Multi-Port Spread Spectrum Clocking Support In IDT PCIe® Gen2 Switches Application Note AN-715

PublishTime：2022-05-16

●Introduction
■The PCI Express Base Specification states that the reference clocks for ports at the two ends of a PCI Express Link must be modulated such that they never exceed a total of 600 ppm (parts per million) difference. PCI Express specifications also permit use of SSC (Spread Spectrum Clocking) with +0 to -5000 ppm modulation of clock frequency at a modulation rate not to exceed 33KHz. The 600 ppm rule applies even when SSC is used. If SSC is desired in PCIe based systems, the implication of the 600 ppm rule to simple system implementations is that if the reference clock used by a port at one end of a Link is SSC then the port at the other end of the link must use the same clock source. For a simple PCIe Switch, this implies that if one or more devices connected to the switch are clocked with SSC, then the switch and all devices connected to the switch must use the same SSC source. This may not be feasible in many system topologies for a variety of reasons. A solution to this problem is to create a PCIe Switch which can isolate the clocking domains on a per port basis such that each Switch port can function properly with a potentially unique SSC source required by and shared with its link partner. The Switch port and its Link partner can be logically thought of as functioning within their own local clocking domain, unconnected to the rest of the Switch device/ports in terms of clocking.
■The IDT PCIe Gen2 System Interconnect and Inter-Domain Switch families offer devices which implement local port clocking functionality described above. This application note describes various clocking topologies which can be implemented using these Switches. IDT System Interconnect devices which offer this local port clocking functionality include the 89H64H16[A]G2, 89H48H12[A|B]G2, 89H34H16G2, 89H32H8G2 and 89H22H16G2. All of the IDT PCIe Gen2 Inter-Domain Switches offer local port clocking. Examples of such Inter-Domain Switch are the 89H32NT24[A|B]G2, 89H24NT24[A]G2, 89H32NT8[A|B]G2, 89H16NT16[B]G2, 89H12NT12G2, 89H16NT2G2, etc. For simplicity, this document often uses a representative product name “PES64H16AG2” or similar shorthand to represent all products with this local port clocking capability.
■Note that there is a slight difference between the implementations of local port clocking in the System Interconnect switches and the Inter-Domain Switches. In the System Interconnect Switches it is possible to have a unique local port clock for each port, while in the Inter-Domain Switches one local port clock is shared by two to four ports.

Brand of Product：	IDT
Part#：	89H64H16G2 、 89H64H16AG2 、 89H48H12G2 、 89H34H16G2 、 89H32H8G2 、 89H22H16G2 、 89H48H12AG2 、 89H48H12BG2 、 89H32NT24G2 、 89H24NT24G2 、 89H32NT8G2 、 89H16NT16G2 、 89H12NT12G2 、 89H16NT2G2 、 89H32NT24AG2 、 89H32NT24BG2 、 89H24NT24AG2 、 89H32NT8AG2 、 89H32NT8BG2 、 89H16NT16BG2 、 PES64H16AG2 More Part#
Product Category：	PCIe® Gen2 Switches 、 PCIe Switch More
Application：	More
Data Type：	Application note & Design Guide More
Safety & Environment Standards：	More
Operating Temperature：	Please see the document for details
Qualification Grade：	More
Qualification Approval：	More
Package：
Language：	English Chinese Chinese and English Japanese
Date of Creation：	October 26, 2009
Version Number：
Data Encoding：	AN-715
Data Size：	431 KB

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