Cyclone® V Device Handbook: Volume 1: Device Interfaces and Integration

ID 683375
Date 10/18/2023
Document Table of Contents Guidelines

When implementing clock switchover in Cyclone® V PLLs, use the following guidelines:

  • Automatic clock switchover requires that the inclk0 and inclk1 frequencies be within 20% of each other. Failing to meet this requirement causes the clkbad[0] and clkbad[1] signals to not function properly.
  • When using manual clock switchover, the difference between inclk0 and inclk1 can be more than 100% (2×). However, differences in frequency, phase, or both, of the two clock sources will likely cause the PLL to lose lock. Resetting the PLL ensures that you maintain the correct phase relationships between the input and output clocks.
  • Both inclk0 and inclk1 must be running when the extswitch signal goes high to initiate the manual clock switchover event. Failing to meet this requirement causes the clock switchover to not function properly.
  • Applications that require a clock switchover feature and a small frequency drift must use a low-bandwidth PLL. When referencing input clock changes, the low-bandwidth PLL reacts more slowly than a high-bandwidth PLL. When switchover happens, a low-bandwidth PLL propagates the stopping of the clock to the output more slowly than a high-bandwidth PLL. However, be aware that the low-bandwidth PLL also increases lock time.
  • After a switchover occurs, there may be a finite resynchronization period for the PLL to lock onto a new clock. The time it takes for the PLL to relock depends on the PLL configuration.
  • The phase relationship between the input clock to the PLL and the output clock from the PLL is important in your design. Assert areset for at least 10 ns after performing a clock switchover. Wait for the locked signal to go high and be stable before re-enabling the output clocks from the PLL.
  • The VCO frequency gradually decreases when the current clock is lost and then increases as the VCO locks on to the backup clock, as shown in the following figure.
Figure 71. VCO Switchover Operating Frequency