Timing Relationship between Signals

The signal launched at one end of the PCB trace takes finite amount of time when traveling from one end to another end of the trace. The signal typically takes 150 ps to travel 1 inch of trace. If your trace is 8 inches long, it will take 1200 ps to travel from one end of the trace to the other end. In many buses, clock and data need to preserve a definite timing relationship. If the receiver captures data at the rising edge of the clock, then the data must arrive and become stable slightly before the rising edge of the clock.
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