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Chapter 5: Clock Networks and PLLs in Cyclone IV Devices
5–27
Hardware Features
October 2012
Altera Corporation
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
.
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
.
Figure 5–16. Counter Cascading
C0
C1
C2
C3
C4
VCO O
u
tp
u
t
VCO O
u
tp
u
t
VCO O
u
tp
u
t
VCO O
u
tp
u
t
VCO O
u
tp
u
t
VCO O
u
tp
u
t
Содержание Cyclone IV
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Страница 164: ...III 2 Section III System Integration Cyclone IV Device Handbook March 2016 Altera Corporation Volume 1...
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