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ELEC 7770 Advanced VLSI Design Spring 2014 Gate Delay and Circuit Timing

This course covers the delay of a transition, gate delay, circuit timing, and timing analysis and optimization in VLSI design.

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ELEC 7770 Advanced VLSI Design Spring 2014 Gate Delay and Circuit Timing

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  1. ELEC 7770Advanced VLSI DesignSpring 2014 Gate Delay and Circuit Timing Vishwani D. Agrawal James J. Danaher Professor ECE Department, Auburn University, Auburn, AL 36849 vagrawal@eng.auburn.edu http://www.eng.auburn.edu/~vagrawal/COURSE/E7770_Spr16/course.html ELEC 7770: Advanced VLSI Design (Agrawal)

  2. Delay of a Transition VDD Ron ic(t) vi (t) vo(t) CL R = large Ground CL = Total load capacitance for gate; includes transistor capacitances of driving gate + routing capacitance + transistor capacitances of driven gates; obtained by layout analysis. ELEC2200-002 Lecture 8

  3. Charging of a Capacitor R = Ron t = 0 v(t) i(t) C = CL VDD Charge on capacitor, q(t) = C v(t) Current, i(t) = dq(t)/dt = C dv(t)/dt ELEC2200-002 Lecture 8

  4. i(t) = C dv(t)/dt = [VDD – v(t)] /R dv(t) dt ∫───── = ∫ ──── VDD – v(t) RC – t ln [VDD – v(t)] = ── + A RC Initial condition, t = 0, v(t) = 0 → A = ln VDD – t v(t) = VDD[1 – exp(───)] = 0.5VDD RC t = 0.69 RC ELEC2200-002 Lecture 8

  5. Delay: Definitions VDD GND 90% VDD A Fall time 10% VDD Time B Gate delay A NOT gate 1→0 VDD GND 0→1 90% VDD B Rise time 10% VDD Rise time is the time a signal takes to rise from 10% to 90% of its peak value. Fall time is the time a signal takes to drop from 90% to 10% of its peak value. Delay of a gate or circuit is the time interval between the input crossing 50% of peak value and the output crossing 50% of peak value. ELEC2200-002 Lecture 8 Time

  6. Inverter: Idealized Input VDD GND INPUT Gate delay VDD 0.5VDD GND OUTPUT time t= 0 0.69CR ELEC2200-002 Lecture 8

  7. Timing of a Digital Circuit FF FF Primary Inputs Primary Outputs Combinational circuit (Gates interconnected without feedback) FF FF Clock FF FF Most digital circuits are clocked synchronous finite state machines (FSM). ELEC2200-002 Lecture 8

  8. Timing Paths Input Signal changes Output Observation instant Transient region Comb. logic Inputs Synchronized With clock Outputs time Clock period ELEC 7770: Advanced VLSI Design (Agrawal)

  9. Timing Analysis and Optimization • Timing analysis • Dynamic analysis: Simulation. • Static timing analysis (STA): Vector-less topological analysis of circuit. • Timing optimization • Performance • Clock design • Other forms of design optimization • Chip area • Testability • Power ELEC 7770: Advanced VLSI Design (Agrawal)

  10. Circuit Delays • Switching or inertial delay is the interval between input change and output change of a gate: • Depends on input capacitance, device (transistor) characteristics and output capacitance of gate. • Also depends on input rise or fall times and states of other inputs (second-order effects). • Approximation: fixed rise and fall delays (or min-max delay range, or single fixed delay) for gate output. • Propagation or interconnect delay is the time a transition takes to travel between gates: • Depends on transmission line effects (distributed R, L, C parameters, length and loading) of routing paths. • Approximation: modeled as lumped delays for gate inputs. ELEC 7770: Advanced VLSI Design (Agrawal)

  11. Spice • Circuit/device level analysis • Circuit modeled as network of transistors, capacitors, resistors and voltage/current sources. • Node current equations using Kirchhoff’s current law. • Analysis is accurate but expensive • Used to characterize parts of a larger circuit. • Original references: • L. W. Nagel and D. O. Pederson, “SPICE – Simulation Program With Integrated Circuit Emphasis,” Memo ERL-M382, EECS Dept., University of California, Berkeley, Apr. 1973. • L. W. Nagel, SPICE 2, A Computer program to Simulate Semiconductor Circuits, PhD Dissertation, University of California, Berkeley, May 1975. ELEC 7770: Advanced VLSI Design (Agrawal)

  12. Ca Logic Model of MOS Circuit VDD pMOS FETs a Da c Dc a b Db c Cc b Daand Dbare interconnect or propagation delays Dcis inertial delay of gate Cb nMOS FETs Cd Ca , Cb , Cc and Cd are node capacitances ELEC 7770: Advanced VLSI Design (Agrawal)

  13. Spice Characterization ELEC 7770: Advanced VLSI Design (Agrawal)

  14. Spice Characterization (Cont.) ELEC 7770: Advanced VLSI Design (Agrawal)

  15. Complex Gates: Switch-Level Partitions Internal switching nodes not seen by logic simulator G2 G1 G3 Circuit partitioned into channel-connected components for Spice characterization. Reference: R. E. Bryant, “A Switch-Level Model and Simulator for MOS Digital Systems,” IEEE Trans. Computers, vol. C-33, no. 2, pp. 160-177, Feb. 1984. ELEC 7770: Advanced VLSI Design (Agrawal)

  16. Interconnect Delay: Elmore Delay Model 2 R2 C2 1 R1 s 4 R4 C4 C1 R3 3 R5 Shared resistance: R45 = R1 + R3 R15 = R1 R34 = R1 + R3 C3 5 C5 W. Elmore, “The Transient Response of Damped Linear Networks with Particular Regard to Wideband Amplifiers,” J. Appl. Phys., vol. 19, no.1, pp. 55-63, Jan. 1948. ELEC 7770: Advanced VLSI Design (Agrawal)

  17. Elmore Delay Formula N Delay at node k = 0.69 ΣCj × Rjk j=1 where N = number of capacitive nodes in the network Example: Delay at node 5 = 0.69 [ R1 C1 + R1 C2 + (R1+R3)C3 + (R1+R3)C4 (R1+R3+R5)C5 ] ELEC 7770: Advanced VLSI Design (Agrawal)

  18. Event Propagation Delays Single lumped inertial delay modeled for each gate PI transitions assumed to occur without time skew Path P1 1 3 1 0 2 4 6 P2 1 2 3 0 P3 5 2 0 ELEC 7770: Advanced VLSI Design (Agrawal)

  19. Circuit Outputs Clock period Final value Initial value Slow transitions Fast transitions time Initial value Final value Each path can potentially produce one signal transition at the output. The location of an output transition in time is determined by the delay of the path. ELEC 7770: Advanced VLSI Design (Agrawal)

  20. Delay and Discrete-Event Simulation(NAND gate) Transient region a Inputs b c (CMOS) c (zero delay) c (unit delay) Logic simulation X rise=5, fall=5 c (multiple delay) Unknown (X) c (minmax delay) min =2, max =5 5 Time units 0 ELEC 7770: Advanced VLSI Design (Agrawal)

  21. Event-Driven Simulation(Example) Activity list d, e f, g g Scheduled events c = 0 d = 1, e = 0 g = 0 f = 1 g = 1 a =1 e =1 t = 0 1 2 3 4 5 6 7 8 2 c =1→0 g =1 2 2 d = 0 Time stack 4 f =0 b =1 g 4 8 0 Time, t ELEC 7770: Advanced VLSI Design (Agrawal)

  22. Time Wheel (Circular Stack) max Current time pointer t=0 Event link-list 1 2 3 4 5 6 7 ELEC 7770: Advanced VLSI Design (Agrawal)

  23. Timing Design and Delay Test • Timing simulation: • Critical paths are identified by static (vector-less) timing analysis tools like Primetime (Synopsys). • Timing or circuit-level simulation using designer-generated functional vectors verifies the design. • Layout optimization: Critical path data are used in placement and routing. Delay parameter extraction, timing simulation and layout are repeated for iterative improvement. • Testing: Some form of at-speed test is necessary. Critical paths and all gate transition delays are tested. ELEC 7770: Advanced VLSI Design (Agrawal)

  24. Static Timing Analysis (STA) Flip-flops Combinational circuit Flip-flops Flip-flops Finds maximum and minimum delays between all clocked flip-flops. ELEC 7770: Advanced VLSI Design (Agrawal)

  25. Early References T. I. Kirkpatrick and N. R. Clark, “PERT as an Aid to Logic Design,” IBM J. Res. Dev., vol. 10, no. 2, pp. 135-141, March 1966. R. B. Hitchcock, Sr., “Timing Verification and the Timing Analysis Program,” Proc. 19th Design Automation Conf., 1982, pp. 594-604. V. D. Agrawal, “Synchronous Path Analysis in MOS Circuit Simulator,” Proc. 19th Design Automation Conf., 1982, pp. 629-635. ELEC 7770: Advanced VLSI Design (Agrawal)

  26. Basic Ideas • Adopted from project management • Frederick W. Taylor (1856-1915) • Henry Gantt (1861-1919) • PERT – Program Evaluation and Review Technique • CPM – Critical Path Method ELEC 7770: Advanced VLSI Design (Agrawal)

  27. A Gantt Chart in Microsoft Excel ELEC 7770: Advanced VLSI Design (Agrawal)

  28. Using a Gantt Chart Track progress of subtasks and project. Assess resource needs as a function of time. ELEC 7770: Advanced VLSI Design (Agrawal)

  29. PERT (Program Evaluation and Review Technique) Chart MilestonesActivities ELEC 7770: Advanced VLSI Design (Agrawal)

  30. Example: Thesis Research Begin Defense done Analysis completed 2, 3, 4 2, 2, 2 2, 4, 6 weeks 4, 4, 4 4, 5, 6 Thesis Draft done 1, 2, 3 Problem selected Background study completed Thesis submitted 3, 4, 5 5, 7, 9 4, 4, 4 1,2,3 2, 4, 6 minimum average maximum Draft revisions Program and Experiment completed ELEC 7770: Advanced VLSI Design (Agrawal)

  31. Critical Path Critical path is path of maximum average delay (26 weeks). Begin Defense done Analysis completed 2, 3, 4 2, 2, 2 2, 4, 6 weeks 4, 4, 4 4, 5, 6 Thesis Draft done 1, 2, 3 Problem selected Background study completed Thesis submitted 3, 4, 5 5, 7, 9 4, 4, 4 1,2,3 2, 4, 6 minimum average maximum Draft revisions Program and Experiment completed ELEC 7770: Advanced VLSI Design (Agrawal)

  32. Timing Analysis Using PERT H. Chang and S. S. Sapatnekar, “Statistical Timing Analysis Considering Spatial Correlations Using a Single PERT_Like Traversal,” Proc. International Conf. on Computer-Aided Design, 2003, pp. 621-625. ELEC 7770: Advanced VLSI Design (Agrawal)

  33. Large Circuit Timing Analysis • Determine gate delays: • From layout analysis, or use approximate delays: • Gate delay increases in proportion to number of fanouts (increased capacitance) • Delay decreases in proportion to increase in gate size (reduced transistor channel resistance) • Purpose of analysis is to verify timing behavior – determine maximum speed of operation. • Methods of analysis: • Circuit simulation – most accurate, expensive (Spice program) • Static timing analysis (STA) – most efficient, approximate ELEC2200-002 Lecture 8

  34. Static Timing Analysis (STA) • Combinational logic for critical path delays. • Circuit represented as an acyclic directed graph (DAG). • Gates characterized by delays; gate function ignored. • No inputs are used – worst-case analysis – static analysis (simulation would be dynamic). ELEC2200-002 Lecture 8

  35. Combinational Circuit of an FSM A 1 H 1 H 1 Gate delay B 1 E 4 G 1 Fanout = 4 C 2 J 1 F 2 D 1 Input to Output delay must not exceed clock period ELEC2200-002 Lecture 8

  36. Static Timing Analysis (STA) Step 1 Levelize circuit. Initialize arrival times at primary inputs to 0. 0 0 A 1 H 1 0 0 B 1 E 4 G 1 0 0 C 2 J 1 F 2 0 0 D 1 Level of a gate is one greater than the maximum of fanin gate levels Level 0 1 2 3 4 5 ELEC2200-002 Lecture 8

  37. Static Timing Analysis (STA) Step 2 Determine output arrival times of gates in level order. 0 0 1 A 1 10 H 1 0 0 1 B 1 6 E 4 9 G 1 0 0 C 2 2 9 J 1 F 2 0 0 8 1 D 1 Arrival time at a gate output = maximum of input arrivals + gate delay Level 0 1 2 3 4 5 ELEC2200-002 Lecture 8

  38. Static Timing Analysis (STA) Step 3 Trace critical paths from the output with longest arrival time. 0 0 1 A 1 10 H 1 0 0 1 B 1 6 E 4 9 G 1 0 0 C 2 2 9 J 1 F 2 0 0 8 1 D 1 Critical path: C, E, F, G, H; delay = 10 Level 0 1 2 3 4 5 ELEC2200-002 Lecture 8

  39. Characteristics of STA • Linear time analysis, Complexity is O(n), n is number of gates and interconnects. • Variations: • Find k longest paths: • S. Kundu, “An Incremental Algorithm for Identification of Longest (Shortest) Paths,”Integration, the VLSI Journal, vol. 17,  no. 1, pp. 25-35, August 1994. • Find worst-case delays from an input to all outputs. • Linear programming methods. ELEC 7770: Advanced VLSI Design (Agrawal)

  40. Algorithms for Directed Acyclic Graphs (DAG) • Graph size: n = |V| + |E|, for |V| vertices and |E| edges. • Levelization: O(n) (linear-time) algorithm finds the maximum (or minimum) depth. • Path counting: O(n2) algorithm. Number of paths can be exponential in n. • Finding all paths: Exponential-time algorithm. • Shortest (or longest) path between two nodes: • Dijkstra’s algorithm: O(n2) • Bellman-Ford algorithm: O(n3) ELEC 7770: Advanced VLSI Design (Agrawal)

  41. References • Delay modeling, simulation and testing: • M. L. Bushnell and V. D. Agrawal, Essentials of Electronic Testing for Digital, Memory and Mixed-Signal VLSI Circuits, Springer, 2000. • Analysis and Design: • G. De Micheli, Synthesis and Optimization of Digital Circuits, McGraw-Hill, 1994. • N. Maheshwari and S. S. Sapatnekar, Timing Analysis and Optimization of Sequential Circuits, Springer, 1999. • PrimeTime (Static timing analysis tool): • H. Bhatnagar, Advanced ASIC Chip Synthesis, Second Edition, Springer, 2002 ELEC 7770: Advanced VLSI Design (Agrawal)

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