Modeling designs helps engineers verify whether their circuit will function as intended. Modeling programs simulate circuit behavior under different conditions and in accordance with the design requirements, allowing engineers to better plan and build circuits.
Every SPICE simulation needs to have a source or input signal. To analyze how the circuit will behave in real life, this input must be modeled accurately. If not modeled accurately, functionality issues can go undetected until far later in the design process, wasting time and money. To efficiently and accurately create a digital stimulus SPICE model, the intended input signal must be interpreted by identifying the type and signal values for replication.
What is a Digital Stimulus?
A digital stimulus generates input signals that emulate the behavior of digital logic levels. These signals are used to test and analyze the functionality of digital circuits which include gates, flip-flops, latches, counters, or entire digital systems.
What is Needed to Create a Digital Stimulus?
The digital stimulus model produces sequences of binary states (logic 0 and 1), based on specified timing and waveform characteristics. To create a digital stimulus SPICE Model, there are a few items that must be defined:
Stimulus
- What are the times and binaries associated with the stimulus?
Electrical Characteristics
- What is the resistance at the output pin if connected to an analog device?
- What are the high and low output voltage levels?
This information must be incorporated into the SPICE simulation model which can be achieved by manually creating or editing a text file. Keep in mind if the source signal created is not the intended outcome, values will need to be edited manually. This manual process to produce the desired source signal is time consuming and increases the likelihood of errors; however, the PSpice Modeling App provides a fast, easily configurable, and fully integrated method to create a digital stimulus SPICE model for simulation.
Creating a Digital Stimulus SPICE Model with PSpice
The digital stimulus modeling application quickly creates a digital stimulus SPICE model with a wizard-based approach. This application models the digital stimulus source using a stimulus file for a maximum of 32 output nodes. To model a digital stimulus, specify the names of the signals in the stimulus file, which are to be referenced by the device. The signal names correspond to the nodes connected to the device.
The stimulus file should contain the outputs as well as the time and sequence of binaries required to create the stimulus in the following format:
- The first row is reserved for output bit names
- The first column is reserved for time
- The second column is reserved for binaries
The necessary source specifications are pre-defined, and users can easily input desired parameters required such as:
Number of Output Nodes
The total number of outputs defined in the stimulus file.
Output Resistance (ROUT)
Specify output pin series resistance in ohms, if connected to an analog device.
Max High Voltage
Define the output high voltage (V), if connected to an analog device.
Max Low Voltage
Define the output low voltage (V), if connected to an analog device.
Using the inputted information above, the PSpice Modeling App generates a schematic symbol and automatically associates the newly created digital stimulus SPICE model without leaving the OrCAD Capture environment. The PSpice Modeling App also automatically manages the simulation profile configuration, eliminating any library set up for simulation.
