126.96.36.199. Automatic Switchover
M-Series I/O PLLs support a fully configurable clock switchover capability.
When the current reference clock is not present, the clock sense block automatically switches to the backup clock for I/O PLL reference. You can select a clock source as the backup clock by connecting it to the inclk1 port of the I/O PLL in your design.
The clock switchover circuit sends out three status signals—clkbad0, clkbad1, and activeclock—from the I/O PLL to implement a custom switchover circuit in the logic array.
In automatic switchover mode, the clkbad0 and clkbad1 signals indicate the status of the two clock inputs. When they are asserted, the clock sense block detects that the corresponding clock input has stopped toggling. These two signals are not valid if the frequency difference between inclk0 and inclk1 is greater than 20%.
The activeclock signal indicates which of the two clock inputs (inclk0 or inclk1) is being selected as the reference clock to the I/O PLL. When the frequency difference between the two clock inputs is more than 20%, the activeclock signal is the only valid status signal.
Use the switchover circuitry to automatically switch between inclk0 and inclk1 when the current reference clock to the I/O PLL stops toggling. You can switch back and forth between inclk0 and inclk1 any number of times when one of the two clocks fails and the other clock is available.
For example, in applications that require a redundant clock with the same frequency as the reference clock, the switchover state machine generates a signal (clksw) that controls the multiplexer select input. In this case, inclk1 becomes the reference clock for the I/O PLL.
When using automatic clock switchover mode, the following requirements must be satisfied:
- Both clock inputs must be running when the FPGA is configured.
- The period of the two clock inputs can differ by no more than 20%.
- The input clocks must meet the input jitter specifications and I/O standard specifications.
Glitches in the input clock may be seen as a greater than 20% difference in frequency between the input clocks.
If the current clock input stops toggling while the other clock is also not toggling, switchover is not initiated and the clkbad[0..1] signals are not valid. If both clock inputs are not the same frequency, but their period difference is within 20%, the clock sense block detects when a clock stops toggling. However, the I/O PLL may lose lock after the switchover is completed and needs time to relock.
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