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Signal Integrity Simulation - Getting started: Part 2

Moderator
Moderator
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Hello and welcome back to the Hardware simulation blog series.

In the earlier posts, we discussed the basics of Signal Integrity (SI) Simulations, the models required to run them, and the basics of HyperLynx®.

If you haven't gone through them, I would highly recommend going over those posts before proceeding with this entry. 

The World of Hardware Simulation

Signal Integrity Simulation - Getting started: Part 1

In this entry, we will cover the available waveform viewer options and how SI simulations can be used to correlate with hardware measurements. HyperLynx®  provides two different waveform viewers to view the simulation results.

Oscilloscope Viewer:

The Oscilloscope Viewer provides simulation options in an Oscilloscope window. It is good for new users as it is quick and easy to use with interactive simulations.

There are options in the Oscilloscope viewer for both non-sweep and sweep run simulations.

fig 1.jpg

The Digital Oscilloscope dialog box looks similar to an Oscilloscope with the knobs and dials.  

fig 2.jpg

Operation:

Set it to either Standard or Eye Diagram to view results as standard waveforms or as an Eye Diagram.

Stimulus:

The stimulus can be set to Global for a simple stimulus, with either the Edge or Oscillator options set as required.

For Per Net/pin basis, per pin stimulus can be set in the Assign Stimulus dialog box.

If using the Eye Diagram operation, Eye Mask can be set for Global or Per Net/Pin options.

Simulator:

Select Auto to have the tool pick the appropriate simulator automatically. Manually set the simulator to HyperSim/ADMS/HSPICE if using a specific model of this kind. 

Time resolution:

Used to set the simulation time step size. Auto is the default, but it can be manually set for any value between 0.001ps and 1000ps.

SPICE Options:

If there are SPICE models, use this option to specify parameters or include files.

IC modeling:

This is used to set the IBIS IC model corner to be used in simulation.

Show Window:

Disable or Enable options here to include the Overview pane, Readout text (settings displayed on the Top Main Screen), and Loaded/Previous/Latest results to be viewed.

The Probes can be set to Always at the Pin/ Always at the die/Per each IC model’s setting.

Pin Tree:

Used to Enable and disable probes, edit the waveform color by double-clicking the color square, display waveforms for the latest/previous simulations, and manually add differential probes by double-clicking <Insert diff probe>.

Visibility:

Choose to display Voltage or Current waveforms.

Thresholds for:

Used to display IBIS model threshold voltages (such as Vinh, Vmeas, and Vdiff) as dashed lines in the Main screen.

Select between All IC Pins/<no pin selected>/<reference designator.pin>.

Vertical:

Position – Used to move the waveform and the ground marker in the main screen up or down relative to the grid.

Scale – Used to set the Vertical scale.

Horizontal:

Delay – Used to move the completed simulation waveform to the edge of the screen based on the setting.

Scale – Used to set the Horizontal scale.

Measurements:

Used to automatically measure waveforms for the “Entire” simulation time or a specific “Region” of the waveforms.

You can use the Waveform option to select the waveform of interest to measure.

Save/Load:

Used to save the waveforms or load a saved waveform file into the Waveform viewer.

Copy to Clip:

Copies the waveform so that it can be pasted into another Windows application

Erase:

Erases latest, previous, and loaded waveforms

Print:

Prints the waveform

* Note: Some options might not be available, or restrictions can apply if using the Eye Diagram Operation.

 Interactive Sweeps can be done to sweep a specific model for different parameters to view the effects.

fig 3.jpg

Below is an example of trying to sweep the Differential Termination resistor from 80 ohms to 120 ohms in increments of 10 ohms.

fig 4.jpg

EZWave Viewer:

Provides simulation options with results displayed in the EZWave viewer and also the measurement spreadsheet.

Good for experienced users as it supports multiple simulations and multiple options.                         

The four options available in EZWave viewer for non-sweep and sweep run simulations are as follows:

fig 5.jpgfig 6.jpgfig 7.jpgfig 8.jpg

The screen capture below shows the options available to plot in the EZWave viewer for Interactive simulation, and the resulting window.

fig 9.jpg

The screen capture below shows the options available to plot in the EZWave viewer for Interactive simulation with measurements:

fig 10.jpgfig 11.jpg

The viewer can be complicated to read when there are multiple waveforms with different options selected.

It will require some familiarization to get used to viewing the results.

The viewer gives you the flexibility to run multiple simulations and heavy run simulations and so is preferred by experienced users. 

Using IBIS Simulations to correlate with Hardware Measurements

 

Today’s high-speed PCBs are often more than 10 layers in thickness, in addition to having complex routing.

During board bring-up, the signal of interest is often probed at the signal via on the board or a test point on the line.

Due to the signal being probed away from the receiver and the effect of the probe, reflections might be observed on the probed signal and tend to be misconstrued as signal quality being poor.

IBIS simulations provide a good method to correlate with on-board oscilloscope measurements.

In the below example, the signal of interest is probed at a test point. Adding the probe model to the interface schematic, the simulation waveforms show the probed signal which can seem like a poor-quality signal.

However that is not the case as the signal at the receiver is much cleaner.

fig 12.jpg

fig 13.jpg

Today's high-speed PCBs incorporate micro vias, buried vias, and blind vias on signals to meet the data rates, making the probing of these signals in a lab impossible due to non-accessibility on the board.

Signal Integrity simulations are the only means for an engineer to debug and sign off on issues with these signals.