In the beginning there was transient simulation. Ensuring functional chip-to-chip links in a system meant performing a time-domain simulation with a SPICE simulator. The result was a time-domain waveform that was evaluated during post processing for signal integrity (SI). The main task was to measure the eye diagram, usually assuming a perfect clock as the phase reference. Due to rising signaling speeds and decreasing timing margins, the task was made more challenging when it became necessary to account for other effects.