EP2AGX95EF29C6N Datasheet, PDF (143/380 Pages) Altera Corporation – Device Interfaces and Integration

Chapter 5: Clock Networks and PLLs in Arria II Devices
PLLs in Arria II Devices
5–35
The VCO frequency reported by the Quartus II software is the value after the
post-scale counter divider (K).
Each PLL has one pre-scale counter (N) and one multiply counter (M) with a range of
1 to 512 for both M and N. The n counter does not use duty-cycle control because the
only purpose of this counter is to calculate frequency division. There are seven generic
post-scale counters in each PLL that can feed GCLKs, RCLKs, or external clock
outputs. These post-scale counters range from 1 to 512 with a 50% duty cycle setting.
The high- and low-count values for each counter ranges from 1 to 256. The sum of the
high- and low-count values chosen for a design selects the divide value for a given
counter.
The Quartus II software automatically chooses the appropriate scaling factors
according to the input frequency, multiplication, and division values entered into the
ALTPLL megafunction.
Post-Scale Counter Cascading
Arria II PLLs support post-scale counter cascading to create counters larger than 512.
This is automatically implemented in the Quartus II software by feeding the output of
one C counter into the input of the next C counter, as shown in Figure 5–32.
Figure 5–32. Counter Cascading for Arria II Devices
VCO Output
C0
VCO Output
C1
VCO Output
C2
VCO Output
C3
VCO Output
C4
VCO Output
Cn
(1)
Note to Figure 5–32:
(1) For Arria II GX devices, n = 6. For Arria II GZ devices, n = 6 or 9.
from preceding
post-scale counter
When cascading post-scale 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 = 40 and C1 = 20, the
cascaded value is C0  C1 = 800.
1 Post-scale counter cascading is set in the configuration file. You cannot accomplish
post-scale counter cascading with PLL reconfiguration.
July 2012 Altera Corporation
Arria II Device Handbook Volume 1: Device Interfaces and Integration