PCIe goes Clockless -- Achieving independent spread-spectrum clocking without SSC isolation

Reginald Conley, PLX Technology
EETimes (7/5/2012 11:25 AM EDT)

PCI Express (PCIe) has established itself as the IO interconnect of choice for communication within the server and PC environment. Today, an emerging trend among designers is extending PCIe beyond the PC/server while maintaining the advantages of simplicity, bandwidth, scalability, low power and cost. One of the major system-level challenges in extending PCIe outside the box has been clock distribution between separated domains.

While many PCIe devices can operate asynchronously, these applications use constant-frequency clocking. The challenge gets more complicated when spread spectrum clocking (SSC) is needed. In systems required to operate with SSC, the only available option has been clock isolation. This method adds complexity in component count, clock fidelity, and media selection. In addition, the cable itself must operate in a constant frequency clock (CFC) domain. This CFC domain can still represent a significant source of electromagnetic interference (EMI).

Let’s look at the importance of introducing independent SSC operation to the ecosystem of PCIe and some of the cross technology advantages in performance, simplicity and cost that it can deliver.

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