AR# 32625: Serial Transceivers - How to use the reference clock in fabric.
Serial Transceivers - How to use the reference clock in fabric.
Virtex-5 GTP/GTX: There are dedicated clock paths to supply the GTP and GTX transceivers with a high quality reference clock. Unfortunately, there is no direct routing that can be utilized to use this clock in fabric. This answer record discusses methods by which the user application can gain access to a version of the dedicated reference clock.
Virtex-6 GTX: The REFCLK can be used in the fabric directly without having to go through the GTX.
The reference clock routing provides dedicated paths between the reference clock input pins and the CLKIN port of the GTP or GTX transceivers. There is also an output from the transceivers called REFCLKOUT which is a buffered version of CLKIN that can be used in fabric.
In the case where transceivers in the design are using the appropriate clock, it is a simple matter to use REFCLKOUT from those transceivers as a clock source for any other portion of the design.
Cases where the desired clock is not being used by a transceiver, the only way to pipe the clock into fabric is to instantiate a GTP, provide CLKIN with the clock in question and use REFCLKOUT as the fabric source for this clock. To avoid unnecessary power usage, RXPOWERDOWN and TXPOWERDOWN can be tied to 2'b11.