12. dynamic cmos logic...ee 382m.7 { vlsi i fall 2011 october 10, 2011 ece department, university of...
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Introduction to VLSI Design, VLSI I, Fall 201112. Dynamic CMOS Logic 1
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12. Dynamic CMOS Logic
J. A. Abraham
Department of Electrical and Computer EngineeringThe University of Texas at Austin
EE 382M.7 – VLSI IFall 2011
October 10, 2011
ECE Department, University of Texas at Austin Lecture 12. Dynamic CMOS Logic J. A. Abraham, October 10, 2011 1 / 23
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Dynamic Logic
Dynamic gates use a clocked pMOS pullup
Two modes of operation: precharge and evaluate
ECE Department, University of Texas at Austin Lecture 12. Dynamic CMOS Logic J. A. Abraham, October 10, 2011 1 / 23
Department of Electrical and Computer Engineering, The University of Texas at AustinJ. A. Abraham, October 10, 2011
Introduction to VLSI Design, VLSI I, Fall 201112. Dynamic CMOS Logic 2
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The “Foot” Transistor
What if pulldown network is ON during precharge?
Use series evaluation transistor to prevent fight betweenpMOS and nMOS transistors
ECE Department, University of Texas at Austin Lecture 12. Dynamic CMOS Logic J. A. Abraham, October 10, 2011 2 / 23
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Logical Effort
ECE Department, University of Texas at Austin Lecture 12. Dynamic CMOS Logic J. A. Abraham, October 10, 2011 3 / 23
Department of Electrical and Computer Engineering, The University of Texas at AustinJ. A. Abraham, October 10, 2011
Introduction to VLSI Design, VLSI I, Fall 201112. Dynamic CMOS Logic 3
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Monotonicity
Dynamic gates require monotonicallyrising inputs during evaluation
0→ 00→ 11→ 1But not 1→ 0
ECE Department, University of Texas at Austin Lecture 12. Dynamic CMOS Logic J. A. Abraham, October 10, 2011 4 / 23
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Monotonicity Woes
But dynamic gates produce monotonically falling outputsduring evaluation
Illegal for one dynamic gate to drive another!
ECE Department, University of Texas at Austin Lecture 12. Dynamic CMOS Logic J. A. Abraham, October 10, 2011 5 / 23
Department of Electrical and Computer Engineering, The University of Texas at AustinJ. A. Abraham, October 10, 2011
Introduction to VLSI Design, VLSI I, Fall 201112. Dynamic CMOS Logic 4
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Domino Gates
Follow dynamic stage with inverting static gate
Dynamic/static pair is called domino gateProduces monotonic outputs
ECE Department, University of Texas at Austin Lecture 12. Dynamic CMOS Logic J. A. Abraham, October 10, 2011 6 / 23
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Domino Optimizations
Each domino gate triggers next one, like a string of dominostoppling overGates evaluate sequentially, precharge in parallelEvaluation is more critical than prechargeHI-skewed static stages can perform logic
ECE Department, University of Texas at Austin Lecture 12. Dynamic CMOS Logic J. A. Abraham, October 10, 2011 7 / 23
Department of Electrical and Computer Engineering, The University of Texas at AustinJ. A. Abraham, October 10, 2011
Introduction to VLSI Design, VLSI I, Fall 201112. Dynamic CMOS Logic 5
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Dual-Rail Domino
Domino only performs noninverting functions:
AND, OR but not NAND, NOR, or XOR
Dual-rail domino solves this problem
Takes true and complementary inputsProduces true and complementary outputs
sig h sig l Meaning
0 0 Precharged
0 1 ‘0’
1 0 ‘1’
1 1 Invalid
ECE Department, University of Texas at Austin Lecture 12. Dynamic CMOS Logic J. A. Abraham, October 10, 2011 8 / 23
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Example: AND/NAND
Given A h, A l, B h, B l
Compute Y h = A * B, Y l = ∼(A * B)
Pulldown networks are conduction complements
ECE Department, University of Texas at Austin Lecture 12. Dynamic CMOS Logic J. A. Abraham, October 10, 2011 9 / 23
Department of Electrical and Computer Engineering, The University of Texas at AustinJ. A. Abraham, October 10, 2011
Introduction to VLSI Design, VLSI I, Fall 201112. Dynamic CMOS Logic 6
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Example: XOR/XNOR
Sometimes possible to share transistors
Sharing works well in implementations of symmetric functionsSee papers on “relay logic” published over 50 years ago
ECE Department, University of Texas at Austin Lecture 12. Dynamic CMOS Logic J. A. Abraham, October 10, 2011 10 / 23
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Leakage
Dynamic node floats high during evaluationTransistors are leaky (Ioff 6= 0)Dynamic value will leak away over timeFormerly milliseconds, now nanoseconds!
Use keeper to hold dynamic nodeMust be weak enough not to fight evaluation
Leakage Power!
ECE Department, University of Texas at Austin Lecture 12. Dynamic CMOS Logic J. A. Abraham, October 10, 2011 11 / 23
Department of Electrical and Computer Engineering, The University of Texas at AustinJ. A. Abraham, October 10, 2011
Introduction to VLSI Design, VLSI I, Fall 201112. Dynamic CMOS Logic 7
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Charge Sharing
Dynamic gates suffer from charge sharing
Vx = Vy =Cy
Cx + CyVDD
ECE Department, University of Texas at Austin Lecture 12. Dynamic CMOS Logic J. A. Abraham, October 10, 2011 12 / 23
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Secondary Precharge
Solution: add secondary precharge transistors
Typically need to precharge every other node
Big load capacitance on Y helps as well
ECE Department, University of Texas at Austin Lecture 12. Dynamic CMOS Logic J. A. Abraham, October 10, 2011 13 / 23
Department of Electrical and Computer Engineering, The University of Texas at AustinJ. A. Abraham, October 10, 2011
Introduction to VLSI Design, VLSI I, Fall 201112. Dynamic CMOS Logic 8
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Noise Sensitivity
Dynamic gates are very sensitive to noiseInputs: VIH ≈ Vtn
Outputs: floating output susceptible noiseNoise sources
Capacitive crosstalkCharge sharingPower supply noiseFeedthrough noiseAnd more!
Chip power supply voltage map
when executing a program
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Alternating N & P Domino Logic
ECE Department, University of Texas at Austin Lecture 12. Dynamic CMOS Logic J. A. Abraham, October 10, 2011 15 / 23
Department of Electrical and Computer Engineering, The University of Texas at AustinJ. A. Abraham, October 10, 2011
Introduction to VLSI Design, VLSI I, Fall 201112. Dynamic CMOS Logic 9
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Cascade Voltage Switch Logic (CVSL)
ECE Department, University of Texas at Austin Lecture 12. Dynamic CMOS Logic J. A. Abraham, October 10, 2011 16 / 23
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Dynamic CVSL XOR Gate
ECE Department, University of Texas at Austin Lecture 12. Dynamic CMOS Logic J. A. Abraham, October 10, 2011 17 / 23
Department of Electrical and Computer Engineering, The University of Texas at AustinJ. A. Abraham, October 10, 2011
Introduction to VLSI Design, VLSI I, Fall 201112. Dynamic CMOS Logic 10
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Dual-Rail Domino Full Adder Design
Very fast, but large and power hungry
Used in very fast multipliers
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“Manchester” Adders
ECE Department, University of Texas at Austin Lecture 12. Dynamic CMOS Logic J. A. Abraham, October 10, 2011 19 / 23
Department of Electrical and Computer Engineering, The University of Texas at AustinJ. A. Abraham, October 10, 2011
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Domino Summary
Domino logic is attractive for high-speed circuits
1.5 2x faster than static CMOS
Many Challenges
MonotonicityLeakageCharge sharingNoise
Used in previous generation high-performance microprocessorsand in some recent embedded processors
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Domino Logic in Current Designs
Domino design from Intrinsity used in 1-GHz 0.75W ARMCortex A8 from Samsung (Intrinsity later acquired by Apple)Fast Domino (called “Fast14 NDL”) gates are insertedselectively into critical speed paths, with custom SRAMs andoptimized synthesized logic elsewhereStandard power saving techniques are also usedDomino gates are clocked by multiphase clocksA type of “super-pipeline” where the domino footers form thebarrier for the pipeline operation
(Source: Electronic Design – Embedded, August 29, 2009)
ECE Department, University of Texas at Austin Lecture 12. Dynamic CMOS Logic J. A. Abraham, October 10, 2011 21 / 23
Department of Electrical and Computer Engineering, The University of Texas at AustinJ. A. Abraham, October 10, 2011
Introduction to VLSI Design, VLSI I, Fall 201112. Dynamic CMOS Logic 12
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Intrinsity OR/NOR Implementation with “N-nary Logic”
2-bit function using 1-out-of-4 signals
Ref: U. S. Patent 6066965, Method and apparatus for a N-naryLogic Circuit Using 1 of 4 Signals
ECE Department, University of Texas at Austin Lecture 12. Dynamic CMOS Logic J. A. Abraham, October 10, 2011 22 / 23
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Intrinsity XOR/Equivalence Implementation
Using 1-out-of-2 signals
Ref: U. S. Patent 6066965, Method and apparatus for a N-naryLogic Circuit Using 1 of 4 Signals
ECE Department, University of Texas at Austin Lecture 12. Dynamic CMOS Logic J. A. Abraham, October 10, 2011 23 / 23
Department of Electrical and Computer Engineering, The University of Texas at AustinJ. A. Abraham, October 10, 2011
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