The fractional divide in UltraScale™ and UltraScale+™ devices (where the resolution is 0.125) has been replaced in the Versal ACAP by a fractional sigmadelta module (SDM) in the feedback path. The additional 6bit resolution divider allows generation of finer granularity output clocks than was possible with UltraScale devices. The best jitter performance of an MMCM is obtained when using it in integer mode. When the fractional part of M+F is set to 0, integer mode is automatically chosen. When running the MMCM in fractional mode make sure the VCO runs at the maximum possible frequency for the application. Following examples are provided to highlight the mechanism while the Clock Wizard will automatically deal with this when fractional output frequencies are required. Use attributes for fractional divide:
CLKFBOUT_FRACT : integer := 0;
CLKFBOUT_MULT : integer := 42;
To put the focus on the fractional divide, the steps to calculate M, D, and other values are omitted from the following examples:
Example 1
Assume F_{CLKOUT} must be 296.703 MHz for an F_{CLKIN} of 27 MHz.
 D = 10 and M = 109


 F_{VCO} = 2967.046875 MHz from:
 F_{CLKOUT} is then 296.7046875 MHz (rounded to 296.705 MHz)
The previous calculation does not use the highest possible VCO frequency to generate the output clock frequency of 296.703 MHz. Recalculate for the highest possible VCO frequency.
 D = 14 and M = 153


 F_{VCO}= 4153.78125 MHz
 F_{CLKOUT} is then 296.69866 MHz (rounded to 296.699)
The first calculation can achieve 296.7046875 MHz and the second can achieve 296.69866 MHz. Both settings are slightly off from the target frequency of 296.703 MHz, which is due to the resolution of the fractional mode. To improve the frequency error, higher reference frequencies must be used.
Example 2
 D = 1, M = 100, and O = 9

 F_{VCO}= 3015 MHz from:
F_{CLKOUT} is then 335 MHz
Example 3


 Desired F_{CLKOUT} is 212.3457 MHz for an F_{CLKIN} of 50 MHz
 F_{VCOmax} = 4320 MHz
 The highest possible VCO frequency is obtained for a O value of:
 _{rounddown}(3420/212.3457) = 20


 Assume D = 1


 F must be an integer value, therefore take the _{rounddown}F_{IDEAL} or F = 60
Redo the calculations to get the real F_{CLKOUT}:
212.3457 MHz was required.