ECE260B CSE241A Winter Design Styles Multi-Vdd/ Vth Designs. Website: / vlsicad.ucsd.edu/ courses/ ece260bw05

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1 ECE260B CSE241A Winter 2005 Design Styles Multi-Vdd/ Vth Designs Website: / courses/ ece260bw05 ECE 260B CSE 241A Design Styles 1

2 The Design Problem Source: sematech97 A growing gap between design complexity and design productivity ECE 260B CSE 241A Design Styles 2

3 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 3

4 Behavioral Description of Accumulator entity accumulator is port ( DI : in integer; DO : inout integer := 0; CLK : in bit ); end accumulator; architecture behavior of accumulator is begin process(clk) variable X : integer := 0; -- intermediate variable begin if CLK = '1' then X < = DO + D1; DO <= X; end if; end process; end behavior; ECE 260B CSE 241A Design Styles 4 Design described as set of input-output relations, regardless of chosen implementation Data described at higher abstraction level ( integer )

5 Structural Description of Accumulator entity accumulator is port ( -- definition of input and output terminals DI: in bit_vector(15 downto 0) -- a vector of 16 bit wide DO: inout bit_vector(15 downto 0); CLK: in bit ); end accumulator; architecture structure of accumulator is component reg -- definition of register ports port ( DI : in bit_vector(15 downto 0); DO : out bit_vector(15 downto 0); CLK : in bit ); end component; component add -- definition of adder ports port ( IN0 : in bit_vector(15 downto 0); IN1 : in bit_vector(15 downto 0); OUT0 : out bit_vector(15 downto 0) ); end component; -- definition of accumulator structure signal X : bit_vector(15 downto 0); begin add1 : add port map (DI, DO, X); -- defines port connectivity reg1 : reg port map (X, DO, CLK); end structure; ECE 260B CSE 241A Design Styles 5 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

6 Implementation Methodologies Digital Circuit Implementation Approaches Semi-custom Custom Cell-Based Standard Cells Compiled Cells ECE 260B CSE 241A Design Styles 6 Macro Cells Array-Based Pre-diffused (Gate Arrays) Pre-wired (FPGA)

7 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 7

8 Symbolic Layout V D D 3 O ut In 1 Dimensionless layout entities Only topology is important Final layout generated by compaction program G N D Stick diagram of inverter ECE 260B CSE 241A Design Styles 8

9 Standard Cells Organized in rows Cells made as full custom by Logic Cell Feedthrough Cell 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 9 Rows of Cells vendor (not user) Routing Channel Functional Module (RAM, multiplier, ) Routing channel requirements are reduced by presence of more interconnect layers

10 Standard Cell Example [Brodersen92] ECE 260B CSE 241A Design Styles 10

11 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 11

12 Automatic Cell Generation Random-logic layout generated by CLEO cell compiler (Digital) ECE 260B CSE 241A Design Styles 12

13 Module Generators Compiled Datapath buffer adder reg1 reg0 bus2 mux bus0 bus1 routing area feed-through bit-slice Advantages: One-dimensional placement/routing problem ECE 260B CSE 241A Design Styles 13

14 Macrocell-Based Design Predefined macro blocks (up, RAM, etc.) Macro blocks made as full custom by vendor (IP blocks) All layers customized Digital and some analog Macrocell 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 14 Interconnect Bus Routing Channel

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

16 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 16 rows of uncommitted cells routing channel

17 Gate Array Primitive Cells polysilicon In1 In 2 In3 In4 VD D metal possible contact GND Out Uncommited Cell ECE 260B CSE 241A Design Styles 17 Committed Cell (4-input NOR)

18 Sea-of-gate Primitive Cells O x id e - i s o l a t io n PM O S PMOS NM OS NM OS NM OS Using oxide-isolation ECE 260B CSE 241A Design Styles 18 Using gate-isolation

19 Sea-of-gates Random Logic Memory Subsystem LSI Logic LEA300K (0.6 µm CMOS) ECE 260B CSE 241A Design Styles 19

20 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 ECE 260B CSE 241A Design Styles 20 Courtesy Altera Corp.

21 Programmable Logic Devices PLA ECE 260B CSE 241A Design Styles 21 PROM PAL

22 EPLD Block Diagram Primary inputs Macrocell Courtesy Altera Corp. ECE 260B CSE 241A Design Styles 22

23 Field-Programmable Gate Arrays - Fuse-based I/O B u ffe r s P r o g r a m / T e s t / D ia g n o s t i c s V e r ti c a l ro u te s I/O B u ffe rs I/O B u ffe r s Standard-cell like floorplan R o w s o f lo g i c m o d u le s R o u tin g c h a n n e ls I/O B u ffe r s ECE 260B CSE 241A Design Styles 23

24 Interconnect P r o g r a m m e d in t e r c o n n e c t io n I n p u t/o u tp u t p in C e ll A n tifu s e H o riz o n ta l tra c k s V e r t ic a l t r a c k s ECE 260B CSE 241A Design Styles 24 Programming interconnect using anti-fuses

25 Field-Programmable Gate Arrays - RAM-based CLB CLB switching matrix Horizontal routing channel Interconnect point CLB CLB Vertical routing channel ECE 260B CSE 241A Design Styles 25

26 RAM-based FPGA - Basic Cell (CLB) C o m b in a tio n a l lo g ic S to ra g e e l e m e n ts R A B /Q 1 /Q 2 D Any function of up to 4 variables C /Q 1 /Q 2 R in F F D G B /Q 1 /Q 2 C /Q 1 /Q 2 F CE D A Q 1 Any function of up to 4 variables R G F Q 2 G D E D CE G C lo c k C E Courtesy of Xilinx ECE 260B CSE 241A Design Styles 26

27 RAM-based FPGA Xilinx XC4025 ECE 260B CSE 241A Design Styles 27

28 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 28

29 Global Signaling and Layout Global signaling and layout optimization Multi-Vdd Static power analysis Multi-Vth + Vdd + sizing ECE 260B CSE 241A Design Styles 29 D. Sylvester, DAC-2001

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

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

32 Multi-Vdd Global signaling and layout optimization Multi-Vdd Static power analysis Multi-Vth + Vdd + sizing ECE 260B CSE 241A Design Styles 32 D. Sylvester, DAC-2001

33 Multi-Vdd Status Idea: Incorporate two Vdd s to reduce dynamic power Limited to a few recent Japanese multimedia processors Example 0.3 µm, 75MHz, 3.3V media processor (Toshiba) - Total power savings of 47% in logic, 69% in clock 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 Mentality: today s high performance parts aren t limited by power ECE 260B CSE 241A Design Styles 33 D. Sylvester, DAC-2001

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

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

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

37 Multi-Vdd Instead of Sizing Power ~C Vdd2 f, where f is fixed Key: Reducing gate width impacts power sub-linearly Interconnect capacitance is not affected Reducing supply voltage cuts power quadratically All capacitive loads have lower voltage swing How can we minimize delay penalty at low Vdd? ECE 260B CSE 241A Design Styles 37 D. Sylvester, DAC-2001

38 Challenges For Multi-Vdd Area overhead 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 Additional shielding required between Vdd,low and Vdd,high signals? Including clock network ECE 260B CSE 241A Design Styles 38 D. Sylvester, DAC-2001

39 Static Power Global signaling and layout optimization Multi-Vdd Static power Multi-Vth + Vdd + sizing ECE 260B CSE 241A Design Styles 39 D. Sylvester, DAC-2001

40 Static Power Why do we care about static power in non-portable devices? Standby power is wasted -- leaves fewer Watts for computation Worsens reliability by raising die temperatures Leakage current is a function of Vth and subthreshold swing (Ss) (x10 at operating vs. room temp!) V th I off Ss A/ m Ss expected to remain at mv/dec (room temp) Device technology may cut this by ~ 20% Vth reductions are mandated by scaling Vdd V has been around VddD./5 Sylvester, DAC-2001 th Design Styles 40 ECE 260B CSE 241A

41 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 3~x Ioff per node Reference ITRS node Tox (Å) (electrical) Vdd Ion (µa/ µm) Ioff (na/ µm) Intel, Samsung, NEC, TI, Intel, NEC, (physical) ITRS (physical) ITRS (physical) ITRS (physical) ITRS (uses high-k) ECE 260B CSE 241A Design Styles 41 D. Sylvester, DAC-2001 Working numbers

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

43 Can Gate-length biasing help leakage reduction? Reduce leakage? Leakage Delay Variation of leakage and delay (each normalized to 1) for an NMOS device in an industrial 130nm technology Gate-length (nm) Reduce leakage variability? Leakage Variability Leakage Biasing Gate-length ECE 260B CSE 241A Design Styles 43 Leakage Leakage Variability Gate-length

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

45 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 45

46 Device-Level Leakage Reduction Leakage saving with a delay penalty of up to 10% (Simplified device level biasing) Low Vt 20 Nom Vt High Vt INVX4 ECE 260B CSE 241A Design Styles 46 NANDX4 BUFX4 ANDX6

47 Circuit level Bias gate-length for non-critical cells Library extended with each cell having a biased version Benefits analyzed in conjunction with Multi-VT assignment and in isolation SVT-SGL DVT-SGL SVT-DGL DVT-DGL ECE 260B CSE 241A Design Styles 47

48 Normalized Leakage Results: Leakage Reduction SVT-SGL SVT-DGL DVT-SGL DVT-DGL c5315 c6288 c7552 alu128 With less than 2.5% delay penalty Design Compiler used for VT assignment and gate-length biasing Better results expected with Duet (academic sizer from Michigan) ECE 260B CSE 241A Design Styles 48

49 Results: Leakage Variability Leakage distribution for the testcase alu128 Traces shown Unbiased circuit Technology level biasing Uniform biasing 60.00% Percentage Reduction in Leakage Spread 50.00% 40.00% 30.00% 20.00% 10.00% 0.00% c5315 ECE 260B CSE 241A Design Styles 49 c6288 c7552 alu128

50 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 50

51 Multi-Vth + Vdd + Sizing Global signaling and layout optimization Multi-Vdd Static power analysis Multi-Vth + Vdd + sizing ECE 260B CSE 241A Design Styles 51 D. Sylvester, DAC-2001

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

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

54 Multi-Vdd + Vth Negates Delay Penalty Delay ~CVdd/Ion Scenarios Constant Vth (current paradigm) Scale Vth to maintain constant static power Scale Vth to reduce static power linearly with Vdd Delay penalty is substantially offset I is very sensitive to V on th at Vdd < 1V Pstatic reduces with Vdd due to linear term and smaller Ioff (Ion and DIBL ) ECE 260B CSE 241A Design Styles 54 D. Sylvester, DAC-2001

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

56 Summary Power density must saturate to maintain affordable packaging options 50 W/ cm2 means W for future large MPUs Dynamic thermal management saves 25% on packaging power budget Multi-Vdd will leverage multiple Vth s to offset delay penalty at low Vdd More widespread re-assignment to Vdd,low Use Vdd first instead of re-sizing to take advantage of large path slacks Anticipated power savings of 50-80% Static power also addressed through multi-vth + Vdd + sizing Vth difficult to control in ultra-short channels DAC-2001 Intra-cell V assignment D.+Sylvester, MTCMOS/variants + sleep modes CSE 241A Design Styles ECE 260B th 56

57 Next Week: Project Meetings ECE 260B CSE 241A Design Styles 57 D. Sylvester, DAC-2001