In a simulation, the rise rail overshoot is nearly -2V for some signals. I am trying to understand the exact meaning of this parameter and how it correlates to the actual overshoot of this signal.
The measurement is pretty simple, but you need to be aware of whether the report is showing measured overshoot or overshoot margin. The rail overshoot is simply the peak voltage compared to the power supply level.
The power supply specified in the receiver IBIS model is 1.2V. The voltage reaches up to 1.289V, so the overshoot is 0.089V. If the measured overshoot is negative then the peak did not pass the voltage rail value. This often happens with a net that has pull-down or pull-up termination.
The overshoot margin is the allowed overshoot (specified in the receiver model or by the simulation setup) minus the measured overshoot. Margins should be calculated such that a positive value is good. If the margin value is -2V then this means that the peak measured voltage is 2V beyond the allowed limit. In the case above, if we have an allowed limit of 400mV of overshoot then the margin would be 400mV - 89mV = 311mV.
Retrieving data ...