When using the LogiCORE Tri-mode Ethernet MAC Core in Virtex-4 with the RGMII interface, DCMs are instantiated in the top-level wrapper. How do the new Virtex-4 timing parameters affect this DCM? Is the DCM_STANDBY macro required?
For details on how different Virtex-4 stepping levels affect your design when targeting Virtex-4 devices, refer to (Xilinx Answer 21605).
For information on the new Virtex-4 timing parameters, see (Xilinx Answer 21127). Notice that the DCM_STANDBY macro is required to meet the DCM_INPUT_CLOCK_STOP and DCM_RESET requirements. The DCM_STANDBY macro can be manually instantiated or automatically inserted by ISE depending on the stepping level of the device that is being targeted, as indicated in (Xilinx Answer 21605).
If it is determined that manual insertion of the macro is required, refer (Xilinx Answer 21127) for more information on how to use the macro.
When used with the RGMII interface, the design example holds the DCM in reset whenever operating at 10/100 speeds. This will exceed the 10 second reset limit described by the DCM_RESET requirement, mentioned in the Answer Records above. Therefore, the DCM_STANDBY macro is necessary for this case.