The Circuit Simulations You Need in VLSI Layout and Design

December 4, 2020 Cadence PCB Solutions

Key Takeaways

  • VLSI layout combines a huge number of circuits into a larger integrated circuit.

  • This design methodology starts with building fundamental circuit blocks and integrating them into a larger system.

  • When  designing circuit blocks before VLSI layout, a set of circuit simulations are used to optimize each circuit block.

VLSI layout silicon wafer

VLSI layout techniques are used to create these ICs on an Si wafer.

Every integrated circuit contains millions of transistors and other fundamental circuit elements, all of which need to be carefully designed and optimized to provide the desired electrical behavior. In a typical integrated circuit, each block in the device needs to be carefully built up from smaller circuits and, eventually, individual components embedded on a silicon wafer. Each portion of the design needs to be carefully optimized before circuit blocks are combined together in very large-scale integration (VLSI) layout software.

Simulation tools can be an important part of design optimization as designers can experiment with different components and topologies before integrating circuit blocks into a physical layout. When circuit simulations are needed as part of VLSI layout, the right set of circuit simulation tools will aid design optimization using parameter sweeps. Here’s what goes into circuit simulations as part of VLSI design and layout.

What Circuit Simulations Are Needed in VLSI?

When building up circuit blocks for an integrated circuit, each circuit will need its own set of simulations. Everything from electrical behavior to noise and thermal behavior in the system should be simulated to ensure the design can be properly optimized. Analog simulations for large VLSI layouts are time consuming, so individual circuit blocks are simulated before being connected into a larger system.

In VLSI layout, each fundamental circuit block gets combined together into larger blocks by cascading inputs and outputs between each block. In the 1-bit accumulator circuit shown below, the outputs from one accumulator block would be connected to the inputs of another accumulator block. The bus outputs would then be connected together on a single line in the integrated circuit and routed into another integrated circuit block.

VLSI layout

Accumulator circuit for a VLSI layout shown in Virtuoso.

In this simple example and in other VLSI designs, the important circuit simulations that should be performed include:

  • Output signals produced for a given set of inputs (both for the bottom output group and the bus output in the above example)

  • Electrical behavior of each transistor-based circuit that makes up the larger circuit

  • Total resistive power consumed by the circuit block

Each area listed above is quite broad, but they can be addressed with some simple simulation techniques as part of VLSI design. In each of these areas, circuit simulations are easiest when created with a SPICE-based simulator. This simulation tool has been the industry standard for decades, including for integrated circuit design. Some of the important simulation tasks that can be performed in a SPICE-based simulator are summarized below.

 

Electrical behavior

Type of simulation

Notes

Switching behavior for digital circuits

Transient analysis

Watch for underdamped oscillations during switching.

Signal transmission between circuit blocks

Transient analysis and frequency sweeps

The goal is to examine input/output impedances as part of matching for microwave circuits.

Transistor linear range

DC load line vs. base/gate current

Determine the base/gate current limit where saturation occurs.

Resonances and bandwidth

Transfer function (Bode plot)

Any resonances and the circuit’s bandwidth can be seen directly in a Bode plot.

Power consumption

Transient analysis

Dynamic power consumption can be determined from transient analysis for digital or wideband analog signals.

Circuit stability

Pole-zero analysis

This quickly summarizes whether electrical behavior will be stable (e.g., positive vs. negative feedback) for a given input signal.

Once you’ve performed these simulation tasks and determined electrical behavior in important portions of the circuit, you can begin cascading circuit blocks together to examine how signals propagate throughout an integrated circuit. This is the first step in examining how signals will transfer through a real integrated circuit made up of multiple interconnected circuit blocks. The other important aspect of circuit simulation in VLSI layout is circuit optimization, where circuit simulation can be used to determine the best design parameters for each circuit block.

Circuit Optimization and VLSI Layout

An important part of using circuit simulations is design optimization. This is a broad field of electronics design, and many unique optimization methodologies have been developed for specific structures in PCBs and ICs (e.g., transmission lines, impedance matching networks, and decoupling capacitor selection). The goal in circuit optimization is to balance different electrical objectives, such as maximized power efficiency, minimized impedance deviation, or maximized signal bandwidth.

These design goals are often in competition, and manually simulating every parameter value in your circuits is time consuming without the right set of circuit simulation tools. Instead of doing each SPICE simulation in your VLSI circuit blocks manually, you can use parameter sweeps in your SPICE simulator. This tool will iterate through a range of parameter values in your circuit, and the electrical behavior can be plotted graphically. You can then determine the parameter set that produces the desired electrical behavior graphically or using regression.

Parameter sweep VLSI layout

Parameter sweep results for an LC network in an MMIC. Using parameter sweeps helps you optimize electrical behavior of your circuit blocks as you prepare for VLSI layout.

When you need to perform circuit simulations for your VLSI layout, use the front-end design software from Cadence to start creating your circuit schematics and access simulation tools. The PSpice Simulator application includes an array of integrated circuit simulation features that are ideal for advanced electronics. You’ll have a complete set of tools to create and simulate your circuits in the time domain and frequency domain. You’ll also be able to perform parameter sweeps and tolerance analyses as part of design optimization for your circuits.

If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts.

 

About the Author

Cadence PCB solutions is a complete front to back design tool to enable fast and efficient product creation. Cadence enables users accurately shorten design cycles to hand off to manufacturing through modern, IPC-2581 industry standard.

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