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Progettazione di circuiti e sistemi VLSI Anno Accademico 2007-2008 Lezione 15 Riepilogo 1 Challenges in Digital Design “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 ? …and There’s a Lot of Them! 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 10,000 100 100,000 1,0001 10 10,000 x 0.1 100 xx 0.01 10 xx x 1 1,000 21%/Yr. compound Productivity growth rate x 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 Courtesy, ITRS Roadmap 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 Cost of Integrated Circuits • NRE (non-recurrent engineering) costs – design time and effort, mask generation – one-time cost factor • Recurrent costs – silicon processing, packaging, test – proportional to volume – proportional to chip area Mapping between analog and digital signals V “ 1” V OH V V IH out Slope = -1 OH Undefined Region V “ 0” V Slope = -1 IL V OL OL V IL V IH V in A Modern CMOS Process gate-oxide TiSi2 AlCu SiO2 Tungsten poly p-well n+ SiO2 n-well p+ p-epi p+ Dual-Well Trench-Isolated CMOS Process CMOS Process at a Glance Define active areas Etch and fill trenches Implant well regions Deposit and pattern polysilicon layer Implant source and drain regions and substrate contacts Create contact and via windows Deposit and pattern metal layers Design Rules • Interface between designer and process engineer • Guidelines for constructing process masks • Unit dimension: Minimum line width – scalable design rules: lambda parameter – absolute dimensions (micron rules) Progettazione di circuiti e sistemi VLSI Anno Accademico 2007-2008 Lezione 3 Dispositivi e modelli Diode Model RS + VD - ID CD Current-Voltage Relations Long-Channel Device A model for manual analysis Dynamic Behavior of MOS Transistor G CGS CGD D S CGB CSB B CDB Gate Capacitance G G CGC CGC D S Cut-off G CGC D S Resistive D S Saturation Most important regions in digital design: saturation and cut-off Impact of Interconnect Parasitics • Interconnect parasitics – reduce reliability – affect performance and power consumption • Classes of parasitics – Capacitive – Resistive – Inductive CMOS Inverter Load Characteristics ID n PMOS Vin = 0 Vin = 2.5 Vin = 0.5 Vin = 2 Vin = 1 Vin = 1.5 Vin = 1.5 Vin = 1 Vin = 1.5 Vin = 2 Vin = 2.5 NMOS Vin = 1 Vin = 0.5 Vin = 0 Vout CMOS Inverter VTC NMOS off PMOS res 2.5 Vout 2 NMOS s at PMOS res 1 1.5 NMOS sat PMOS sat 0.5 NMOS res PMOS sat 0.5 1 1.5 2 NMOS res PMOS off 2.5 Vin CMOS Inverter Propagation Delay Approach 1 VDD tpHL = CL Vswing/2 Iav CL Vout ~ Iav Vin = V DD CL kn VDD CMOS Inverter Propagation Delay Approach 2 VDD tpHL = f(R on.CL) = 0.69 RonCL Vout ln(0.5) Vout CL Ron 1 VDD 0.5 0.36 Vin = V DD RonCL t Inverter Chain In Out CL If CL is given: - How many stages are needed to minimize the delay? - How to size the inverters? May need some additional constraints. Apply to Inverter Chain In Out 1 2 N CL tp = tp1 + tp2 + …+ tpN C gin, j 1 t pj ~ RunitCunit 1 C gin , j N N C gin, j 1 , C gin, N 1 C L t p t p , j t p 0 1 C j 1 i 1 gin, j Optimal Tapering for Given N Delay equation has N - 1 unknowns, Cgin,2 – Cgin,N Minimize the delay, find N - 1 partial derivatives Result: Cgin,j+1/Cgin,j = Cgin,j/Cgin,j-1 Size of each stage is the geometric mean of two neighbors C gin, j C gin, j 1C gin, j 1 - each stage has the same effective fanout (Cout/Cin) - each stage has the same delay Optimum Delay and Number of Stages When each stage is sized by f and has same eff. fanout f: f N F CL / Cgin,1 Effective fanout of each stage: f NF Minimum path delay t p Nt p 0 1 N F / Optimum Effective Fanout f Optimum f for given process defined by f exp 1 f fopt = 3.6 for =1 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. Progettazione di circuiti e sistemi VLSI Anno Accademico 2007-2008 Lezione 6 La logica combinatoria Combinational vs. Sequential Logic In Combinational Logic Circuit In Out Combinational Logic Circuit State Combinational Output = f(In) Sequential Output = f(In, Previous In) Out Static CMOS Circuit At every point in time (except during the switching transients) each gate output is connected to either VDD or Vss via a low-resistive path. The outputs of the gates assume at all times the value of the Boolean function, implemented by the circuit (ignoring, once again, the transient effects during switching periods). This is in contrast to the dynamic circuit class, which relies on temporary storage of signal values on the capacitance of high impedance circuit nodes. Constructing a Complex Gate VDD VDD C F SN4 F SN1 A SN3 D B C B SN2 A D A B D C F (a) pull-down network (b) Deriving the pull-up network hierarchically by identifying sub-nets A D B C (c) complete gate CMOS Properties • Full rail-to-rail swing; high noise margins • Logic levels not dependent upon the relative device sizes; ratioless • Always a path to Vdd or Gnd in steady state; low output impedance • Extremely high input resistance; nearly zero steady-state input current • No direct path steady state between power and ground; no static power dissipation • Propagation delay function of load capacitance and resistance of transistors Transistor Sizing Rp 2 A Rp B Rn 2 B 2 Rn A Rp 4 B 2 CL Cint Rp 4 Cint A 1 Rn Rn A B CL 1 Transistor Sizing a Complex CMOS Gate A B 8 6 C 8 6 4 3 D 4 6 OUT = D + A • (B + C) A D 2 1 B 2C 2 Fast Complex Gates: Design Technique 1 • Transistor sizing – as long as fan-out capacitance dominates • Progressive sizing InN CL MN In3 M3 C3 In2 M2 C2 In1 M1 C1 Distributed RC line M1 > M2 > M3 > … > MN (the mos closest to the output is the smallest) Can reduce delay by more than 20%; decreasing gains as technology shrinks Fast Complex Gates: Design Technique 2 • Transistor ordering critical path In3 1 M3 charged CL In2 1 M2 C2 charged In1 M1 01 C1 charged delay determined by time to discharge CL, C1 and C2 critical path 01 In1 M3 CLcharged In2 1 M2 C2 discharged In3 1 M1 C1 discharged delay determined by time to discharge CL Logical Effort CL Delay k RunitCunit 1 Cin p g f p – intrinsic delay (3kRunitCunit) - gate parameter f(W) g – logical effort (kRunitCunit) – gate parameter f(W) f – effective fanout Normalize everything to an inverter: ginv =1, pinv = 1 Divide everything by inv (everything is measured in unit delays inv) Assume = 1. Delay in a Logic Gate Gate delay: d=h+p effort delay intrinsic delay Effort delay: h=gf logical effort effective fanout = Cout/Cin Logical effort is a function of topology, independent of sizing Effective fanout (electrical effort) is a function of load/gate size Logical Effort • Inverter has the smallest logical effort and intrinsic delay of all static CMOS gates • Logical effort of a gate presents the ratio of its input capacitance to the inverter capacitance when sized to deliver the same current • Logical effort increases with the gate complexity Logical Effort Logical effort is the ratio of input capacitance of a gate to the input capacitance of an inverter with the same output current VDD A VDD A 2 2 B F 2 F A A VDD B 4 A 4 2 F 1 A B Inverter g=1 1 B 2 2-input NAND g = 4/3 2-input NOR g = 5/3 1 Add Branching Effort Branching effort: b Con path Coff path Con path Summary Sutherland, Sproull Harris Ratioed Logic VDD Resistive Load VDD Depletion Load RL PDN VSS (a) resistive load PMOS Load VSS VT < 0 F In1 In2 In3 VDD F In1 In2 In3 PDN VSS (b) depletion load NMOS F In1 In2 In3 PDN VSS (c) pseudo-NMOS Goal: to reduce the number of devices over complementary CMOS Active Loads VDD Depletion Load VDD PMOS Load VT < 0 VSS F In1 In2 In3 PDN VSS depletion load NMOS F In1 In2 In3 PDN VSS pseudo-NMOS Pass-Transistor Logic Inputs B Switch Out A Out B Network B • N transistors • No static consumption Dynamic CMOS • In static circuits at every point in time (except when switching) the output is connected to either GND or VDD via a low resistance path. – fan-in of n requires 2n (n N-type + n P-type) devices • Dynamic circuits rely on the temporary storage of signal values on the capacitance of high impedance nodes. – requires on n + 2 (n+1 N-type + 1 P-type) transistors Dynamic Gate Clk Clk Mp off Mp on Out In1 In2 In3 Clk CL PDN A C B Me Clk Two phase operation Precharge (Clk = 0) Evaluate (Clk = 1) 1 Out ((AB)+C) off Me on Cascading Dynamic Gates V Clk Mp Clk Mp Out1 Me Clk Out2 In In Clk Clk Me Out1 VTn V Out2 t Only 0 1 transitions allowed at inputs! Differential (Dual Rail) Domino off Mp Mkp Clk Out = AB 1 on Mkp 0 Clk Mp 1 A !A 0 !B B Clk Me Solves the problem of non-inverting logic Out = AB NORA Logic Clk In1 In2 In3 Clk Mp 11 10 Out1 PDN Clk Me In4 In5 PUN 00 01 Clk Me to other PDN’s WARNING: Very sensitive to noise! Mp Out2 (to PDN) to other PUN’s