5–26
Chapter 5: Clock Networks and PLLs in the Cyclone III Device Family
PLL Reconfiguration
■
Keep the secondary PLL in a reset state until the primary PLL has locked to ensure
the phase settings are correct on the secondary PLL.
You cannot connect either of the
inclk
ports of any PLLs in the cascaded scheme
to clock outputs from PLLs in the cascaded scheme.
■
PLL Reconfiguration
PLLs use several divide counters and different VCO phase taps to perform frequency
synthesis and phase shifts. In Cyclone III device family PLLs, 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
Cyclone III Device Handbook
Volume 1
July 2012 Altera Corporation
ALTERA [ ALTERA CORPORATION ]
