Benton H. Calhoun Jan M. Rabaey Low Power Design Essentials ©2008 Chapter 9 Optimizing Standby Memory

Low Power Design Essentials © Chapter Outline  Memory in Standby  Voltage Scaling  Body Biasing  Periphery

Low Power Design Essentials © Memory Dominates Processor Area  SRAM is a major source of static power in ICs, especially for low power applications  Special memory requirement: need to retain state in standby  Metrics for standby: –1. Leakage power –2. Energy overhead for entering/leaving standby –3. Timing/area overhead BL WL M1 M2 M3 M4 M5 M6 Q QB

Low Power Design Essentials © Reminder of “Design Time” Leakage Reduction  Design-time techniques (Ch 7) also impact leakage –High V TH transistors –Different precharge voltages –Floating BLs  This Chapter: adaptive methods that uniquely address memory standby power

Low Power Design Essentials © The Voltage Knobs  Changing internal voltages has different impact on leakage of various transistors in cell  Voltage changes accomplished by playing tricks with peripheral circuits [Ref: Y. Nakagome, IBM’03] Offset voltage,  (V) Leakage reduction (ratio) L = 90 nm, t OX = 2 nm V DD = 1 V S = 100 mV/decade K = 0.2 V 1/2, 2  = 0.6 V = 0.05 V DD 0 0 -- 0 0 -- ++ 0 V DD -  (DIBL) NMOS V DD 0 0 ++ C B1 B2 A1 A2

Low Power Design Essentials © Lower V DD in Standby  Basic Idea: Lower V DD lowers leakage –sub-threshold leakage –GIDL –gate tunneling  Question: What sets the lower limit? [Ref: K. Flautner, ISCA ’02] V DD V DDlow V DD_SRAM drowsy SRAM V DD V DDH V DDL Active mode Standby mode Example

Low Power Design Essentials © Limits to V DD Scaling: DRV Data Retention Voltage (DRV): Voltage below which a bitcell loses its data That is, the supply voltage at which the Static Noise Margin (SNM) of the SRAM cell in standby mode reduces to zero. [Ref: H. Qin, ISQED ’04] V (V) V 2 VTC 1 2 V DD =0.18V V DD =0.4V 130 nm CMOS

Low Power Design Essentials © Power savings of DRV Supply Voltage (V) Leakage Current (μA) Measured DRV range More than 90% reduction in leakage power with 350mV standby V DD (100mV guard band). Test chip in 130 nm CMOS technology with built-in voltage regulator 1.4 mm IP Module of 4kB SRAM [Ref: H. Qin, ISQED’04]

Low Power Design Essentials © DRV and Transistor Sizes Width Scaling Factor DRV (mV) MaMa MpMp MnMn Model With M a, M p and M n the access transistor, PMOS pull-up and NMOS pull-down, respectively [Ref: H. Qin, Jolpe ’06]

Low Power Design Essentials © Impact of Process “Balance” Stronger PMOS or NMOS (S P,S N ) in sub- threshold lowers SNM even for typical cell [Ref: J. Ryan, GLSVLSI’07]

Low Power Design Essentials © Impact of Process Variations on DRV DRV Spatial Distribution DRV histogram for 32 kBit SRAM  DRV varies widely from cell to cell  Most variations random with some systematic effects (e.g. module boundaries)  DRV histogram has long tail 130 nm CMOS [Ref: H. Qin, ISQED’04]

Low Power Design Essentials © Impact of Process Variations on DRV [Ref: J. Wang, CICC’07] DRV (mV) Frequency nm tail90 nm tail Other sources of variation: Global variations, data values, temperature (weak), bit-line voltage (weak ) DRV distribution for 90 nm and 45 nm CMOS © IEEE 2007

Low Power Design Essentials © DRV Statistics for an Entire Memory  DRV distribution is neither normal nor lognormal  CDF model of DRV distribution (F DRV (x) = 1- P(SNM < 0, V DD =x)) [Ref: J. Wang, ESSCIRC 2007] Worst DRV (mV) Memory size  Model Normal LogNormal Monte-Carlo © IEEE 2007

Low Power Design Essentials © Reducing the DRV Chip DRV 1.Cell optimization 2.ECC (Error Correcting Codes) 3.Cell optimization + ECC

Low Power Design Essentials © Lowering the DRV Using ECC Error Correction Challenges  Maximize correction rate  Minimize timing overhead  Minimize area overhead  Hamming [31, 26, 3] achieves 33% power saving  Reed-Muller [256, 219, 8] achieves 35% power saving  Hamming [31, 26, 3] achieves 33% power saving  Reed-Muller [256, 219, 8] achieves 35% power saving Data P Write Read ECC Encoder ECC Encoder ECC Decoder ECC Decoder Data In Data Out SRAM with ECC D P Data Correction [Ref: A. Kumar, ISCAS’07]

Low Power Design Essentials © Combining Cell Optimization and ECC A B C D 650mV 320mV 255mV 50X Standard Optimized Optimized+ECC V DD (V) Normalized SRAM leakage current Original SRAM Optimized SRAM w/ ECC [Ref: A. Kumar, ISCAS’07]

Low Power Design Essentials © How to Approach the DRV Safely? Core Cells Failure Detectors Sub-V T Controller V DD VCTRL voltages “1” “0” “1” “0” Adjustable Power Supply Reset Using “canary cells” to set the standby voltage in closed-loop [Ref: J. Wang, CICC’07]

Low Power Design Essentials © How to Approach the DRV Safely? Multiple sets of canary cells [Ref: J. Wang, CICC’07] 128Kb SRAM ARRAY Canary Replica & test circuit 0.6% area overhead in 90nm test chip Mean DRV of Canary Cells (V) More reliable Less power Failure Threshold SRAM cell DRV Histogram VCTRL(V) © IEEE 2007

Low Power Design Essentials ©  Raise bitcell V SS in standby (e.g. 0 to 0.5V)  Lower BL voltage in standby (e.g. 1.5V to 1V) Raising V SS [Ref: K. Osada, JSSC’03] Lower voltage  less gate leakage and GIDL ‘0’ is 0.5V Lower V DS  less sub- V TH leakage (DIBL) Negative V BS  reduces sub-V TH leakage 1.0V WL=0V 1.5V 0.5V ‘0’ ‘1’

Low Power Design Essentials © Body Biasing  Reverse Body Bias (RBB) for leakage reduction –Move FET source (as in raised V SS ) –Move FET body  Example: Whenever WL is low, apply RBB 0V V DD 0V V DD 0V V DD 2V DD -V DD ActiveStandby WL V DD,V SS V PB,V NB BL BLB WL V DD V SS V PB V NB [Ref: H. Kawaguchi, VLSI Symp. 98]

Low Power Design Essentials © Combining Body Biasing and Voltage Scaling 0V V DD 0V V DD 0V V DD 2V DD -V DD ActiveStandby WL V DD,V SS V PB,V NB BL BLB WL V DD V SS V PB V NB [Ref: A. Bhavnagarwala, SOC’00]

Low Power Design Essentials © Combining Raised VSS and RBB 28X savings in standby power reported BL BLB WL V DD V SS [Ref: L. Clark, TVLSI’04] V PB V NB

Low Power Design Essentials © Voltage Scaling in and Around the Bitcell [1] K. Osada et al. JSSC 2001 [2] N. Kim et al. TVLSI 2004 [3] H. Qin et al. ISQED 2004 [4] K. Kanda et al. ASIC/SOC 2002 [5] A. Bhavnagarwala et al. SymVLSIC 2004 [6] T. Enomoto et al. JSSC 2003 [7] M. Yamaoka et al. SymVLSIC 2002 [8] M. Yamaoka et al. ISSC 2004 [9] A. Bhavnagarwala et al. ASIC/SOC 2000 [10] K. Itoh et al. SymVLSIC 1996 [11] H. Yamauchi et al. SymVLSIC 1996 [12] K. Osada et al. JSSC 2003 [13] K. Zhang et al. SymVLSIC 2004 [14] K. Nii et al. ISSCC 2004 [15] A. Agarwal et al. JSSC 2003 [16] K. Kanda et al. JSSC 2004 Large number of reported techniques

Low Power Design Essentials © Periphery Breakdown  Periphery leakage often not ignorable –Wide transistors to drive large load capacitors –Low V TH transistors to meet performance specs  Chapter 8 techniques for logic leakage reduction equally applicable, but …  Task made easier than for generic logic because of well-defined structure and signal patterns of periphery –e.g. decoders output 0 in standby  Lower peripheral V DD can be used, but need fast level-conversion to interface with array

Low Power Design Essentials © Summary and Perspectives  SRAM standby power is leakage dominated  Voltage knobs are effective to lower power  Adaptive schemes must account for variation to allow outlying cells to function  Combined schemes are most promising –e.g. Voltage scaling and ECC  Important to assess overhead! –Need for exploration and optimization framework, in the style we have defined for logic

Low Power Design Essentials © References Books and Book Chapters:  K. Itoh, M. Horiguchi, and H. Tanaka, Ultra-Low Voltage Nano-Scale Memories, Springer  T. Takahawara and K. Itoh, “Memory Leakage Reduction,” in Leakage in Nanometer CMOS Technologies, S. Narendra, Ed, Chapter 7, Springer Articles:  A. Agarwal, L.Hai, K. Roy, “A single-V/sub t/ low-leakage gated-ground cache for deep submicron,” IEEE Journal of Solid State Circuits, pp , Febr  A. Bhavnagarwala, A. Kapoor, A.; J. Meindl, “Dynamic-threshold CMOS SRAM cells for fast, portable applications,” Proceedings IEEE ASIC/SOC Conference, pp , Sept  A. Bhavnagarwala et all, “A transregional CMOS SRAM with single, logic V/sub DD/ and dynamic power rails,” Proceedings IEEE VLSI Circuits Symposium, pp , June  L. Clark., M. Morrow, and W. Brown, “Reverse-body bias and supply collapse for low effective standby power,” IEEE Transactions on VLSI, pp , Sep  T. Enomoto, Y. Ota, and H. Shikano, “A self-controllable voltage level (SVL) circuit and its low- power high-speed CMOS circuit applications, “ IEEE Journal of Solid State Circuits, “ Vol. 38, Issue 7, pp , July  K. Flautner et al., “Drowsy Caches: Simple Techniques for Reducing Leakage Power., Proceedings ISCA 2002, pp , Anchorage, May  K. Itoh et al, “A deep sub-V, single power-supply SRAM cell with multi-VT, boosted storage node and dynamic load, Proceedings VLSI Circuits Symposium, pp , June,1996.  K. Kanda, T. Miyazaki, S. Min, H. Kawaguchi, T. Sakurai, “Two orders of magnitude leakage power reduction of low voltage SRAMs by row-by-row dynamic Vdd control (RRDV) scheme,” Proceedings IEEE ASIC/SOC Conference, pp , Sept

Low Power Design Essentials © References (cntd)  K. Kanda, et al., “90% write power-saving SRAM using sense-amplifying memory cell,” IEEE Journal of Solid-State Circuits, pp.927 – 933, June 2004  H. Kawaguchi, Y. Itaka and T. Sakurai, “Dynamic Leakage Cut-off Scheme for Low-Voltage SRAMs,” Proceedings VLSI Symposium, pp , June  A. Kumar et al, “Fundamental Bounds on Power Reduction during Data-Retention in Standby SRAM,” Proceedings ISCAS 2007, pp , May  N.Kim, K. Flautner, D. Blaauw, and T. Mudge, “Circuit and microarchitectural techniques for reducing cache leakage power,” IEEE Transactions on VLSI, pp , Feb  Y. Nakagome et al.. “Review and prospects of low-voltage RAM circuits,” IBM J. R & D, vol. 47. no. 516, pp , Sep. /Nov  K. Osada, “Universal-Vdd V 32-kB cache using a voltage-adapted timing-generation scheme and a lithographically symmetrical cell, “ IEEE Journal of Solid State Circuits, pp , Nov  K. Osada et al, “16.7-fA/cell tunnel-leakage-suppressed 16-Mb SRAM for handling cosmic-ray- induced multierrors,” IEEE Journal of Solid State Circuits, pp , Nov  H. Qin, et al., “SRAM leakage suppression by minimizing standby supply voltage,” Proceedings ISQED, pp ,  H. Qin, R. Vattikonda, T.Trinh, Y. Cao, and J. Rabaey, “SRAM Cell Optimization for Ultra-Low Power Standby,” Journal on Low Power Electronics, Vol. 2 No3, pp. 401–411, December  J. Ryan, J. Wang, and B. Calhoun, "Analyzing and Modeling Process Balance for Sub-threshold Circuit Design“ Proceedings GLSVLSI, pp , March  J. Wang and B. Calhoun, "Canary Replica Feedback for Near-DRV Standby VDD Scaling in a 90nm SRAM“, Proceedings Custom Integrated Circuits Conference (CICC), pages 29-32, September 2007.

Low Power Design Essentials © References (cntd)  J. Wang, A. Singhee, R. Rutenbar, and B. Calhoun, "Statistical Modeling for the Minimum Standby Supply Voltage of a Full SRAM Array“, Proceedings European Solid State Circuits Conference (ESSCIRC), pages , September  M. Yamaoka et al. “0.4-V logic library friendly SRAM array using rectangular-diffusion cell and delta-boosted-array-voltage scheme, Proceedings VLSI Circuits Symposium, pp , June  M. Yamaoka, et al, “A 300MHz 25/spl  A/Mb leakage on-chip SRAM module featuring process- variation immunity and low-leakage-active mode for mobile-phone application processor,” Proceedings IEEE Solid-State Circuits Conference, pp , Febr  K. Zhang et al., “SRAM design on 65nm CMOS technology with integrated leakage reduction scheme,” Proceedings VLSI Circuits Symposium, 2004, pp , June 2004.