Intel® Cyclone® 10 LP Core Fabric and General Purpose I/Os Handbook

ID 683777
Date 2/15/2023
Document Table of Contents Manual Override

If you are using the automatic switchover, you must switch input clocks with the manual override feature with the clkswitch input.

The following figure shows an example of a waveform illustrating the switchover feature when controlled by clkswitch. In this case, both clock sources are functional and inclk0 is selected as the reference clock. A low-to-high transition of the clkswitch signal starts the switchover sequence. The clkswitch signal must be high for at least three clock cycles (at least three of the longer clock period if inclk0 and inclk1 have different frequencies). On the falling edge of inclk0, the reference clock of the counter, muxout, is gated off to prevent any clock glitching. On the falling edge of inclk1, the reference clock multiplexer switches from inclk0 to inclk1 as the PLL reference, and the activeclock signal changes to indicate which clock is currently feeding the PLL.

In this mode, the activeclock signal mirrors the clkswitch signal. As both blocks are still functional during the manual switch, neither clkbad signals go 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 the automatic switch only works depending on the availability of the clock that is switched to. If the clock is unavailable, the state machine waits until the clock is available.

When CLKSWITCH = 1, it overrides the automatic switchover function. As long as clkswitch signal is high, further switchover action is blocked.

Figure 51. Example of Clock Switchover Using the clkswitch (Manual) Control