220.127.116.11. Automatic Switchover with Manual Override
In automatic switchover with manual override mode, you can use the clkswitch 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 the switchover using the clkswitch signal. The automatic clock sense circuitry cannot monitor clock input (inclk0 and inclk1) frequencies with a frequency difference of more than 20%.
This feature is useful when clock sources originate from multiple cards on the backplane, requiring a system-controlled switchover between frequencies of operation.
You must choose the backup clock frequency and set the M, N, and C counters so that the VCO operates within the recommended frequency range.
The following figure shows a clock switchover waveform controlled by the clkswitch signal. In this case, both clock sources are functional and inclk0 is selected as the reference clock. The clkswitch signal goes high, which starts the switchover sequence. On the falling edge of inclk0, the counter’s reference clock, muxout, is gated off to prevent clock glitching. On the falling edge of inclk1, the reference clock multiplexer switches from inclk0 to inclk1 as the PLL reference. The activeclock signal is asserted to indicate the clock that is currently feeding the PLL, which is inclk1.
In automatic override with manual switchover mode, the activeclock signal mirrors the clkswitch 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 clkswitch signal does not cause the circuit to switch back from inclk1 to inclk0. When the clkswitch signal goes high again, the process repeats.
The clkswitch 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.