Post-Scale Counter Cascading; Programmable Duty Cycle; Pll Control Signals - Altera Cyclone IV Device Handbook

Chapter 5: Clock Networks and PLLs in Cyclone IV Devices
Hardware Features

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. Counter Cascading
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
User
October 2012 Altera Corporation
VCO Output
VCO Output
VCO Output
VCO Output
VCO Output
VCO Output
Guide.
Figure 5–16.
C0
C1
C2
C3
C4
ALTPLL Megafunction
5–27
Cyclone IV Device Handbook,
Volume 1