The MMCM has some restrictions that must be adhered to. These are summarized in the MMCM electrical specifications in the UltraScale device data sheets [Ref 5] . In general, the major limitations are VCO operation range, input frequency, duty cycle programmability, and phase shift. In addition, there are connectivity limitations to other clocking elements (pins, GTs, and clock buffers). Cascading MMCMs can only occur through the clock routing network.