## Definition

Circuit simulation is a process in which a model of an electronic circuit is created and analyzed using various software algorithms, which predict and verify the behavior and performance of the circuit. Since fabrication of electronic circuits, especially integrated circuits (ICs), is expensive and time-consuming, it is faster and more cost-effective to verify the behavior and performance of the circuit using a circuit simulator before fabrication.

There are different types of circuit simulators catering to varied needs across the accuracy-performance/capacity spectrum. At one end of the spectrum are analog simulators that solve accurate representations of the electronic circuits. They offer high accuracy and are commonly used to simulate small circuits. At the other end of the spectrum are digital simulators that use functional representations of electronic circuits, typically described using hardware description languages (HDL). These offer the highest performance and capacity, but at relatively lower levels of accuracy. Digital simulators are commonly used to simulate very large circuits.

## Types of Circuit Simulation

There are three basic types of circuit simulation: analog, digital, and mixed-mode.

Analog circuit simulation involves the use of highly accurate models (i.e., representations) of the electronic circuit to achieve high accuracy. The models include non-linear, linear, and simpler table-based representations of the various electronic devices in the circuit. Analog simulation can run in different modes. These include AC (frequency domain), DC (non-linear quiescent), and transient (time-domain). All analog simulators employ algorithms to mathematically analyze the behavior of the electronic circuit in these different modes. They all share the quality of solving matrices to predict the performance of the electronic circuit. Signals are propagated as continuously varying values.

There are two primary types of analog circuit simulators: SPICE and FastSPICE. SPICE simulators use highly accurate non-linear and linear models of electronic devices to analyze the behavior of the circuit. SPICE simulators employ many different integration methods, such as Forward Euler, Backward Euler, and Newton-Raphson as well as matrix decomposition techniques to compute the response of the entire circuit (i.e., mathematical representation) at every single time point in the simulation period of interest.

By contrast, FastSPICE simulators use simpler table-model representations of electronic devices to analyze circuit behavior. They employ sophisticated algorithms to reduce the complexity of the circuit and partition the circuit based on various criteria, essentially creating a simpler and more modular circuit representation. This representation is then selectively evaluated at a given time point in the simulation period of interest, a process that greatly improves the performance and capacity of the simulation. FastSPICE simulators offer various simulation knobs to help balance the tradeoffs between simulation accuracy and performance.

Digital circuit simulation involves the use of simpler models of the electronic circuit. These models are typically created using HDL. In digital simulation, rather than propagating continuously varying signals, a few discrete voltage levels (primarily logic 0 and logic 1) are propagated. The methods to propagate these signals have varying degrees of accuracy regarding the propagation delay of the logic levels through the circuit.  This method of simulation allows much larger circuits to be simulated in less time with fewer compute resources when compared to analog simulation.

Mixed-mode circuit simulation combines the analog and digital simulation approaches. The circuit is partitioned between the two regimes to support the correct level of analysis detail for each part of the circuit. Analog simulators (SPICE and FastSPICE) are used for the analog analyses and a digital simulator is used for the digital analyses. This method of simulation allows for much larger circuits to be simulated in less time with fewer compute resources when compared to analog simulation.

## Benefits of Circuit Simulation

Circuit simulation provides a critical view into the behavior of electronic circuits. Given the expense and time involved in fabricating electronic circuits, especially ICs, it’s much more practical to validate circuit behavior and performance via circuit simulation prior to manufacturing.

Some of the specific areas of validation include:

• Memory Performance. The read and write access times and latency of memory devices are built from analog circuit simulation of the bit cells and read/write paths inside these memories.
• Overall Digital Simulation Accuracy. Digital circuit simulators model the propagation of voltage for logic level 1 and logic level 0. Analog circuit simulation is used to determine the time it takes for a circuit to transition between these voltage levels. This forms the basis for the overall accuracy of the digital circuit simulator.
• Noise and Crosstalk. Higher level models for noise and crosstalk are developed based on the detailed circuit level analysis of these parasitic effects from analog circuit simulations.
• Optimization of High-Frequency and High-Power Circuits. These types of circuits must undergo detailed continuous time analysis to determine their behavior and performance criteria. Analog circuit simulation delivers these important analyses.
• The overall performance and behavior of complex digital circuits (core processors and AI accelerators are examples) are verified with digital circuit simulation.

## Circuit Simulation and Synopsys

Synopsys offers a range of analog and digital circuit simulation solutions for a diverse set of use cases and circuit types.

Analog simulation tools include:

• PrimeSim Continuum: Unified workflow of Synopsys SPICE and FastSPICE simulators
• PrimeSim HSPICE: The gold standard in SPICE simulation
• PrimeSim XA: Industry-leading FastSPICE simulator targeting SRAM and custom digital designs
• PrimeSim SPICE: Industry’s highest performance SPICE simulator targeting analog, RF, and SerDes designs
• PrimeSim Pro: Industry-leading FastSPICE simulator targeting DRAM and Flash designs
• PrimeSim Custom Fault: High-performance analog fault simulator targeting full-chip functional safety, test coverage analysis, and silicon failure analysis
• PrimeWave Design Environment: Newly architected design environment providing comprehensive analysis, improved productivity, and ease of use

Digital simulation tools include: