IBIS Model Ramp Keyword Data

Ramp Data

By Timothy Coyle

Understanding Legacy Ramp Data in IBIS

In the earlier versions of the IBIS Specification the VT waveform data did not exist. A simpler method called Ramp Data was used to estimate the slew rates of the signal edges and this in turn was used by simulators to turn the pullup and pulldown IV curves on and off. As we have seen previously the VT data is a more accurate data set to use but the Ramp Data is still an IBIS requirement so we are going to go over it.

The Required Ramp Data

From the image above it can be seen why the VT data waveform is more accurate than the Ramp Data because the Ramp Data only allows for a linear approximation of the signal edges. The Ramp Data is calculated for both the rising and falling edge. For the Rising Edge the reference load is R_fixture to Ground (pullup on) and for the Falling Edge the reference load is R_fixture to Vcc (pulldown on) using 20 to 80 transition measurement.

One of the reasons the IBIS Specification still requires the Ramp Data even though most IBIS models now use the VT data waveforms for better accuracy is to stay backwards compatible. Plus some EDA simulators still use the Ramp Data transition time variables to estimate the edge rates of the driver for determining the fastest edge rate in the system link for transmission line analysis.

Accuracy Issues to Watch for In Ramp Data

Since the Ramp Data must be calculated from the VT data curves there are often errors with the Ramp Data in an IBIS file.

The first thing to verify is that the measured 20 to 80 voltage change parameter dV matches the actual VT waveform. The second thing to verify is that the measured 20 to 80 time change parameter dt matches the actual VT waveform. Often times the biggest error is that the Ramp Data is improperly calculated from the VT data or the wrong VT waveforms are used for the calculation.

Another issue to watch out for is the ramp rate order. If the design is a CMOS design where in most cases the Minimum process corner is the ‘slow’ corner and the Maximum process corner is the ‘fast’ corner than the dt values should reflect that with the Maximum being the faster than the Minimum since it is slower.

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