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

ID 683777
Date 2/15/2023
Public
Document Table of Contents

4.2.9.1. Automatic Clock Switchover

The Intel® Cyclone® 10 LP 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 PLL reference. The clock switchover circuit also sends out three status signals—clkbad0, clkbad1, and activeclock—from the PLL to implement a custom switchover circuit. You can select a clock source at the backup clock by connecting it to the inclk1 port of the PLL in your design.

Figure 49. Automatic Clock Switchover Circuit Block DiagramThis figure shows a block diagram of the automatic switchover circuit built into the PLL.

There are two ways to use the clock switchover feature:

  • Use the switchover circuitry for switching from inclk0 to inclk1 running at the same frequency. For example, in applications that require a redundant clock with the same frequency as the reference clock, the switchover state machine generates a signal that controls the multiplexer select input. In this case, inclk1 becomes the reference clock for the PLL. This automatic switchover can switch back and forth between the inclk0 and inclk1 clocks any number of times, when one of the two clocks fails and the other clock is available.
  • Use the clkswitch input for user- or system-controlled switch conditions. This is possible 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 because the automatic clock-sense circuitry cannot monitor primary and secondary clock frequencies with a frequency difference of more than 20%. This feature is useful when clock sources can originate from multiple cards on the backplane, requiring a system-controlled switchover between frequencies of operation. Choose the secondary clock frequency so the VCO operates in the recommended frequency range. Also, set the M, N, and C counters accordingly to keep the VCO operating frequency in the recommended range.
Figure 50. Example of Automatic Switchover After Loss of Clock DetectionThis figure shows an example waveform of the switchover feature when using automatic loss of clock detection. In this example, the inclk0 signal remains low. After the inclk0 signal remains low for approximately two clock cycles, the clock sense circuitry drives the clkbad[0] signal high. Since the reference clock signal is not toggling, the switchover state machine controls the multiplexer through the clksw signal to switch to the backup clock, inclk1.