The PCI Express specification, section 18.104.22.168.1, describes Spread Spectrum Clocking (SSC) as follows:
The data rate can be modulated from +0% to -0.5% of the nominal data rate frequency, at a
modulation rate in the range not exceeding 30 kHz - 33 kHz. The +/- 300 ppm requirement still holds, which requires the two communicating Ports to be modulated such that they never exceed a total of 600 ppm difference.
For most implementations, this places the requirement that both Ports require the same bit rate clock source when the data is modulated with an SSC.
Are the Xilinx Virtex-5, Virtex-4, and Virtex-II Pro devices capable of supporting this requirement?
The Virtex-5, Virtex-4, and Virtex-II Pro devices are designed to support the PCI Express SSC requirement.
When using a Virtex-4 or Virtex-II Pro, if you see that the design is encountering a large number of phase errors because of the DCM's response to SSC, please try to increase the FACTORY_JF setting to X'FFFF.
For more information on FACTORY_JF, please see (Xilinx Answer 13756).
IMPORTANT: For Virtex-5, Virtex-4, and Virtex-II Pro Express add-in cards to work in an open system, they must be clocked off of the central motherboard clock resource so that they can take advantage of the SSC down spread and be frequency locked with the motherboard clock (i.e., synchronously clocked).
Add-in cards clocked with their own on-board oscillator (i.e., asynchronously clocked) will NOT work in open systems.
For more information on this, please refer to (Xilinx Answer 18329).