When I run through PAR or perform a static timing analysis (Timing Analyzer or TRACE) on my design, the following warning occurs:
"WARNING:Timing:2797 - The input clock user_CLKIN_IBUFG to DCM u1 has a period
(frequency) specification of 100000 ps (10.00 Mhz). This violates the
maximum period (minimum frequency) of 41670 ps (24.00 Mhz)."
In most cases, the minimum frequency of a DCM is 24 MHz in low frequency mode. However, when CLKFX is used without CLKFB, the minimum frequency is 1 MHz. The above warning message is generated regardless of the CLKFB usage. Why?
This warning can be safely ignored if you are using only DFS outputs (CLKFX and CLKFX180) without a feedback on the DCM.
This issue will be fixed in the next major design tools release.