code Procedure

for(VDD from 0.1 to 3.3) for(Vt from 0.1 to 0.7) for each gate for(W from 1 to 100) calculate delay if lower than Dmax pick W calculate total power dissipation

• Gives optimal VDD, Vt and W for all gates such that timing is met. NB: Pwr - f(switching activity)
• Complexity depends on number of gates and quantization of parameters
• Binary Search technique is used for large circuits
Results

Circuit A: s349 - 226 gates, depth 28 Benchmark: Vdd=3.3V, Vt=0.7V  = 0.5 Optimum: Vdd=0.7V, Vt=0.1V Power Savings of 54x

Circuit B: s526 - 596 gates, depth 18 Benchmark: Vdd=2.5V, Vt=0.7V  = 0.005 Optimum: Vdd=0.4V, Vt=0.13V Power Savings of 18x

Problems for Future Work
• Perception - Static power is Bad!
• Shutdown
• Variation of low Vt due to process issues.- Drags improvements from 20x to 6x with 50% variation- How do we get a reliable and efficient low Vt?
• System on a chip supplies - want > 1V. Noise issues.
• Variation of Switching activities and their effect.
• Low Vt - Great for pass logic! Exploit the benefits.
• Multiple Vt circuits - advantages?
Primary References
• “Simultaneous Power Supply, Threshold Voltage, and Transistor Size Optimization for Low-Power Operation of CMOS Circuits,” Pant, De, Chatterjee, IEEE Trans. on VLSI Systems, Vol. 6, No. 4, Dec 1998
• “Predictive System Shutdown and Other Architectural Techniques for Energy Efficient Programmable Computation,” Srivastava, Chandrakasan, Brodersen, IEEE Trans. on VLSI Systems Vol. 4, No.1, March 1996
Appendix: More Results

Circuit A: s349 - 226 gates, depth 28 Benchmark: Vdd=3.3V, Vt=0.7V  =0.5 Optimum 1: Vdd=0.7V, Vt=0.1V Savings of: Power 54x Area of 64%  =0.005 Optimum 2: Vdd=0.6V, Vt=0.1V Savings of: Power 27x Area of 59%

Circuit B: s526 - 596 gates, depth 18 Benchmark: Vdd=2.5V, Vt=0.7V  = 0.5 Optimum 1: Vdd=0.3V, Vt=0.1V Savings of: Power 67x Area of 8% = 0.005 Optimum 2: Vdd=0.4V, Vt=0.13V Savings of: Power 18x Area of 20%