Jennifer Winikus Computer Engineering Seminar Michigan Technological University February 10,2011 2/10/2011J Winikus EE5900 1

Presentation based on : 2/10/2011J Winikus EE Gregory Chen, Dennis Sylvester, David Blaauw, and Trevor Mudge IEEE Transactions on Very Large Scale Integration(VLSI) Systems, Vol. 18 No. 11, November 2010

 SRAM Performance Background  SRAM 6T and 8T designs  Experiment Methodology  Experimental Results  Conclusions 2/10/2011J Winikus EE5900 3

 Reduction in energy consumption is a high priority objective for electronics  RAM cells are optimized by utilizing transistors instead of flip flops to improve density  Transistors have increase in “failure” as V DD is minimized near and below V TH  Simulations of 8T and 6T SRAM at various sizes, V DD, V TH performed to analyze for future applications 2/10/2011J Winikus EE5900 4

 RAM is the acronym for Random Access Memory  Utilized by computers and digital devices as a temporary memory storage to store data as it is processed  RAM and Processor speeds are the main components in the speed of a computer except for saving functions that the hard drive capabilities factor in 2/10/2011J Winikus EE5900 5

 Acronym for Static Random Access Memory  Denser then flip flops  Faster then DRAM  Holds the data as long as power is applied  Form arrays on the RAM chips 2/10/2011J Winikus EE5900 6

 Three power regions exist for Transistors: triode, saturation and cut-off  Near Threshold region is classified as between 400 and 700 mV 2/10/2011J Winikus EE Power Regions in MOSFET

 The shifting of V TH for each transistor independently  Reduces Static Noise Margin  Causes mismatches within cells  More predominate influence on system at smaller V DD  Robustness is reduced with Random Dopant Fluctuation 2/10/2011J Winikus EE Chen, Gregory, et al. "Yield-Driven Near-Threshold SRAM Design." IEEE TRANSACTIONS ON VERY LARGE SCALE INTEGRATION (VLSI) SYSTEMS (2010):

 Researchers at MIT have established that in general the relationship between total energy, energy leakage and Voltage in terms of the energy per ALU cycle  ALU cycles are computer architecture’s actions  ALU’s include read and write 2/10/2011J Winikus EE Advances in Ultra-Low-Voltage Design Joyce Kwong, Anantha P. Chandrakasan, Massachusetts Institute of Technology

 Designs are based around desired read and write speeds and stability  Cross coupled invertors are responsible for holding the state 2/10/2011J Winikus EE

 The voltage threshold in the MOSFET in which below the threshold the transistor is essentially off, only leakage current passes between the drain and source 2/10/2011J Winikus EE

 Read is performed by prechargin the cell and floating the bitline  Write is performed by driving opposite values to the bitline overriding the wordline 2/10/2011J Winikus EE Chen, Gregory, et al. "Yield-Driven Near-Threshold SRAM Design." IEEE TRANSACTIONS ON VERY LARGE SCALE INTEGRATION (VLSI) SYSTEMS (2010):

 Read Upset  The voltage applied to the node holding the zero value is flipped due to noise  Dependent on the pass gate strength  Write  Timing  Hold  Static Noise Margin over powers the cross- coupled invertors causing the state being stored to flip 2/10/2011J Winikus EE

 Useful for independent read-write ports, done by having adding two additional stacked negative channel FETS to the 6 cell to isolate the read and write ports from each other 2/10/2011J Winikus EE Chen, Gregory, et al. "Yield-Driven Near-Threshold SRAM Design." IEEE TRANSACTIONS ON VERY LARGE SCALE INTEGRATION (VLSI) SYSTEMS (2010):

 The addition of a separate Read Bitline eliminates the read failure that occurs in the 6T cell  Benefit of having the separate ports is that the remaining 6 transistors can be sized and doped to get the desired write stability  Failure mechanisms are otherwise the same as the 6T 2/10/2011J Winikus EE

 An alternative to sizing modification  The premise is the modification of voltage thresholds for behaviors such that failures are prevented  In a duel port structure the word line voltage can be reduced when read process is performed  The implementation is done through addition of diodes  More complex drivers and decoding is needed with this methodology 2/10/2011J Winikus EE

 Modification of the threshold voltage of the transistors for the design specific goals  In application the tuning of MOSFETs are done by exciting dopants in the drain and source with laser pulses to achieve desired threshold voltages 2/10/2011J Winikus EE

 8T SRAM is typically 33% larger then a 6T cell  V TH is optimized separately for the SRAM and the Logic to improve robustness and performance  Sizing alterations can be minimized by utilizing Assist Circuits 2/10/2011J Winikus EE

 The experiment methodology is an incremental conditioning structure  Only Word Line Drivers, Bit Line Drivers and bit cells are considered 2/10/2011J Winikus EE Chen, Gregory, et al. "Yield-Driven Near- Threshold SRAM Design." IEEE TRANSACTIONS ON VERY LARGE SCALE INTEGRATION (VLSI) SYSTEMS (2010):

 Monte Carlo  To account for high yeild systems like 8-kb SRAM with a 99% yeild, the failure rate is on the order of 10 -7, which requires atleast 10 million simulations  SRAM with caches larger then 8-kb have smaller failure rates in which more simulations would be needed  Definition: Monte Carlo is the art of approximating an expectation by the sample mean of a function of simulated random variables(UC Berkley)  Importance Sampling  A unique probability density function is chosen for each transistor  Choosing a relative range to choose the samples from 2/10/2011J Winikus EE

 Based on the percent error seen over many iterations, using the importance sampling 20,000 samples is sufficient for accurate results  Monte Carlo sampling would require samples for accurate results 2/10/2011J Winikus EE Chen, Gregory, et al. "Yield-Driven Near-Threshold SRAM Design." IEEE TRANSACTIONS ON VERY LARGE SCALE INTEGRATION (VLSI) SYSTEMS (2010):

 The selected threshold voltages are chosen by choosing random values from within the sample probability distribution in which the normal distribution is shifted to induce a higher probability of failure  Selectively choosing the region speeds up analysis by eliminating samples from regions that are not of interest of the experiment 2/10/2011J Winikus EE Chen, Gregory, et al. "Yield-Driven Near-Threshold SRAM Design." IEEE TRANSACTIONS ON VERY LARGE SCALE INTEGRATION (VLSI) SYSTEMS (2010):

 Measured using Static Noise Margin or Corner Cases  Determines if optimization is complete 2/10/2011J Winikus EE

2/10/2011J Winikus EE Chen, Gregory, et al. "Yield-Driven Near-Threshold SRAM Design." IEEE TRANSACTIONS ON VERY LARGE SCALE INTEGRATION (VLSI) SYSTEMS (2010):

 Activity Factor is the average fraction of bit cells accessed per cycle  V min is the voltage in which the energy needed per operation is minimized 2/10/2011J Winikus EE

 Robustness is controlled to be constant through the modification of the size of the Bit Cells.  This allows for V TH tuning 2/10/2011J Winikus EE Chen, Gregory, et al. "Yield-Driven Near-Threshold SRAM Design." IEEE TRANSACTIONS ON VERY LARGE SCALE INTEGRATION (VLSI) SYSTEMS (2010):

 L1- small in size, high in activity, total energy is mostly dynamic  L2- larger in size then L1, lower activity 2/10/2011J Winikus EE

2/10/2011J Winikus EE Chen, Gregory, et al. "Yield-Driven Near-Threshold SRAM Design." IEEE TRANSACTIONS ON VERY LARGE SCALE INTEGRATION (VLSI) SYSTEMS (2010): The energy reduction capability by tuning is most significantly seen in the 8T, a 61% reduction, including sizing as well results in an 83% energy reduction for the 8T cell

 The activity behavior of the circuit is a substantial controlling factor in V min and E min 2/10/2011J Winikus EE Chen, Gregory, et al. "Yield-Driven Near-Threshold SRAM Design." IEEE TRANSACTIONS ON VERY LARGE SCALE INTEGRATION (VLSI) SYSTEMS (2010):

 The relationship produced between activity factor and V min demonstrates that the less activity the higher the V min 2/10/2011J Winikus EE Chen, Gregory, et al. "Yield-Driven Near-Threshold SRAM Design." IEEE TRANSACTIONS ON VERY LARGE SCALE INTEGRATION (VLSI) SYSTEMS (2010):

 Of the components that assist circuit has been applied the only is beneficial in the sub threshold region for the overdriven word line 2/10/2011J Winikus EE Chen, Gregory, et al. "Yield-Driven Near-Threshold SRAM Design." IEEE TRANSACTIONS ON VERY LARGE SCALE INTEGRATION (VLSI) SYSTEMS (2010):

 Performance is compromised with voltage reduction, but is minimized with the use of assists 2/10/2011J Winikus EE Chen, Gregory, et al. "Yield-Driven Near-Threshold SRAM Design." IEEE TRANSACTIONS ON VERY LARGE SCALE INTEGRATION (VLSI) SYSTEMS (2010):

 The parabolic response of the energy cost per bit access for the write line displays the correspondence with the V min as below the approximate value the energy increases again  Below about 650 mV assist drooping is no longer considered to assist the circuit  Leakage increases in a near exponential behavior below V min 2/10/2011J Winikus EE Chen, Gregory, et al. "Yield-Driven Near-Threshold SRAM Design." IEEE TRANSACTIONS ON VERY LARGE SCALE INTEGRATION (VLSI) SYSTEMS (2010):

 Assist circuit design benefits in this result for minimum voltage capabilities  Energy cost increases with activity 2/10/2011J Winikus EE Chen, Gregory, et al. "Yield-Driven Near-Threshold SRAM Design." IEEE TRANSACTIONS ON VERY LARGE SCALE INTEGRATION (VLSI) SYSTEMS (2010):

 With low failure rates, Importance Sampling is more efficient then Monte Carlo Sampling  Scaling to 300mV for 8T cells achieves 83% energy reduction  In effort to reduce power and maintain function-ability size is the compromise or through the addition of assist circuit  Assist modification of certain functional components is beneficial in failure prevention as voltage decreases 2/10/2011J Winikus EE

 In the computer based 21 st Century there is the drive for faster computing, smaller devices and less power consumption  The results display that at this point tuning threshold voltages is the most promising advancement to forward the consumer desires  The results express the trade off that exists, size or power, speed or power 2/10/2011J Winikus EE

 Chen, Gregory, et al. "Yield-Driven Near-Threshold SRAM Design." IEEE TRANSACTIONS ON VERY LARGE SCALE INTEGRATION (VLSI) SYSTEMS (2010):  R.P. Lu, A.D. Ramirez, B.W. Offord, and S.D. Russell of SPAWAR Systems Center Pacific for the Office of Naval Research.“Threshold Voltage Tuning of Metal-Gate MOSFETs Using an Excimer Laser”.  Advances in Ultra-Low-Voltage Design.Joyce Kwong, Anantha P. Chandrakasan, Massachusetts Institute of Technology bac26c8/index.jsp?&pName=sscs_level1_article&path=sscs/08Fall&f ile=Kwong.xml&xsl=article.xsl bac26c8/index.jsp?&pName=sscs_level1_article&path=sscs/08Fall&f ile=Kwong.xml&xsl=article.xsl  Eric C. Anderson, 1999, UC Berkeley. “Lecture Notes for Stat 578C” tes.pdf 2/10/2011J Winikus EE

2/10/2011J Winikus EE