The MMCM reference clock can be dynamically switched using the CLKINSEL pin. The switching is done asynchronously. After the clock switches, the MMCM is likely to lose LOCKED and automatically lock onto the new clock. Therefore, after the clock switches, the MMCM must be reset. The MMCM clock MUX switching is shown in the following figure. The CLKINSEL signal directly controls the MUX. No synchronization logic is present.
Figure 1. Input Clock Switching
Missing Input Clock or Feedback Clock
When the input clock or feedback clock is lost, the CLKINSTOPPED or CLKFBSTOPPED status signal is asserted. The MMCM deasserts the LOCKED signal. After the clock returns, the CLKINSTOPPED signal is deasserted and a RESET must be applied.