5–16
Chapter 5: Clock Networks and PLLs in the Cyclone III Device Family
Hardware Features
Post-Scale Counter Cascading
Cyclone III device family PLLs 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 5–12.
Figure 5–12. 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.
Cyclone III Device Handbook
Volume 1
July 2012 Altera Corporation
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