ECE 260B – CSE 241A Design Styles 1http://vlsicad.ucsd.edu ECE260B – CSE241A Winter 2005 Design Styles Multi-Vdd/Vth Designs Website:

ECE 260B – CSE 241A Design Styles 2http://vlsicad.ucsd.edu The Design Problem Source: sematech97 A growing gap between design complexity and design productivity

ECE 260B – CSE 241A Design Styles 3http://vlsicad.ucsd.edu Design Methodology Design process traverses iteratively between three abstractions: behavior, structure, and geometry More and more automation for each of these steps

ECE 260B – CSE 241A Design Styles 4http://vlsicad.ucsd.edu Behavioral Description of Accumulator Design described as set of input-output relations, regardless of chosen implementation Data described at higher abstraction level (“integer”)

ECE 260B – CSE 241A Design Styles 5http://vlsicad.ucsd.edu Structural Description of Accumulator Design defined as composition of register and full-adder cells (“netlist”) Data represented as {0,1,Z} Time discretized and progresses with unit steps Description language: VHDL Other options: schematics, Verilog

ECE 260B – CSE 241A Design Styles 6http://vlsicad.ucsd.edu Implementation Methodologies

ECE 260B – CSE 241A Design Styles 7http://vlsicad.ucsd.edu Full Custom  Hand drawn geometry  All layers customized  Digital and analog  Simulation at transistor level  High density  High performance  Long design time Magic Layout Editor (UC Berkeley)

ECE 260B – CSE 241A Design Styles 8http://vlsicad.ucsd.edu Symbolic Layout Stick diagram of inverter Dimensionless layout entities Only topology is important Final layout generated by “compaction” program

ECE 260B – CSE 241A Design Styles 9http://vlsicad.ucsd.edu Standard Cells Routing channel requirements are reduced by presence of more interconnect layers  Organized in rows  Cells made as full custom by vendor (not user)  All layers customized  Digital with possible special analog cells  Simulation at gate level (digital)  Medium-high density  Medium-high performance  Reasonable design time

ECE 260B – CSE 241A Design Styles 10http://vlsicad.ucsd.edu Standard Cell — Example [Brodersen92]

ECE 260B – CSE 241A Design Styles 11http://vlsicad.ucsd.edu Standard Cell - Example 3-input NAND cell (from Mississippi State Library) characterized for fanout of 4 and for three different technologies

ECE 260B – CSE 241A Design Styles 12http://vlsicad.ucsd.edu Automatic Cell Generation Random-logic layout generated by CLEO cell compiler (Digital)

ECE 260B – CSE 241A Design Styles 13http://vlsicad.ucsd.edu Module Generators — Compiled Datapath

ECE 260B – CSE 241A Design Styles 14http://vlsicad.ucsd.edu Macrocell-Based Design Macrocell Interconnect Bus Routing Channel  Predefined macro blocks (uP, RAM, etc.)  Macro blocks made as full custom by vendor (IP blocks)  All layers customized  Digital and some analog  Simulation at behavior or gate level  High density  High performance  Short design time  Use standard on-chip busses  “System on a chip” (SOC)

ECE 260B – CSE 241A Design Styles 15http://vlsicad.ucsd.edu Macrocell Design Methodogoly Video-encoder chip [Brodersen92] SRAM Routing Channel Data paths Standard cells Floorplan: Defines overall topology of design, relative placement of modules, and global routes of busses, supplies, and clocks

ECE 260B – CSE 241A Design Styles 16http://vlsicad.ucsd.edu Gate Array  Predefined transistors connected via metal  Two types: channel based, sea of gates  Only metal layers customized  Fixed array sizes  Digital cells in library  Simulation at gate level (digital)  Medium density  Medium performance  Reasonable design time

ECE 260B – CSE 241A Design Styles 17http://vlsicad.ucsd.edu Gate Array — Primitive Cells Uncommited Cell Committed Cell (4-input NOR)

ECE 260B – CSE 241A Design Styles 18http://vlsicad.ucsd.edu Sea-of-gate Primitive Cells Using oxide-isolationUsing gate-isolation

ECE 260B – CSE 241A Design Styles 19http://vlsicad.ucsd.edu Sea-of-gates Random Logic Memory Subsystem LSI Logic LEA300K (0.6  m CMOS)

ECE 260B – CSE 241A Design Styles 20http://vlsicad.ucsd.edu Prewired Arrays  Programmable logic blocks  Programmable connections between logic blocks  No layers customized (standard devices)  Digital only  Low-medium performance  Low-medium density  Programmable: SRAM, EPROM, Flash, Anti-fuse, etc.  Easy and quick design changes  Cheap design tools  Low development cost  High device cost  NOT a real ASIC Courtesy Altera Corp.

ECE 260B – CSE 241A Design Styles 21http://vlsicad.ucsd.edu Programmable Logic Devices PLAPROM PAL

ECE 260B – CSE 241A Design Styles 22http://vlsicad.ucsd.edu EPLD Block Diagram Macrocell Courtesy Altera Corp. Primary inputs

ECE 260B – CSE 241A Design Styles 23http://vlsicad.ucsd.edu Field-Programmable Gate Arrays - Fuse-based Standard-cell like floorplan

ECE 260B – CSE 241A Design Styles 24http://vlsicad.ucsd.edu Interconnect Programming interconnect using anti-fuses

ECE 260B – CSE 241A Design Styles 25http://vlsicad.ucsd.edu Field-Programmable Gate Arrays - RAM-based

ECE 260B – CSE 241A Design Styles 26http://vlsicad.ucsd.edu RAM-based FPGA - Basic Cell (CLB) Courtesy of Xilinx

ECE 260B – CSE 241A Design Styles 27http://vlsicad.ucsd.edu RAM-based FPGA Xilinx XC4025

ECE 260B – CSE 241A Design Styles 28http://vlsicad.ucsd.edu High Performance Devices  Mixture of full custom, standard cells and macro’s  Full custom for special blocks: Adder (data path), etc.  Macro’s for standard blocks: RAM, ROM, etc.  Standard cells for non critical digital blocks

ECE 260B – CSE 241A Design Styles 29http://vlsicad.ucsd.edu Global Signaling and Layout  Global signaling and layout optimization  Multi-V dd  Static power analysis  Multi-V th + V dd + sizing D. Sylvester, DAC-2001

ECE 260B – CSE 241A Design Styles 30http://vlsicad.ucsd.edu Global Signaling  Current global signaling paradigm  insert large static CMOS repeaters to reduce wire RC delay  Impending problems: l Too many repeaters -180nm processors: 22K repeaters (Itanium), 70K (Power4) -Project 1-1.5M repeaters at 45-65nm technologies l Too much power -Many large repeaters = significant static and dynamic power l Too much noise -Repeater clustering complicates power distribution -Inductive coupling across wide bus structures D. Sylvester, DAC-2001

ECE 260B – CSE 241A Design Styles 31http://vlsicad.ucsd.edu Cell Layout Optimization  Advanced layout techniques must allow l Continuous individual device sizing l Variable p/n ratios l Tapered FET stacking sizes l Arbitrary V th assignments within gates  First cut: Cadabra  15-22% power reduction using 1 st two approaches under fixed footprint constraint GDSII Import Compact fixed width Ref: Hurat, Cadabra Optimize specific instances of standard gates D. Sylvester, DAC-2001

ECE 260B – CSE 241A Design Styles 32http://vlsicad.ucsd.edu Multi-Vdd  Global signaling and layout optimization  Multi-V dd  Static power analysis  Multi-V th + V dd + sizing D. Sylvester, DAC-2001

ECE 260B – CSE 241A Design Styles 33http://vlsicad.ucsd.edu Multi-Vdd Status  Idea: Incorporate two V dd ’s to reduce dynamic power  Limited to a few recent Japanese multimedia processors Example – 0.3  m, 75MHz, 3.3V media processor (Toshiba) -Total power savings of 47% in logic, 69% in clock l Dynamic voltage scaling of mobile processors -Transmeta Crusoe, Intel Speedstep, etc. -Not considered in this talk  Very powerful technique currently applied only in low-performance designs l Mentality: today’s high performance parts aren’t “limited” by power D. Sylvester, DAC-2001

ECE 260B – CSE 241A Design Styles 34http://vlsicad.ucsd.edu Lower Power Via Rich Replacement  Media processors and other low speed designs have many non-critical paths l 60-70% of paths have delay  half the clock period l After replacement, most paths become near critical  What about high-speed microprocessors? % of total paths Path delay (normalized to clock period) D. Sylvester, DAC-2001

ECE 260B – CSE 241A Design Styles 35http://vlsicad.ucsd.edu Similar Story For High-Performance  IBM 480 MHz PowerPC shows over 50% of paths have delay less than half the clock period l Implies that high-performance designs can benefit from multi-V dd Ref: Akrout, JSSC98 D. Sylvester, DAC-2001

ECE 260B – CSE 241A Design Styles 36http://vlsicad.ucsd.edu Resizing Is Not The Right Answer  Post-synthesis optimizations resize gates to recover power on non-critical paths l Looks similar to pre- and post-replacement figures in media processor… Before post- synthesis resizing After post- synthesis resizing Ref: Sirichotiyakul, DAC99 This is the wrong approach for nanometer design! D. Sylvester, DAC-2001

ECE 260B – CSE 241A Design Styles 37http://vlsicad.ucsd.edu Multi-Vdd Instead of Sizing  Power ~ C V dd 2 f, where f is fixed  Key: Reducing gate width impacts power sub-linearly l Interconnect capacitance is not affected  Reducing supply voltage cuts power quadratically l All capacitive loads have lower voltage swing  How can we minimize delay penalty at low V dd ? D. Sylvester, DAC-2001

ECE 260B – CSE 241A Design Styles 38http://vlsicad.ucsd.edu Challenges For Multi-Vdd  Area overhead l Toshiba reported 7% rise in area due to placement restrictions, level converters, additional power grid routing  EDA tool support for the above issues (placement, dual power routing)  Noise analysis l Additional shielding required between Vdd,low and Vdd,high signals? l Including clock network D. Sylvester, DAC-2001

ECE 260B – CSE 241A Design Styles 39http://vlsicad.ucsd.edu Static Power  Global signaling and layout optimization  Multi-V dd  Static power  Multi-V th + V dd + sizing D. Sylvester, DAC-2001

ECE 260B – CSE 241A Design Styles 40http://vlsicad.ucsd.edu Static Power  Why do we care about static power in non-portable devices? l Standby power is “wasted” -- leaves fewer Watts for computation l Worsens reliability by raising die temperatures  Leakage current is a function of V th and subthreshold swing (S s ) (x10 at operating vs. room temp!)  S s expected to remain at mV/dec (room temp) l Device technology may cut this by ~20%  V th reductions are mandated by scaling V dd l V th has been around V dd /5 D. Sylvester, DAC-2001

ECE 260B – CSE 241A Design Styles 41http://vlsicad.ucsd.edu Current Status  No sub-1V technologies demonstrate good on/off current performance (yet – expect improvements in production)  Oxide scaling is running out of steam; overall ~3x I off per node (physical)50ITRS (uses high-k)45ITRS (physical)70ITRS (physical)100ITRS (physical)100NEC, Intel, TI, NEC, Samsung, Intel,00 I off (nA/  m) I on (  A/  m) V dd T ox (Å) (electrical)ITRS node Reference Working numbers D. Sylvester, DAC-2001

ECE 260B – CSE 241A Design Styles 42http://vlsicad.ucsd.edu Leakage Suppression Approaches  Dual-V th (most common) l Low-V th on critical paths, high-V th off l Only cost is additional masks  MTCMOS l Series inserted high-V th device cuts leakage current when off (sleep mode) l Delay and area penalties, control device sizing is critical  Other techniques l Substrate biasing to control V th l Dual-V th domino -Use low-V th devices only in evaluate paths Pull Up Pull Down Parasitic Node Vcontrol Vout Vdd High Vth Device D. Sylvester, DAC-2001

ECE 260B – CSE 241A Design Styles 43http://vlsicad.ucsd.edu Can Gate-length biasing help leakage reduction?  Reduce leakage? Variation of leakage and delay (each normalized to 1) for an NMOS device in an industrial 130nm technology  Reduce leakage variability? Biasing

ECE 260B – CSE 241A Design Styles 44http://vlsicad.ucsd.edu Gate-length Biasing  First proposed by Sirisantana et al. l Comparative study of effect of doping, tox and gate-length l Large bias used, significant slow down  Small bias l Little reduction in leakage beyond 10% bias while delay degrades linearly l Preserves pin compatibility  Technique applicable as post-RET step  Salient features l Design cycle not interfered l Zero cost (no additional masks)

ECE 260B – CSE 241A Design Styles 45http://vlsicad.ucsd.edu Granularity  Technology-level All devices in all cells have one biased gate-length  Cell-level All devices in a cell have one biased gate-length  Device-level All devices have independent biased gate-length Simplification: In each cell, NMOS devices have one gate-length and PMOS devices have another

ECE 260B – CSE 241A Design Styles 46http://vlsicad.ucsd.edu Device-Level Leakage Reduction

ECE 260B – CSE 241A Design Styles 47http://vlsicad.ucsd.edu Circuit level  Bias gate-length for non-critical cells  Library extended with each cell having a biased version  Benefits analyzed in conjunction with Multi-V T assignment and in isolation l SVT-SGL l DVT-SGL l SVT-DGL l DVT-DGL

ECE 260B – CSE 241A Design Styles 48http://vlsicad.ucsd.edu Results: Leakage Reduction With less than 2.5% delay penalty Design Compiler used for V T assignment and gate-length biasing Better results expected with Duet (academic sizer from Michigan)

ECE 260B – CSE 241A Design Styles 49http://vlsicad.ucsd.edu Results: Leakage Variability Leakage distribution for the testcase alu128 Traces shown Unbiased circuit Technology level biasing Uniform biasing Percentage Reduction in Leakage Spread

ECE 260B – CSE 241A Design Styles 50http://vlsicad.ucsd.edu Futures  Construction of effective biasing based leakage optimization heuristics  Gate-length selection at true device-level granularity  Evaluation of gate-length biasing at future technology nodes

ECE 260B – CSE 241A Design Styles 51http://vlsicad.ucsd.edu Multi-Vth + Vdd + Sizing  Global signaling and layout optimization  Multi-V dd  Static power analysis  Multi-V th + V dd + sizing D. Sylvester, DAC-2001

ECE 260B – CSE 241A Design Styles 52http://vlsicad.ucsd.edu Multi-Everything  Need an approach that selects between speed, static power, and dynamic power  Should be scalable to nanometer design l Rules out dual-V th domino or other dynamic logic families (low supplies kill performance advantages)  Techniques mentioned so far l Flexible, optimized cell layouts l Multi-V dd l Dual-V th  Put them all together D. Sylvester, DAC-2001

ECE 260B – CSE 241A Design Styles 53http://vlsicad.ucsd.edu Multi-Vdd Can Leverage Vth’s  Existing designs using multi-V dd do not alter V th in low- V dd cells l Highly sub-optimal, delay is fully penalized l Limits cell replacement  limits power savings  Much better solution: reduce V th in low-V dd cells to carefully balance delay, static power, and dynamic power l Enforce technology scaling within a chip – whenever we reduce V dd, we also reduce V th to maintain speed D. Sylvester, DAC-2001

ECE 260B – CSE 241A Design Styles 54http://vlsicad.ucsd.edu Multi-Vdd + Vth Negates Delay Penalty Delay ~ CV dd /I on  Scenarios l Constant V th (current paradigm) l Scale V th to maintain constant static power l Scale V th to reduce static power linearly with V dd  Delay penalty is substantially offset I on is very sensitive to V th at V dd < 1V P static reduces with V dd due to linear term and smaller I off (I on and DIBL  ) D. Sylvester, DAC-2001

ECE 260B – CSE 241A Design Styles 55http://vlsicad.ucsd.edu Now Add Sizing  Multi-V dd + multi-V th + sizing/cell layout optimization attacks power from many angles (multi-dimensional)  Depending on criticality and switching activities, non- critical gates can be: l Assigned Vdd,low l Assigned Vdd,low + lower Vth l Assigned Vth,high l Downsized (at the individual transistor level if advantageous) l Assigned Vdd,low and upsized -For gates that cannot tolerate Vdd,low delay, this can be power efficient l And others D. Sylvester, DAC-2001

ECE 260B – CSE 241A Design Styles 56http://vlsicad.ucsd.edu Summary  Power density must saturate to maintain affordable packaging options l 50 W/cm 2 means W for future large MPUs l Dynamic thermal management saves 25% on packaging power budget  Multi-V dd will leverage multiple V th ’s to offset delay penalty at low V dd l More widespread re-assignment to Vdd,low l Use V dd first instead of re-sizing to take advantage of large path slacks l Anticipated power savings of 50-80%  Static power also addressed through multi-V th + V dd + sizing l V th difficult to control in ultra-short channels l Intra-cell V th assignment + MTCMOS/variants + sleep modes D. Sylvester, DAC-2001

ECE 260B – CSE 241A Design Styles 57http://vlsicad.ucsd.edu Next Week: Project Meetings D. Sylvester, DAC-2001