4.2.10. Clock Switchover
The clock switchover feature allows the PLL to switch between two reference input clocks. Use this feature for clock redundancy or for a dual-clock domain application where a system turns to the redundant clock if the previous clock stops running. The design can perform clock switchover automatically when the clock is no longer toggling or based on a user control signal, extswitch.
Intel® Arria® 10 PLLs support the following clock switchover modes:
- Automatic switchover—The clock sense circuit monitors the current reference clock. If the current reference clock stops toggling, the reference clock automatically switches to inclk0 or inclk1 clock.
- Manual clock switchover—Clock switchover is controlled using the extswitch signal. When the extswitch signal pulse stays low for at least three clock cycles for the inclk being switched to, the reference clock to the PLL is switched from inclk0 to inclk1, or vice-versa.
- Automatic switchover with manual override—This mode combines automatic switchover and manual clock switchover. When the extswitch signal goes low, it overrides the automatic clock switchover function. As long as the extswitch signal is low, further switchover action is blocked.