Download On Clock Network Design for Sub

On Clock Network Design for Subthreshold Circuitry
Yanqing Zhang
University of Virginia
[email protected]
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Outline
 Introduction and Motivation
 Problem Analysis
 Slew Aware Clock Design for Sub-threshold
 A Clock Buffer Design for Sub-threshold
 Clock Topology Discussion for Sub-threshold
 Summary
 Questions
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Outline
 Introduction and Motivation
 Problem Analysis
 Slew Aware Clock Design for Sub-threshold
 A Clock Buffer Design for Sub-threshold
 Clock Topology Discussion for Sub-threshold
 Summary
 Questions
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Intro: Sub-threshold Operation
 Voltage scaling leads to quadratic savings in Pdyn
 Pdyn=α0-1CeffVDD2f
 Scaling is pushed into sub-threshold(sub-VT)
region
 Transistor devices not “on”…
 …but still operational
 Sacrificing performance(speed) for low
power/energy
 Sub-VT characterized by weak drive and long
delay
 And great savings in dynamic power/energy
 Wide application space
 Mobile electronics
 Environmental/body sensor nodes
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Sub-VT
Intro: Clock Networks for Digital Circuits
 Conventionally, H-trees provide:
 Optimal slew(10%-90% VDD transition time in clock signal)
 Optimal skew(phase difference between clock signal at difference registers)
 Slew and skew affect timing constraints
CAUSES TIMING VIOLATIONS
REG
REG
logic
D Q
CLK
D Q
CLK
tslew
CLK’
CLK
CLK’
δ
CLK’
Ideal Clock
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Example H-tree
Practical Clock Example
Intro: Clock Networks for Digital Circuits
 Timing yield dependant on clock network
 Setup constraint
 Hold constraint
TCLK    tc q (tslew )  tlog ic  tsu (t slew )
thold (tslew )    tc q ,cd  tlog ic ,cd
 tc-q, tsu, thold with tslew
HDL Coding
module example(…)
…
endmodule
 Clocks designed via clock synthesis
 Designer has control over tslew,max, Cmax, δmax
Logic Synthesis
Well
defined.
Final Route
Clock Synthesis[ref]
=
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Placement
Outline
 Introduction and Motivation
 Problem Analysis
 Slew Aware Clock Design for Sub-threshold
 A Clock Buffer Design for Sub-threshold
 Clock Topology Discussion for Sub-threshold
 Summary
 Questions
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Sub-threshold Problems for Clock Network
 Setbacks of sub-threshold impose difficulty for quality clock
network
 Exponential dependency on VT variation
W (Vgs Vth Vds )/ s
1
vs
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(1  e Vds /Vth ) I D   Cox (Vgs  VTH ) 2
L
2
 Low drive strengths (~103× less)
I D  Io
 Wire delay no longer a factor
 Wire load still relevant
 Direct Impact on Clock Network
 Both skew and slew
 Slew also increases short circuit power
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Skew degradation with process
variation[3]
Delay(s)
Monte Carlo simulation of delay
exhibiting non-Gaussian distribution
Transistor Vgs-ID curve[1]
Sub-threshold Problems for Clock Network
 Slew a problem in sub-threshold
 Short circuit power(not a focus in super-threshold)
 Low drive strength=less control(10’s ns vs. 1 ps)
Example of register
 Scaling won’t work
 Up to 90% deterministic variation in timing parameters
 Can re-design directly in sub-threshold, but undermines
low power
Scaled Clock Tree
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[2]
[2]
Outline
 Introduction and Motivation
 Problem Analysis
 Slew Aware Clock Design for Sub-threshold
 A Clock Buffer Design for Sub-threshold
 Clock Topology Discussion for Sub-threshold
 Summary
 Questions
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Slew Aware Clock Tree Design for Sub-threshold
 Tighter nodal capacitance constraint CMAX will corral slew
 Slew recovery through buffer is stronger function of CMAX
than input slew
 Therefore, use higher CMAX near clock source, smaller
CMAX at registers
Relationship between slew, CMAX , and power
[2]
Output slew vs. load cap and input slew
[2]
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Resulting optimization
[2]
Resulting clock tree shape
[2]
Outline
 Introduction and Motivation
 Problem Analysis
 Slew Aware Clock Design for Sub-threshold
 A Clock Buffer Design for Sub-threshold
 Clock Topology Discussion for Sub-threshold
 Summary
 Questions
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A Clock Buffer Design for Sub-threshold
 Problem arises from operating in sub-threshold
 Solution: circuit in sub-threshold, clock network not
 Capacitive Boosting:
 Boosts ‘overdrive’ to higher than VDD
 No level converter overhead
Concept of capacitive
boosting[4]
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A Clock Buffer Design for Sub-threshold
 Shortcomings:
 Higher static and leakage power/energy for buffer
 Greater area
 Not integrated into design flow
 Complexities with timing and clock gating(1/2 cycle startup)
Improvement in skew[4]
 Doesn’t save power at lower frequencies for set VDD
 Doesn’t necessarily address process variations
 Advantages:
 Drastic improvement in slew (2.6x at 0.4V, 1 pF load)
 Better overall clock network energy due to fewer # of buffers
 Drastic improvement in skew
Improvement in slew[4]
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Overall improvement in energy
consumption for clock network[4]
Outline
 Introduction and Motivation
 Problem Analysis
 Slew Aware Clock Design for Sub-threshold
 A Clock Buffer Design for Sub-threshold
 Clock Topology Discussion for Sub-threshold
 Summary
 Questions
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Clock Network Design for Sub-threshold
 Conventional tree scrutinized
 Buffers perform poorly, do we really need them?
 Buffers NOT optimal in face of process variations
 Case study to compare buffered vs. non-buffered trees
vs
Buffered vs. non-buffered trees to be compared[3]
Skew and slew improvement with non-buffered trees[3]
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Energy overhead incurred under same nominal skew constraint for
non-buffered trees[3]
Clock Network Design for Sub-threshold
 Measurements show improvement in skew, skew variation, and slew variation without much overhead
 4 orders of magnitude less skew
 28% less slew variation
 4% power overhead
 Drawback: design not suitable across all VDDs
 Drawback: design not suitable across all circuit sizes
 Drawback: design not suitable across all constraints
Skew and slew improvement with non-buffered trees[3]
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Outline
 Introduction and Motivation
 Problem Analysis
 Slew Aware Clock Design for Sub-threshold
 A Clock Buffer Design for Sub-threshold
 Clock Topology Discussion for Sub-threshold
 Summary
 Questions
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Summary
 No universal solution
 Scope and limitations to individual methods
 Slew Aware Clock Tree Design for Sub-threshold:
 Good for slew control by design
 Universal for all clock network topologies
 Always saves something in power
 Does not address variations
 Should be used as an auxiliary method
 A Clock Buffer Design for Sub-threshold:
 Fantastic performance with regards to slew and skew
 Given range of supply voltage that it addresses process variations
 Ridiculous increase in power/energy for single buffer
 Thus restricted to certain frequencies and design sizes
 Clock Network Design for Sub-threshold
 Method address slew, skew, process variations
 Power/energy overhead fluctuates with design characteristics
 What does it all mean?
 Imminently, no universal solution
 We should carefully observe the characteristics of our design, and design accordingly
 Example: Combine papers [3] and [4]?
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References
[1] B. H. Calhoun., A. Wang, N. Verma, A. P. Chandrakasan, "Sub-threshold
Design: The Challenges of Minimizing Circuit Energy," International
Symposium on Low Power Electronics and Design (ISLPED), pp. 366368, October 2006.
[2] J. R. Tolbert, X. Zhao, S. K. Lim, S. Mukhopadhyay, “ Slew-Aware
Clock Tree Design for Reliable Subthreshold Circuits”, ISLPED, August
2009.
[3] Jonggab Kil, et al, “A High-Speed Variation-Tolerant Interconnect
Technique for Sub-threshold Circuits Using Capacitive Boosting,”
ISLPED, 2006, pp. 67-72.
[4] Mingoo Seok, D. Blaauw, D. Sylvester, "Clock Network Design for
Ultra-Low Power Applications,"International Symposium on Low Power
Electronics and Design, Aug, 2010.
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Questions?
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