Power Saving at Architectural Level Xiao Xing March 7, 2005

Purpose of Power Saving In VLSI Circuits For Portability: So that portable Devices Don’t require Batteries That are as Large as A Brief Case. For Cooling: So one does NOT Have to Resort to Expensive Cooling Equipment, that Might Cost more than the Circuit you’re trying to Cool off.

Types of Power Consumption Dynamic Power ( Main type of Power Consumption) Short Circuit Power Static Power [1] –Leakage –Sub-threshold

Power Saving Schemes at Different Levels Transistor Level [Decreasing Transistor & Interconnect Capacitances] Gate-Level [Input Ordering, Tree Vs. Chain] Logic Level [MCML (Low Voltage Swing), Domino (Small Device Count)] Architectural Level [Parallelism, Pipelining, etc] Can Save the Most Power for Suitable Applications [2]

Pipelining to Save Power P Dynamic = C * f * V DD 2 * Alpha [3] Decreasing VDD has the largest Impact on Decreasing Dynamic Power Decreasing VDD should also decrease Leakage Power Sub-Threshold & Short-Circuit (up or down) Power Dissipation might increase, due to the Slightly Increased Device Count (Pipe-Line Registers) Decreasing VDD will also slow down your Circuit, But With Pipelining & Parallelism, This Loss of Speed Can be Compensated.

Pipeline Operation Illustrated

Idea behind Pipelining for Power Saving Pipelining Utilizes Parallelism to Boost the Throughput of the Non-Pipelined Circuit The Throughput Boost can be Nullified by Decreasing VDD of the Pipelined Circuit (The Pipelined Circuit Now has Roughly the Same Throughput as the Non- Pipelined Circuit) But the Decreased VDD  Decreased Dynamic Power Consumption

16 bit value in Read Register 2 Pipelined Data Path for a RISC Micro-Processor

Actual Circuit Utilized To Analyze Pipelining as a Viable Power Saving Scheme A 32-Bit Shift Register –Not Large Scale, Transparent to Implement –32 Flip-Flops, Pipelined to 4 Stages, requiring 3 Extra Flip-flops, with Each Extra Flip-Flop Serving as the Corresponding Pipe- Line Register –Power Ratio is 10+ : 1 (Possibly 1 of the Better Cases, Almost Trivializing the Power by the Pipeline Registers), So Power Saved by Decreasing VDD, should Substantially Out-Weight the Extra Power of the Extra Flip-Flops –Power Ratio Comparable to that of a VLSI with its necessary Pipe-Lined Registers (the # of the FF ‘s Required Generally proportional to the Size of the VLSI Circuit) –Parallel Version, Parallel + Pipelined Version –Layout of the Flip-Flop For Power/Area, Simulation/Estimation –Interested in the Relative % (Should be Applicable to a Bigger Picture) Power Saved

Architecture Analyzed Plain Shift-Register –32 Flip-Flops –VDD at Max (2.5 or 3V for CMOSP18) –Input Rate == 1 Bit Inputted (Processed) Every 32 Clock Cycles –Clock Period decreased to find out the Maximum Operating Frequency (By Looking at Waveform Quality, and Voltage Swing) –Throughput = Input Rate * Frequency = (1 Bit/ 32 Cycles) * (f cycles/second) = x Bit/Second

Architecture Analyzed Pipelined Shift-Reg –35 Flip-Flops –Input Rate == 1 Bit Inputted Every 8 Clock Cycles –VDD, f initially same as that of Plain Version, then Drop to Achieve the same Through-Put 8 Flip-Flops 1 1 1

Architecture Analyzed Parallel Shift-Reg –64 Flip-Flops, 1 Demux, 1 Mux –Input Rate = 2 Bits Inputted Every 32 Clock- Cycles –VDD, f initially same as that of Plain Version, then Drop to Achieve the same Through-Put 32 Flip-Flops De-Mux M u X

Architecture Analyzed Pipelined + Parallel –70 Flips-Flops, 1 Mux, 1 DeMux –Input Rate = 2 Bits Every 8 Clock Cycles –VDD, f initially same as that of Plain Version, then Drop to Achieve the same Through-Put 8 Flip-Flops

Summary The Effectiveness of Architectural Approaches (Pipelining, Full-Parallelism, etc) as Viable Power-Saving Schemes for Digital IC ‘s, will be Simulated on a Smaller Scale. The Resulting Relative Percentage Power- Saved, should be Applicable on a Grander Scale. Pipelining An Average VLSI circuit, May need more than 10% of Hardware/Power for the Pipe- Line Registers (Flip-Flops)

Time Table Feb 1  March 1: Literature Survey March 8  March 12 : Layout March 14  March 18: Simulating Serial & Pipelined Versions Mach 19  March 23: Simulating Parallel & The Combo Version March 24  End of March: Preparing for the Final Presentation April 1 st  April 15: Write up the Final Report

References [1]. Jan. M Raebaey, “Digital Integrated Circuits”, 2 nd Ed., Prentice Hall, 2003 [2]. Jerry Frenkil, “A Multi-Level Approach to Low- Power IC Design”, IEEE Spectrum, Vol 35, Number 2, 1998 [3]. Anantha P. Chandrakasan, “Low Power CMOS Digital Design, IEEE Journal of Solid State Circuits, pp , 1992 [4]. K.K. Parhi, "Low-Power Digital VLSI Approaches", Chapter in Circuits and Systems in the Information Age, Edited by Y. Huang and C. Wei, pp. 3-22, IEEE Press, June 1997 (ISCAS-97 Tutorial Book)

Aside Portability: If your portable device is very power hungry, and Knowing the limited advancement there has been/will be in terms of Battery Capacity, one would need a Very Large Battery to expect it to keep going and going. Intel CPUs getting hotter and hotter than they used to be, and Average House hold Maybe able to afford a CPU, but not necessarily something as Drastic as a Vapor Cooling Computer Case. Application Suitability for Pipelining-For-Power-Saving: 1) Power Consumption of the VLSI being pipelined, must >> the Power Consumption of the Pipeline Registers. 2) Large & Complex Data Dependency  Large & Complex 3) Huge Discrepancy between the delays of the Pipeline stages ( clock Cycles)