Tip: The
report_qor_suggestions Tcl command flags this issue.When configuring an MMCM for frequency synthesis, Xilinx recommends configuring the MMCM to achieve the lowest output jitter on the clocks. Optimize the MMCM settings to run at the highest possible voltage-controlled oscillator (VCO) frequency that meets the allowed operating range for the device. The following equations show the relationship between VCO frequency, M (multiplier), D (divider), and O (output divider) settings to both the input and output clock frequencies:
Tip: You can increase the VCO
frequency by increasing M, lowering D, or both and compensating for the change in
frequency by increasing O. Increases in VCO frequency negatively affects the power
dissipation from the MMCM or PLL.
Different architectures have different VCO frequency maximums. Therefore,
Xilinx recommends regenerating clocking components to be optimal
for your target architecture. Xilinx recommends
using the Clocking Wizard to automatically calculate M and D values along with the VCO
frequency to properly configure an MMCM for the target device.
Tip: When using the
Clocking Wizard from the IP catalog, make sure that Jitter Optimization Setting is
set to the Minimize Output Jitter, which provides the higher VCO frequency. In
addition, performing marginal changes to the desired output clock frequency can
allow for an even higher VCO frequency to further reduce clock
uncertainty.