IBIS Voltage Time Reference Point

VT Data Reference Points

By Timothy Coyle

Setting Your VT Data Reference Point

When generating VT Data for an IBIS model it makes sense that you need to have both a rising and a falling waveform for each specific load that you use. This way when a simulator uses the VT data it can account for both the rising and falling edge of the buffer. The starting point you choose for your VT data simulations does matter and it has to conform to the IBIS Specification guidelines.

Starting VT Data at Time Zero

The IBIS Specification only has one requirement on the VT data reference points: for each rising and falling VT waveform set into the same load the starting time point has to be the same. Again, this makes sense from a simulator point of view where you are trying to re-create the output behavior of the buffer and you want to know what the rising and falling edges look like. You need them to start at the same point otherwise you will not be able to accurately re-create the correct pulse width, edge rate, etc.

Notice that the IBIS Specification does not state that you need to start at a time point of zero. This is often a misconception of the specification because it does not require a time point of zero. The issue here is that historically most EDA simulation tools would give errors if the VT reference point was not zero. In reality, if you are running SPICE simulations to generate the IBIS data there is little reason to not start at time zero. By using a time zero start point for all of your VT waveforms you can ensure that your model data will be compliant with all EDA simulation tools and it requires no book keeping on your end to make sure you have the correct reference points between all VT waveform data pairs.

Why Your VT Time Scale is Relative

We have talked about avoiding over-clocking issues by dealing with lead-in time and dead time and also why starting with a reference time point of zero is the best way to go. So as you can see the actual VT time scale in IBIS is relative in the manner that only the time step resolution matters. So you can shift your VT data along the time axis as long as you maintain the correct time step resolution. The reason this is possible is because an IBIS model does not maintain any internal timing relationships so the actual Time to Clock Output timing parameter is not valid with an IBIS model. We’ll discuss in a later article how an IBIS model is used for timing.

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