5–34 Chapter 5: Clock Networks and PLLs in Cyclone IV Devices
PLL Reconfiguration
Cyclone IV Device Handbook, October 2012 Altera Corporation
Volume 1
PLL Reconfiguration
PLLs use several divide counters and different VCO phase taps to perform frequency
synthesis and phase shifts. In PLLs of Cyclone IV devices, you can reconfigure both
counter settings and phase shift the PLL output clock in real time. You can also change
the charge pump and loop filter components, which dynamically affects PLL
bandwidth. You can use these PLL components to update the output clock frequency,
PLL bandwidth, and phase shift in real time, without reconfiguring the entire FPGA.
The ability to reconfigure the PLL in real time is useful in applications that might
operate at multiple frequencies. It is also useful in prototyping environments,
allowing you to sweep PLL output frequencies and adjust the output clock phase
dynamically. For instance, a system generating test patterns is required to generate
and send patterns at 75 or 150 MHz, depending on the requirements of the device
under test. Reconfiguring PLL components in real time allows you to switch between
two such output frequencies in a few microseconds.
You can also use this feature to adjust clock-to-out (t
CO
) delays in real time by
changing the PLL output clock phase shift. This approach eliminates the need to
regenerate a configuration file with the new PLL settings.
PLL Reconfiguration Hardware Implementation
The following PLL components are configurable in real time:
■ Pre-scale counter (N)
■ Feedback counter (M)
■ Post-scale output counters (
C0–C4
)
■ Dynamically adjust the charge pump current (
I
CP
) and loop filter components
(R, C) to facilitate on-the-fly reconfiguration of the PLL bandwidth
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