Digital Logic Design Simulation Software and PSPICE

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Cadence PCB Solutions

Key Takeaways

Digital logic design simulation software can enable various simulation types, including functional, gate-level, timing, and mixed-signal simulations, catering to different stages of circuit design.
Simulating digital circuits before physical prototyping helps in identifying and rectifying design errors early, reducing development time and costs.
Tools like PSpice enable the integration of digital logic with analog components, allowing for comprehensive mixed-signal simulations within a single environment.

Digital logic components in PSPICE — *An advanced SPICE simulator such as PSPICE features digital logic simulation capabilities.*

The logic behind the use of simulations cannot be disputed. Having the ability to create a virtual exact replication of various components and circuit designs without actual expenditures being incurred is priceless. In addition, the ability to test various designs and configurations through simulations reduces development time,costs, and greatly improves your designs overall. Read on as we discuss digital logic design simulation software, when specific software is used, and how SPICE —and specifically PSpice, should be part of your digital logic design simulation software suite.

What is Digital Circuit Design Simulation?

Digital logic circuit simulation uses mathematical models to replicate the behavior of an electronic device or circuit. The simulation software itself allows for the modeling of circuit operation. In today’s cost-laden design and manufacturing process, software tools for designing and simulating digital circuits have become a key to shortening design times, limiting costs, and improving designs.

When working on integrated circuits or on circuit design in general, logic simulation is used to determine the correctness of the circuit. The three fundamental logic gates are AND, OR, and NOT gates (with NAND and NOR also being extremely common). When determining how to simulate logic, you’ll want to have a circuit and work through the potential outputs of a switch or other gates you’ll be using.

However, digital logic simulation is a vast field, with different digital logic simulators being used for other cases.

Common Types Of Digital Logic Design Simulation Software

Simulation Type	Description & Use Cases	Example
Functional (Logic-Only)	Simulates logic behavior without any timing. Used early in design to validate logical correctness —truth tables, combinational circuits, control logic, etc.	Testing a finite state machine’s behavior using basic logic gates
Gate-Level Simulation	Operates on a synthesized netlist of gates (NAND, NOR, etc.), often with estimated delays. Verifies that the Register Transfer Level (RTL) logic was mapped correctly.	Checking synthesized counter logic from an HDL description
Timing Simulation (Post-Layout)	Includes physical delays from layout and interconnect. Ensures setup and hold timing constraints are met across actual routed paths.	Verifying clock domain crossing after place-and-route
Event-Driven Simulation	Computes only when inputs change, improving performance. Common in HDL-based simulation environments (e.g., Verilog, VHDL).	Simulating a pipelined ALU in Verilog
Cycle-Based Simulation	Advances simulation one clock cycle at a time. Useful when exact timing isn’t required, just correct sequencing.	Running a simplified CPU core model for functional validation
Hardware-Accelerated (Emulation)	Uses FPGAs or dedicated hardware to simulate large designs quickly. Used in late-stage design verification.	Emulating an SoC design on an FPGA platform
Mixed-Signal Simulation	Combines digital logic with analog behavior (voltages, currents). Used when circuits involve ADCs, PLLs, or power-sensitive elements.	Simulating a microcontroller interfacing with a DAC

Digital logic simulation primarily functions through either compiled code or event-driven logic simulation techniques. In logic simulation, designers are looking to determine if their circuit contains any design errors (as is the case in most simulation efforts). SPICE simulators and logic simulators will be looking for digital cell libraries, memory, and AMS or analog and mixed-signal circuits.

What these circuits are looking for is to maintain precision timing to support their proper function. As timing-sensitive designs can be tricky to nail down, especially with the oftentimes painful distance between simulation software and hardware-capabilities, logic simulators are key for the design engineer’s pocket.

The Role of SPICE in Digital Logic Design Simulation

SPICE or Simulation Program with Integrated Circuit Emphasis is the industry standard for open-source analog electronic circuit simulation. Within SPICE, you simply provide a netlist of the circuit you wish to simulate, and SPICE takes care of the rest, allowing you to generate waveform plots for analysis.

Overall, one of the most beneficial aspects of using SPICE is that it allows you to verify the performance of your circuit without touching a single breadboard or physical component. It stands to reason that simulating a circuit’s behavior before actually building it can significantly improve design efficiency and the overall design itself.

PSpice is an advanced, commercial SPICE variant that retains full analog simulation capabilities but also layers on digital (and mixed-signal) engines to help model logic gates, registers, and I/O cells alongside analog circuitry. By combining analog transistor models with event-driven digital elements, PSPICE bridges the gap between purely analog SPICE simulations and pure HDL-based digital logic design simulation software.

Functional Logic Simulation

PSpice contains a digital engine that can model Boolean logic blocks (AND, OR, NAND, etc.), flip-flops, and simple finite-state machines—at a behavioral (logic-only) level. Instead of requiring a timing-annotated netlist like a dedicated gate-level simulator, PSPICE allows you to drop in idealized logic primitives (with default or user-specified propagation delays). This makes it easy to verify combinational and sequential correctness (e.g.,a counter follows the expected sequence) without delving into transistor-by-transistor details. Because these primitives are embedded in the SPICE environment, you can stitch digital outputs directly into analog blocks—something you can’t do in pure HDL simulators.

Gate-Level (Netlist-Based) Simulation (Partial)

While PSpice is primarily an analog solver, it can also accept a limited form of gate-level netlists (e.g., logic gates with lumped delay models) to verify that your synthesized design behaves approximately as intended. You won’t get the full gate-delay accuracy of a dedicated digital gate simulator, but you can import a simplified netlist of gates and connect them to analog input/output buffers, voltage references, or power-rail models. This partial gate-level support is helpful when you need a rough behavioral check of post-synthesis logic interacting with analog front-ends.

digital circuitry timing diagram in PSPICE — *PSPICE enables some modeling of digital circuitry to produce timing diagrams.*

PSpice Mixed-Signal Simulation Capabilities

One of the most powerful PSpice features is its ability to perform mixed-signal simulation, combining analog and digital components in the same simulation environment. This is essential in modern electronics design, where analog and digital blocks often coexist and directly influence one another, such as in sensor interfaces, power management, or data conversion systems.

At its core, PSpice uses a SPICE-based engine to solve continuous-time analog behavior (voltages, currents, transients) while incorporating an event-driven digital logic layer to simulate binary transitions, control logic, and state machines. These two engines communicate in real-time during simulation, allowing users to model:

Analog-to-Digital Interactions: For example, a temperature sensor modeled with analog components feeding a comparator and a digital controller.
Digital-to-Analog Effects: Like a digital pulse-width modulation (PWM) signal driving an analog filter or power transistor.
Threshold-Dependent Logic: Circuits where logic levels are triggered by analog thresholds or where power supply variations influence logic switching behavior.

PSpice includes built-in libraries of digital primitives (gates, flip-flops, multiplexers, etc.) that can be connected directly to analog circuits. Users can define logic delays, set voltage thresholds for logic levels, and observe how analog transients (e.g., supply noise, ringing) impact digital stability.

This makes PSpice ideal for verifying mixed-domain circuits such as:

ADCs and DACs
Clock generation and PLL systems
Level shifters and I/O buffers
Power gating and sleep-mode transitions
Sensor front-ends with digital post-processing

Other Digital Logic Simulators and Associated Uses

Simulation Type	What It Is	When It's Used	Typical Tool Category
Post-Layout Timing Simulation	Simulates precise timing using real routing delays, parasitics (RC), and standard delay format (SDF) back-annotation. Ensures setup/hold timing is met after layout.	Final sign-off before fabrication, after place-and-route is complete.	Static/dynamic timing analyzers
Event-Driven Simulation	Only updates parts of the circuit where signals change, optimizing performance for large digital systems. Typically works at the HDL level (e.g., Verilog, VHDL).	RTL or gate-level functional simulation of large designs, especially for logic correctness.	HDL-based simulators (event-driven simulation engines)
Cycle-Based Simulation	Simulates logic behavior one clock cycle at a time. Ignores intra-cycle delays, emphasizing control flow over timing detail.	Early performance modeling and regression testing for synchronous systems.	Cycle-accurate simulation platforms or virtual prototypes
Hardware-Accelerated Emulation	Uses FPGAs or specialized emulation hardware to run full designs at near-real-time speeds, enabling software co-validation.	Late-stage testing with software stacks or complex I/O before silicon is ready.	FPGA-based prototyping systems and digital hardware emulators

Digital Counter Simulation Tutorial in PSpice

In this tutorial, we’ll walk through how to design and simulate an 8-bit digital counter using PSpice. You’ll learn how to place components, configure a digital clock stimulus, run a transient simulation, and visualize digital waveforms. This simple example demonstrates PSpice’s ability to model digital systems and their timing behavior, ideal for learning how to combine Boolean logic with real clock sources.

Step 1: Launch OrCAD X Capture and Create a New Project

Open OrCAD X Capture from the Cadence program menu.
When prompted, select your license file and click OK.
Go to File > New > Project.
Name your project Counter, and choose a location to save it (e.g., Desktop).
Create a new folder named Counter and select it.
Enable PSpsice Simulation and click OK.
Choose Create a Blank Project and click OK.\

Step 2: Add Components to the Schematic

Go to Place > Components.
Search for and place the following components:

Pull-up resistors
Pull-down resistors
Two 74163 counter ICs (used to create an 8-bit counter)
A digital clock source (DTIM or “digital stimulus”)
Right-click and use End Mode after placing each component.

PSpice simulator editor — *PSpice stimulus editor for configuring a digital signal*

Step 3: Configure the Digital Clock (Stimulus)

Right-click the digital clock source and select Edit Stimulus.
In the Stimulus Editor:

Name the signal (e.g., Digi).
Set Type to Digital Clock.
Set Frequency to 100 Hz.
Set Duty Cycle to 50%.

Click Apply, then OK.

Save the .stl file when prompted, and close the Stimulus Editor.

To hide the stimulus properties on the schematic:

Double-click the stimulus block.
Go to Display → Select Do Not Display → Click OK.

Step 4: Connect the Circuit

Wire up the components to form a working 8-bit counter.

The two 74163 counters should be cascaded.
Connect the digital clock to the clock input of the counters.
Use pull-ups/pull-downs on control pins as needed.

Ensure the circuit resets after 255 counts, cycling back to zero.

PSpice simulation settings window — *PSpice simulation settings for the counter*

Step 5: Set Up and Run the Simulation

Click the New Simulation Profile button.
Name it Counter and click Create.
In the simulation settings:

Choose Transient Analysis.
Set Run Time to 5 seconds.

Under Options > Gate-Level Simulation, change:

DIGINITSTATE to 0 (starts digital states at zero).

Click OK, then run the simulation.

PSpice timing diagram setup — *Adding traces for the timing diagram in PSpice*

Step 6: View the Simulation Output

After the simulation completes, click Add Trace.
Select the output pins:

From U1: QA, QB, QC, QD
From U2: QA, QB, QC, QD

Click OK to plot the digital waveforms.

Use the zoom tool to analyze specific sections of the counter output.

In addition, SPICE also allows you to change component values (resistance, voltage, capacitance) and check frequency response across a range of tolerances or time periods. More advanced SPICE software even lets you run Monte Carlo analysis, perform smoke and stress tests, and create parametric plots.

Enhance your digital logic design simulations with PSpice bundled with OrCAD X, a comprehensive platform that integrates schematic capture and simulation capabilities. Utilize PSpice within OrCAD X to perform detailed digital logic simulations, ensuring accurate and efficient design verification. Experience the benefits firsthand by starting your free trial of OrCAD X today.

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