What is free running clock?

[Altera_Forum]

Hi 

 i came across the free running clock in the PCI Express Userguide. Based on the words, i thought it is a clock that can run with any frequency. However, in the userguide(page 151), it states that 125MHz free running clock. So, I guess that it is the clock that are not derived and connect from oscillator or cyrstal?    Pls correct me if i am wrong.   www.altera.com/literature/ug/ug_pci_express.pdf   Thanks

[Altera_Forum]

generally, a free running clock is not phase locked to other clock sources. 

 In this case it means you need another oscillator to provide that free running clock mentioned in that altera paper.   "free running" does not tell anything else about additional properties of the clock source, e.g. phase noise performance.

[Altera_Forum]

Hello woeichee, 

  I always believed a free-running clock is a clock that will not stop running, i.e. a clock that will always be toggling as long as you want functionality of the transceivers. Please let me know if you have any questions.  Thanks,  Ben