Intel® Agilex™ Clocking and PLL User Guide

ID 683761
Date 11/09/2022

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Document Table of Contents 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, and C counters so that the VCO operates within the recommended operating frequency range. The Intel® Quartus® Prime software notifies you if a given combination of inclk0 and inclk1 frequencies cannot meet this requirement.

Figure 21. Clock Switchover Using the extswitch (Manual) ControlThis figure shows a clock switchover waveform controlled by the extswitch signal. In this case, both clock sources are functional and inclk0 is selected as the reference clock. The switchover sequence starts when the extswitch signal goes low. 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 I/O PLL reference. The activeclock signal changes to indicate the clock which is currently feeding the I/O PLL.

In automatic override with manual switchover mode, the activeclock signal inverts after the extswitch signal transitions from logic high to logic low. Since both clocks are still functional during the manual switch, neither clkbad signal goes high. Because the switchover circuit is negative-edge sensitive, the rising edge of the extswitch signal does not cause the circuit to switch back from inclk1 to inclk0. When the extswitch signal goes low 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.