Fast SPICE Simulators: A Survey

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Fast SPICE Simulators A Survey
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Outline

• How SPICE works
• Fast SPICE simulators in the market
• Nassdas HSIM
• Avant!s Star-SimXT
• and more
• More on HSIM

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SPICE

• Really, its just all about solving KCL and KVL
  equations
• Generally, its a nonlinear differential equation
  (e.g. transistors).
• Bunch of numerical algebra

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First, Linearize it

• At each time step, SPICE builds a small-signal
  model (i.e. linear model) at the operating point

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Matrix Equation

• Construct a matrix equation Axb

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Then, Solve Axb

• Direct Method x A-1b.
• Iterative Method xn1 f(xn) iterate until xn
  converges to the solution.
• Example b 1/a vs bn1 bn(2-a.bn).

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Ways to speed up

• Speed bottlenecks
• Small circuits linearizing
• Large circuits solving equations
• Recent techniques for speed-up
• Look-up table model
• Event-driven algorithm or multi-timestep
  algorithm less computation for inactive
  subcircuits
• Hierarchical simulation save memory and
  computation for redundant circuits.
• Parallel computation

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Cadences Spectre

• 3x faster than SPICE
• Can handle up to 50,000 devices
• Uses the same basic algorithms as SPICE just
  written more carefully with the latest
  algorithmic techniques.
• Behavioral description (Verilog-A)

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Cadences ATS

• Accelerated Transistor-Level Simulator
• 100-1,000x faster than SPICE within 5 error
• Partitions the circuits and uses different
  time-steps exploits circuit latency and
  multi-rate behavior

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Celestrys UltraSim

• 100-1,000x faster than SPICE within 1 error
• Hierarchical simulation adaptive-hierarchy
  compaction technology
• ? crucial for DRAM simulation
• Faster ODE solver engine

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Synopsyss NanoSim

• 10-1,000x faster than SPICE within 2-7 error.
• Used with TimeMill and PowerMill.
• Hierarchical Array Reduction(HAR)
• Analog Circuit Engine(ACE) partitioning and
  synchronization (parallel comp.).
• Verilog-A support

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Nassdas HSIM

• Hierarchical Storage and Isomorphic Matching
• 1,000-10,000x faster than SPICE with
  user-selectable accuracy
• Hierarchical storage and simulation
• Isomorphic matching duplicate simulated circuit
  response for isomorphic subcircuits under same
  conditions.
• Does not use simplified model or simulation
  algorithms.

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Avant!s Star-SimXT

• Successor to Star-Sim (old Anagram)
• 10,000x faster than SPICE with 0 error
• Uses exactly the same models as Hspice (no more
  table-based models).
• Hierarchical simulation (Dynamic-Hierarchy
  Engine)
• Multi-threading simulation kernel (4x speed up on
  multi-CPU machines)

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Some HSIM Tips

• User-Selectable Accuracy
• Logic Functional Verification
• Timing and Power Checks

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Accuracy Options in HSIM

• Can individually set for each subcircuit or
  instance
• .param subcktpll instXpll HSIMparamltvaluegt
• HSIMSPEED choose speed-up mechanisms
• ? 0 (accurate) 6 (fast) (see the manual).
• HSIMSPICE model accuracy
• ? 0 (table model), 1 (DC model), 2 (AC model).
• HSIMANALOG coupling between subcircuits
• ? 0 (no coupling), 1 (coupling within
  hierarchical boundary), 2 (coupling across the
  boundary).

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Functional Verification

• Spice deck
• .param HSIMVECTORFILE hsim.vec
• Vector file (hsim.vec)
• signal clk pd_out10 phdir phwt_0 phwt_14
• phsel_up phsel_dn phwt_up phwt_dn toggle_dir
• period 10
• radix 111111 11111
• io iiiiii ooooo
• 110111 00000
• 010111 00000
• 110111 00000

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Assertion Checks

• Timing checks setup, hold, pulse-width, and
  delay.
• .tcheck check1 setup D x ck r 100ps
• Power checks DC path, excessive current,
  excessive rise and fall time, and high-impedance
  node.
• .pcheck check2 exrf Q rise200ps fall200ps

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Some Observations

• The fundamentals never change still solving the
  equation Axb.
• Fast SPICE is cool, but the market is small
  (200-300 million).
• Will it continue to evolve?
• ? Yes, cheaper than hiring a circuit designer.
• Future directions
• ? More parallel computation
• ? Better user-interface / post-sim analysis