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How to Create an SR Flip-Flop SPICE Model

Modern electronics could not exist without semiconductors and digital electronics but as electronics become faster and more complex, it’s essential to model realistic behavior to ensure design accuracy. At high speeds, digital electronics are susceptible to issues with propagation delay, and any stray capacitance present on the board could lead to attenuation, which could prevent a high input from ever reaching the device’s threshold voltage. PSpice allows you to quickly create a custom SR flip-flop SPICE models for accurate simulation of digital designs with a wizard-based approach.

This quick how-to will provide step-by-step instructions on how to create a clocked SR flip-flop SPICE model in OrCAD PSpice.

To follow along, download the materials provided above the table of contents.

How-To Video

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Creating an SR Flip-Flop SPICE Model

Step 1: Open the provided design in OrCAD PSpice 23.1.

Note: A wizard-based approach to digital models is new in version 23.1. See other new features here.

Activating the Modeling Application in PSpice

Step 2: Select Place > PSpice Part > Modeling Application from the menu.

Step 3: In the Modeling Application, expand Digital > Flip Flops.

Step 4: Select Clocked SR – Set/Reset.

Note: This model contains PRESET and CLEAR pins, which can be used to set the output of the flip-flop without the clock.

Using the Modeling Application in PSpice to create an SR Flip-Flop SPICE Model
Configuring SR Flip-Flop SPICE Model Parameters

Step 5: The Clocked SR Flip-Flop options window opens. Enter the following model parameters:

  • Clock to Output Delay: 15n
  • Set/Reset Delay: 13n
  • Setup Time: 13n
  • Hold Time: 5n
  • Threshold: 1.4
  • Hysteresis Width: 0.4
  • Output Resistance: 1m
  • Input Resistance: 10G

Note: The logic device being modeled has negligible output resistance and requires negligible input current.

Step 6: Click Place.

Step 7: Click to place the flip-flop in the schematic.

Step 8: Select the Voltage/Level Marker button from the toolbar.

Placing Probes in PSpice

Step 9: Click to place probes on the CLK, DATA1, DATA2, and DATA_OUT nets. Right-click and select End Mode when finished.

Running the Simulation

Step 10: Select PSpice > Run from the menu.

Step 11: View the simulation results.

Modifying the Design

Step 12: Back in the schematic, select the DSTM1 stimulus.

Step 13: Hold Alt on the keyboard and click and drag to disconnect the stimulus from the net.

Step 14: Select the DSTM2 stimulus. Hold Alt and click and drag to disconnect the stimulus from the net.

Step 15: Click and drag DSTM1 to connect it to the DATA2 net.

Adjusting the Stimuli for PSpice Simulations

Step 16: Click and drag DSTM2 to connect it to the DATA1 net.

Step 17: Select PSpice > Run from the menu.

Step 18: View the simulation results.

Wrap Up & Next Steps

Quickly create the required SR Flip-Flop SPICE models and simulate accurate digital circuit behavior with the PSpice Modeling Application in OrCAD PSpice. Upgrade to the latest 23.1 release or test out this feature and more with a free trial of OrCAD.

Follow Along, Download the Pre-Packaged Design Files

Table of Contents

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