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Dra. Ruth M. Aguilar Ponce Otoño 2008 Fall 2008 1 Moore’s Law l In 1965, Gordon Moore noted that the number of transistors on a chip doubled every 18 to 24 months. l He made a prediction that semiconductor technology will double its effectiveness every 18 months Fall 2008 2 1959 1960 1961 1962 1963 1964 1965 1966 1967 1968 1969 1970 1971 1972 1973 1974 1975 LOG2 OF THE NUMBER OF COMPONENTS PER INTEGRATED FUNCTION Moore’s Law Fall 2008 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0 Electronics, April 19, 1965. 3 Evolution in Complexity Fall 2008 4 Transistor Counts 1 Billion Transistors K 1,000,000 100,000 10,000 1,000 i486 i386 80286 100 10 Pentium® III Pentium® II Pentium® Pro Pentium® 8086 Source: Intel 1 1975 1980 1985 1990 1995 2000 2005 2010 Projected Fall 2008 Courtesy, Intel 5 Moore’s law in Microprocessors Transistors (MT) 1000 100 2X growth in 1.96 years! 10 486 1 P6 Pentium® proc 386 286 0.1 8086 8085on Lead Transistors Microprocessors double every 2 years 0.01 0.001 1970 Fall 2008 8080 8008 4004 1980 1990 Year Courtesy, Intel 2000 2010 6 Die Size Growth Die size (mm) 100 10 8080 8008 4004 1 1970 8086 8085 1980 286 386 P6 Pentium ® proc 486 ~7% growth per year ~2X growth in 10 years 1990 Year 2000 2010 Die size grows by 14% to satisfy Moore’s Law Fall 2008 Courtesy, Intel 7 Frequency Frequency (Mhz) 10000 Doubles every 2 years 1000 100 486 10 8085 1 0.1 1970 8086 286 P6 Pentium ® proc 386 8080 8008 4004 1980 1990 Year 2000 2010 Lead Microprocessors frequency doubles every 2 years Fall 2008 Courtesy, Intel 8 Power Dissipation Power (Watts) 100 P6 Pentium ® proc 10 8086 286 1 8008 4004 486 386 8085 8080 0.1 1971 1974 1978 1985 1992 2000 Year Lead Microprocessors power continues to increase Fall 2008 Courtesy, Intel 9 Power will be a major problem 100000 18KW 5KW 1.5KW 500W Power (Watts) 10000 1000 Pentium® proc 100 286 486 8086 10 386 8085 8080 8008 1 4004 0.1 1971 1974 1978 1985 1992 Year 2000 2004 2008 Power delivery and dissipation will be prohibitive Fall 2008 Courtesy, Intel 10 Power density Power Density (W/cm2) 10000 1000 100 Rocket Nozzle Nuclear Reactor 8086 Hot Plate 10 4004 P6 8008 8085 Pentium® proc 386 286 486 8080 1 1970 1980 1990 2000 Year 2010 Power density too high to keep junctions at low temp Fall 2008 Courtesy, Intel 11 Not Only Microprocessors Cell Phone Small Signal RF Digital Cellular Market (Phones Shipped) Year Units 1996 48M 1997 86M 1998 162M Power Managemen t 1999 260M 2000 435M (data from Texas Instruments) Fall 2008 Power RF Analog Baseband Digital Baseband (DSP + MCU) 12 Challenges in Digital Design DSM 1/DSM “Macroscopic Issues” “Microscopic Problems” • Time-to-Market • Millions of Gates • High-Level Abstractions • Reuse & IP: Portability • Predictability • etc. • Ultra-high speed design • Interconnect • Noise, Crosstalk • Reliability, Manufacturability • Power Dissipation • Clock distribution. Everything Looks a Little Different ? Fall 2008 …and There’s a Lot of Them! 13 10,000 10,000,000 100,000 100,000,000 Logic Tr./Chip Tr./Staff Month. 1,000 1,000,000 10,000 10,000,000 100 100,000 Productivity (K) Trans./Staff - Mo. Complexity Logic Transistor per Chip (M) Productivity Trends 1,000 1,000,000 58%/Yr. compounded Complexity growth rate 10,00010 100 100,000 1,0001 10 10,000 x 0.1 100 xx 0.01 10 xx x x 1 1,000 21%/Yr. compound Productivity growth rate x 0.1 100 0.01 10 2009 2007 2005 2003 2001 1999 1997 1995 1993 1991 1989 1987 1985 1983 1981 0.001 1 Source: Sematech Complexity outpaces design productivity Fall 2008 Courtesy, ITRS Roadmap 14 Why Scaling? Technology shrinks by 0.7/generation With every generation can integrate 2x more functions per chip; chip cost does not increase significantly Cost of a function decreases by 2x But … How to design chips with more and more functions? Design engineering population does not double every two years… Hence, a need for more efficient design methods Exploit different levels of abstraction Fall 2008 15 Design Abstraction Levels SYSTEM MODULE + GATE CIRCUIT DEVICE G S n+ Fall 2008 D n+ 16 Design Metrics How to evaluate performance of a digital circuit (gate, block, …)? Cost Reliability Scalability Speed (delay, operating frequency) Power dissipation Energy to perform a function Fall 2008 17 Introduction to VLSI-CMOS Integrated circuits: many transistors on one chip. Very Large Scale Integration (VLSI): very many Complementary Metal Oxide Semiconductor Fast, cheap, low power transistors Today: How to build your own simple CMOS chip CMOS transistors Building logic gates from transistors Transistor layout and fabrication Fall 2008 18 Silicon Lattice Transistors are built on a silicon substrate Silicon is a Group IV material Forms crystal lattice with bonds to four neighbors Fall 2008 Si Si Si Si Si Si Si Si Si 19 Dopants Silicon is a semiconductor Pure silicon has no free carriers and conducts poorly Adding dopants increases the conductivity Group V: extra electron (n-type) Group III: missing electron, called hole (p-type) Fall 2008 Si Si Si Si Si Si As Si Si B Si Si Si Si Si - + + - Si Si Si 20 p-n Junctions A junction between p-type and n-type semiconductor forms a diode. Current flows only in one direction Fall 2008 p-type n-type anode cathode 21 nMOS Transistor Four terminals: gate, source, drain, body Gate – oxide – body stack looks like a capacitor Gate and body are conductors SiO2 (oxide) is a very good insulator Called metal – oxide – semiconductor (MOS) capacitor Source Gate Drain Even though gate is Polysilicon no longer made of metal SiO2 n+ n+ p Fall 2008 bulk Si 22 nMOS Operation Body is commonly tied to ground (0 V) When the gate is at a low voltage: P-type body is at low voltage Source-body and drain-body diodes are OFF No current flows, transistor is OFF Source Gate Drain Polysilicon SiO2 0 n+ n+ S p Fall 2008 D bulk Si 23 nMOS Operation Cont. When the gate is at a high voltage: Positive charge on gate of MOS capacitor Negative charge attracted to body Inverts a channel under gate to n-type Now current can flow through n-type silicon from source through channel to drain, transistor is ON Source Gate Drain Polysilicon SiO2 1 n+ n+ S p Fall 2008 D bulk Si 24 pMOS Transistor Similar, but doping and voltages reversed Body tied to high voltage (VDD) Gate low: transistor ON Gate high: transistor OFF Bubble indicates inverted behavior Source Gate Drain Polysilicon SiO2 p+ p+ n Fall 2008 bulk Si 25 Power Supply Voltage GND = 0 V In 1980’s, VDD = 5V VDD has decreased in modern processes High VDD would damage modern tiny transistors Lower VDD saves power VDD = 3.3, 2.5, 1.8, 1.5, 1.2, 1.0, … Fall 2008 26 Transistors as Switches We can view MOS transistors as electrically controlled switches Voltage at gate controls path from source to drain d nMOS pMOS g=1 d d OFF g ON s s s d d d g OFF ON s Fall 2008 g=0 s s 27 CMOS Inverter A VDD Y 0 1 A A Y Y GND Fall 2008 28 CMOS Inverter A VDD Y 0 1 OFF 0 A=1 Y=0 ON A Y GND Fall 2008 29 CMOS Inverter A Y 0 1 1 0 VDD ON A=0 Y=1 OFF A Y GND Fall 2008 30 CMOS NAND Gate A B 0 0 0 1 1 0 1 1 Y Y A B Fall 2008 31 CMOS NAND Gate A B Y 0 0 1 0 1 1 0 1 1 Fall 2008 ON ON Y=1 A=0 B=0 OFF OFF 32 CMOS NAND Gate A B Y 0 0 1 0 1 1 1 0 1 1 Fall 2008 OFF ON Y=1 A=0 B=1 OFF ON 33 CMOS NAND Gate A B Y 0 0 1 0 1 1 1 0 1 1 1 Fall 2008 ON A=1 B=0 OFF Y=1 ON OFF 34 CMOS NAND Gate A B Y 0 0 1 0 1 1 1 0 1 1 1 0 Fall 2008 OFF A=1 B=1 OFF Y=0 ON ON 35 CMOS NOR Gate A B Y 0 0 1 0 1 0 1 0 0 1 1 0 Fall 2008 A B Y 36 3-input NAND Gate Y pulls low if ALL inputs are 1 Y pulls high if ANY input is 0 Fall 2008 37 3-input NAND Gate Y pulls low if ALL inputs are 1 Y pulls high if ANY input is 0 Y A B C Fall 2008 38 CMOS Gate Design Activity: Sketch a 4-input CMOS NAND gate Fall 2008 39 CMOS Gate Design Activity: Sketch a 4-input CMOS NOR gate A B C D Y Fall 2008 40 Complementary CMOS Complementary CMOS logic gates nMOS pull-down network pMOS pull-up network a.k.a. static CMOS Pull-up OFF Pull-up ON Pull-down OFF Z (float) 1 Pull-down ON X (crowbar) Fall 2008 0 pMOS pull-up network inputs output nMOS pull-down network 41 Series and Parallel a a 0 g1 nMOS: 1 = ON pMOS: 0 = ON g2 (a) Parallel: either can be ON (b) a g1 g2 (c) a g1 g2 b (d) Fall 2008 0 1 b b OFF OFF OFF ON a a a a 0 1 1 1 0 1 b b b b ON OFF OFF OFF a a a a 0 0 b 1 b 0 b 1 1 0 g2 a b a g1 a 0 0 b Series: both must be ON a 0 1 1 0 1 1 b b b b OFF ON ON ON a a a a 0 0 0 1 1 0 1 1 b b b b ON ON ON OFF 42 Conduction Complement Complementary CMOS gates always produce 0 or 1 Ex: NAND gate Series nMOS: Y=0 when both inputs are 1 Thus Y=1 when either input is 0 Requires parallel pMOS A Y B Rule of Conduction Complements Pull-up network is complement of pull-down Parallel -> series, series -> parallel Fall 2008 43 Compound Gates Compound gates can do any inverting function Ex: Y A B C D A C A C B D B D (a) A (b) B C D (c) C D A B (d) C D A B A B C D Y A C B D Y (f) (e) Fall 2008 44 Example: O3AI Y A B C D Fall 2008 45 Example: O3AI Y A B C D A B C D Y D A Fall 2008 B C 46 Signal Strength Strength of signal How close it approximates ideal voltage source VDD and GND rails are strongest 1 and 0 nMOS pass strong 0 But degraded or weak 1 pMOS pass strong 1 But degraded or weak 0 Thus nMOS are best for pull-down network Fall 2008 47 Pass Transistors Transistors can be used as switches g s d g s Fall 2008 d 48 Pass Transistors Transistors can be used as switches g=0 g s d s d Input g = 1 Output 0 strong 0 g=1 s d g=0 g s s g=1 Input d d g=1 s Fall 2008 1 d degraded 1 g=0 0 Output degraded 0 g=0 strong 1 49 Transmission Gates Pass transistors produce degraded outputs Transmission gates pass both 0 and 1 well Fall 2008 50 Transmission Gates Pass transistors produce degraded outputs Transmission gates pass both 0 and 1 well Input g a b gb a b gb Fall 2008 g = 0, gb = 1 a b g = 1, gb = 0 0 strong 0 g = 1, gb = 0 a b g = 1, gb = 0 strong 1 1 g g a g b gb Output a b gb 51 Tristates Tristate buffer produces Z when not enabled EN A 0 0 0 1 1 0 1 1 EN Y Y A EN Y A EN Fall 2008 52 Tristates Tristate buffer produces Z when not enabled EN A Y 0 0 Z 0 1 Z 1 0 0 1 1 1 EN Y A EN Y A EN Fall 2008 53 Nonrestoring Tristate Transmission gate acts as tristate buffer Only two transistors But nonrestoring Noise on A is passed on to Y EN A Y EN Fall 2008 54 Tristate Inverter Tristate inverter produces restored output Violates conduction complement rule Because we want a Z output A EN Y EN Fall 2008 55 Tristate Inverter Tristate inverter produces restored output Violates conduction complement rule Because we want a Z output A A A EN Y Y Y EN = 0 Y = 'Z' EN = 1 Y=A EN Fall 2008 56 Multiplexers 2:1 multiplexer chooses between two inputs S S D1 D0 0 X 0 0 X 1 1 0 X 1 1 X Fall 2008 Y D0 0 Y D1 1 57 Multiplexers 2:1 multiplexer chooses between two inputs S S D1 D0 Y 0 X 0 0 0 X 1 1 1 0 X 0 1 1 X 1 Fall 2008 D0 0 Y D1 1 58 Gate-Level Mux Design Y SD1 SD0 (too many transistors) How many transistors are needed? Fall 2008 59 Gate-Level Mux Design Y SD1 SD0 (too many transistors) How many transistors are needed? 20 D1 S D0 D1 S D0 Fall 2008 Y 4 2 4 2 4 2 Y 2 60 Transmission Gate Mux Nonrestoring mux uses two transmission gates Fall 2008 61 Transmission Gate Mux Nonrestoring mux uses two transmission gates S Only 4 transistors D0 Y S D1 S Fall 2008 62 Inverting Mux Inverting multiplexer Use compound AOI22 Or pair of tristate inverters Essentially the same thing Noninverting multiplexer adds an inverter D0 S S D1 D0 D1 S S Y S S S Y S D0 Y S D1 Fall 2008 0 1 63 4:1 Multiplexer 4:1 mux chooses one of 4 inputs using two selects Fall 2008 64 4:1 Multiplexer 4:1 mux chooses one of 4 inputs using two selects Two levels of 2:1 muxes S1S0 S1S0 S1S0 S1S0 Or four tristates D0 S0 D0 0 D1 1 S1 D1 0 Y Y D2 0 D3 1 1 D2 D3 Fall 2008 65 D Latch When CLK = 1, latch is transparent D flows through to Q like a buffer When CLK = 0, the latch is opaque Q holds its old value independent of D a.k.a. transparent latch or level-sensitive latch D Fall 2008 Latch CLK CLK D Q Q 66 D Latch Design Multiplexer chooses D or old Q CLK D 1 CLK Q Q Q D Q 0 CLK CLK CLK Fall 2008 67 D Latch Operation Q Q D CLK = 1 Q D Q CLK = 0 CLK D Q Fall 2008 68 D Flip-flop When CLK rises, D is copied to Q At all other times, Q holds its value a.k.a. positive edge-triggered flip-flop, master-slave flip-flop CLK CLK D Flop D Q Q Fall 2008 69 D Flip-flop Design Built from master and slave D latches CLK CLK CLK QM D CLK QM Latch D Latch CLK CLK CLK CLK Q CLK Fall 2008 Q CLK 70 D Flip-flop Operation D QM Q CLK = 0 D QM Q CLK = 1 CLK D Q Fall 2008 71 Race Condition Back-to-back flops can malfunction from clock skew Second flip-flop fires late Sees first flip-flop change and captures its result Called hold-time failure or race condition CLK1 CLK2 Q1 Flop D Flop CLK1 CLK2 Q2 Q1 Q2 Fall 2008 72 Nonoverlapping Clocks Nonoverlapping clocks can prevent races As long as nonoverlap exceeds clock skew We will use them in this class for safe design Industry manages skew more carefully instead 2 1 QM D 2 2 2 Q 1 1 1 1 2 Fall 2008 73 Gate Layout Layout can be very time consuming Design gates to fit together nicely Build a library of standard cells Standard cell design methodology VDD and GND should abut (standard height) Adjacent gates should satisfy design rules nMOS at bottom and pMOS at top All gates include well and substrate contacts Fall 2008 74 Example: Inverter Fall 2008 75 Example: NAND3 Horizontal N-diffusion and p-diffusion strips Vertical polysilicon gates Metal1 VDD rail at top Metal1 GND rail at bottom 32 l by 40 l Fall 2008 76 Fall 2008 77 Where Does Power Go in CMOS? • Dynamic Power Consumption Charging and Discharging Capacitors • Short Circuit Currents Short Circuit Path between Supply Rails during Switching • Leakage Leaking diodes and transistors Fall 2008 78 Dynamic Power Dissipation Vdd Vin Vout CL 2 dd L Energy/transition = C * V L Power = Energy/transition * f = C * V 2 dd *f Not a function of Ltransistor sizes! dd Need to reduce C , V , and f to reduce power. Fall 2008 79 Modification for Circuits with Reduced Swing Vdd Vdd Vdd -Vt CL E0 1 = CL Vdd Vdd – Vt Can exploit reduced sw ing to low er power (e.g., reduced bit-line swing in memory) Fall 2008 80 Adiabatic Charging 2 2 Fall 2008 2 81 Adiabatic Charging Fall 2008 82 Node Transition Activity and Power Consider switching a CMOS gate for N clock cycles E N = CL V dd2 n N EN : the energy consumed for N clock cycles n(N ): the number of 0->1 transition in N clock cycles EN 2 n N P avg = lim -------- fclk = lim ----------- C Vdd f clk N N N N L 0 1 = n N lim -----------N N P avg = 0 1 C Vdd 2 f clk L Fall 2008 83 Transistor Sizing for Minimum Energy In Out Cg1 1 f Cext Goal: Minimize Energy of whole circuit Design parameters: f and VDD tp tpref of circuit with f=1 and VDD =Vref f F t p t p 0 1 1 f VDD t p0 VDD VTE Fall 2008 84 Transistor Sizing (2) Performance Constraint (=1) tp t pref t p0 t p 0 ref F 2 f f VDD Vref VTE 3 F Vref VDD VTE F 2 f f 1 3 F Energy for single Transition 2 E VDD C g1 1 1 f F 2 VDD 2 2 f F E Eref Vref 4 F Fall 2008 85 Transistor Sizing (3) VDD=f(f) E/Eref=f(f) 4 1.5 3.5 F=1 normalized energy 3 2 vdd (V) 2.5 5 2 1.5 1 10 0.5 20 0 1 2 3 4 f Fall 2008 5 6 7 1 0.5 0 1 2 3 4 5 6 7 f 86 Short Circuit Currents Vd d Vin Vout CL IVDD (mA) 0.15 0.10 0.05 0.0 Fall 2008 1.0 2.0 3.0 Vin (V) 4.0 5.0 87 How to Keep Short-Circuit Currents Low? Short circuit current goes to zero if tfall >> trise, but can’t do this for cascade logic, so ... Fall 2008 88 Minimizing Short Circuit Power 8 7 6 Vdd =3.3 Pnorm 5 4 Vdd =2.5 3 2 1 Vdd =1.5 0 0 1 2 3 4 5 t /t sin sout Fall 2008 89 Leakage Vd d Vout Drain Junction Leakage Sub-Threshold Current Sub-threshold current one of most compelling issues in low-energy circuit design! Sub-Threshold Current Dominant Factor Fall 2008 90 Reverse-Biased Diode Leakage GATE p+ p+ N Reverse Leakage Current + V - dd IDL = JS A 2 JS = JS 1-5pA/ for a 1.2 technology = 10-100 at 25 degCMOS C for 0.25m CMOS mpA/m2 m JS doubles for every 9 deg C! Js double with every 9oC increase in temperature Fall 2008 91 Subthreshold Leakage Component Fall 2008 92 Static Power Consumption Vd d Istat Vout Vin =5V CL Pstat = P(In=1) .Vdd . Istat Wasted •energy … over dynamic consumption Dominates Should be avoided in almost all cases, • Not a function of switching frequency but could help reducing energy in others (e.g. sense amps) Fall 2008 93 Principles for Power Reduction Prime choice: Reduce voltage! Recent years have seen an acceleration in supply voltage reduction Design at very low voltages still open question (0.6 … 0.9 V by 2010!) Reduce switching activity Reduce physical capacitance Device Sizing: for F=20 Fall 2008 fopt(energy)=3.53, fopt(performance)=4.47 94 Digital Electronics Fall 2008 95 Goals of Technology Scaling Make things cheaper: Want to sell more functions (transistors) per chip for the same money Build same products cheaper, sell the same part for less money Price of a transistor has to be reduced But also want to be faster, smaller, lower power Fall 2008 96 Technology Scaling Goals of scaling the dimensions by 30%: Reduce gate delay by 30% (increase operating frequency by 43%) Double transistor density Reduce energy per transition by 65% (50% power savings @ 43% increase in frequency Die size used to increase by 14% per generation Technology generation spans 2-3 years Fall 2008 97 Technology Generations Fall 2008 98 Technology Evolution (2000 data) International Technology Roadmap for Semiconductors Year of Introduction 1999 Technology node [nm] 180 Supply [V] 1.5-1.8 Wiring levels 2000 2001 2004 2008 2011 2014 130 90 60 40 30 1.5-1.8 1.2-1.5 0.9-1.2 0.6-0.9 0.5-0.6 0.3-0.6 6-7 6-7 7 8 9 9-10 10 Max frequency [GHz],Local-Global 1.2 1.6-1.4 2.1-1.6 3.5-2 7.1-2.5 11-3 14.9 -3.6 Max P power [W] 90 106 130 160 171 177 186 Bat. power [W] 1.4 1.7 2.0 2.4 2.1 2.3 2.5 Node years: 2007/65nm, 2010/45nm, 2013/33nm, 2016/23nm Fall 2008 99 Technology Evolution (1999) Fall 2008 100 ITRS Technology Roadmap Acceleration Continues Fall 2008 101 Technology Scaling (1) Minimum Feature Size (micron) 10 10 10 10 2 1 0 -1 -2 10 1960 1970 1980 1990 2000 2010 Year Minimum Feature Size Fall 2008 102 Technology Scaling (2) Number of components per chip Fall 2008 103 Technology Scaling (3) tp decreases by 13%/year 50% every 5 years! Propagation Delay Fall 2008 104 Technology Scaling (4) / 4 x 3 1 0.1 0.01 80 MPU DSP 85 90 Year (a) Power dissipation vs. year. 95 1000 3 10 a rs e y 0.7 100 Power Dissipation (W) 100 rs Power Density (mW/mm2 ) ea x1.4 / 3 y 10 1 1 Scaling Factor (normalized by 4 m design rule ) (b) Power density vs. scaling factor. 10 From Kuroda Fall 2008 105