The possible clock sources for the MMCM include:
• IBUF – Global clock input buffer. The MMCM compensates the delay of this path. IBUF represents a global clock pin in the same region. The IBUF must be located at a global clock pin location.
• BUFGCTRL – Internal global clock buffer. The MMCM does not compensate the delay of this path.
• BUFGCE – Global clock buffer. The MMCM does not compensate the delay of this path.