126.96.36.199. Automatic Switchover with Manual Override
In automatic switchover with manual override mode, you can use the extswitch signal for user- or system-controlled switch conditions. You can use this mode for same-frequency switchover, or to switch between inputs of different frequencies.
For example, if inclk0 is 66 MHz and inclk1 is 200 MHz, you must control switchover using the extswitch signal. The automatic clock-sense circuitry cannot monitor clock input (inclk0 and inclk1) frequencies with a frequency difference of more than 100% (2×).
This feature is useful when the clock sources originate from multiple cards on the backplane, requiring a system-controlled switchover between the frequencies of operation.
You must choose the backup clock frequency and set the M, N, C, and K counters so that the VCO operates within the recommended operating frequency range. The ALTERA_PLL IP Catalog notifies you if a given combination of inclk0 and inclk1 frequencies cannot meet this requirement.
In automatic override with manual switchover mode, the activeclock signal mirrors the extswitch signal. Since both clocks are still functional during the manual switch, neither clkbad signal goes high. Because the switchover circuit is positive-edge sensitive, the falling edge of the extswitch signal does not cause the circuit to switch back from inclk1 to inclk0. When the extswitch signal goes high again, the process repeats.
The extswitch signal and automatic switch work only if the clock being switched to is available. If the clock is not available, the state machine waits until the clock is available.
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