Electronic – Why is length matching performed with the clock trace length as the target length


Length matching is performed mainly to avoid the skew generated among the parallel data lines/data bus width.

All the high speed PCB design guideline suggest performing length matching with the clock trace length as the target length and trace length tolerance of the data, address, command lines has to be maintained with respect to clock signal. May I know the particular reason for this?

As I understand it, the clock appears continuously. There will not be any data loss in the clock, hence it is a periodic signal. Data will consider the positive edge or negative edge of the clock and it will appear initial sequence in the timing waveform.

Any insight would be greatly appreciated.

Best Answer

Data is sent in respect to the clock signal.

Data has to be stable before the clock edge (setup time) and it has to be stable after the clock edge (hold time).

If the clock wiring is too long compared to data, clock will appear too late to be within hold time specs, and if data wiring is too long compared to clock, clock will appear too early to be within setup time specs.