I/O Interfaces & Buses

What is the purpose of spread-spectrum clocking (SSC), used in PCI Express and elsewhere?

  1. ATo raise the clock frequency greatly and double the data rate
  2. BTo vary the clock frequency slightly, spreading electromagnetic emission energy over a wider band and lowering its peak
  3. CTo stop the clock and cut power to zero
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Answer: B. To vary the clock frequency slightly, spreading electromagnetic emission energy over a wider band and lowering its peak

Circuits clocked at a fixed frequency emit sharp noise peaks at that frequency and its harmonics, which can exceed electromagnetic interference limits. SSC slowly sweeps the clock frequency up and down by about 0.5%, spreading the noise energy over neighboring frequencies and lowering the peaks. The receiver's CDR is designed to track this slow variation and still capture the data.

  • A. The variation is about 0.5% and does not raise speed.
  • B. Correct. It makes electromagnetic interference limits easier to meet.
  • C. The clock is not stopped.
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  2. Which describes the credit-based flow control used in PCI Express?
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