EP4CE30F29C7N Datasheet, PDF (89/488 Pages) Altera Corporation – Cyclone IV Device Handbook, Volume 1

Chapter 5: Clock Networks and PLLs in Cyclone IV Devices
Hardware Features
5–27
Post-Scale Counter Cascading
PLLs of Cyclone IV devices support post-scale counter cascading to create counters
larger than 512. This is implemented by feeding the output of one C counter into the
input of the next C counter, as shown in Figure 5–16.
Figure 5–16. Counter Cascading
VCO Output
C0
VCO Output
C1
VCO Output
C2
VCO Output
C3
VCO Output
C4
VCO Output
When cascading counters to implement a larger division of the high-frequency VCO
clock, the cascaded counters behave as one counter with the product of the individual
counter settings.
For example, if C0 = 4 and C1 = 2, the cascaded value is C0 × C1 = 8.
1 Post-scale counter cascading is automatically set by the Quartus II software in the
configuration file. Post-scale counter cascading cannot be performed using the PLL
reconfiguration.
Programmable Duty Cycle
The programmable duty cycle allows PLLs to generate clock outputs with a variable
duty cycle. This feature is supported on the PLL post-scale counters. You can achieve
the duty cycle setting by a low and high time count setting for the post-scale counters.
The Quartus II software uses the frequency input and the required multiply or divide
rate to determine the duty cycle choices. The post-scale counter value determines the
precision of the duty cycle. The precision is defined by 50% divided by the post-scale
counter value. For example, if the C0 counter is 10, steps of 5% are possible for duty
cycle choices between 5 to 90%.
Combining the programmable duty cycle with programmable phase shift allows the
generation of precise non-overlapping clocks.
PLL Control Signals
You can use the pfdena, areset, and locked signals to observe and control the PLL
operation and resynchronization.
f For more information about the PLL control signals, refer to the ALTPLL Megafunction
User Guide.
October 2012 Altera Corporation
Cyclone IV Device Handbook,
Volume 1