Chapter 5: Clock Networks and PLLs in the Cyclone III Device Family
5–15
Hardware Features
Hardware Features
Cyclone III device family PLLs support several features for general-purpose clock
management. This section discusses clock multiplication and division
implementation, phase shifting implementations, and programmable duty cycles.
Clock Multiplication and Division
Each Cyclone III device family PLL provides clock synthesis for PLL output ports
using M/(N*post-scale counter) scaling factors. The input clock is divided by a
pre-scale factor, N, and is then multiplied by the M feedback factor. The control loop
drives the VCO to match fIN (M/N). Each output port has a unique post-scale counter
that divides down the high-frequency VCO. For multiple PLL outputs with different
frequencies, the VCO value is the least common multiple of the output frequencies
that meets its frequency specifications. For example, if output frequencies required
from one PLL are 33 and 66 MHz, the Quartus II software sets the VCO to 660 MHz
(the least common multiple of 33 and 66 MHz in the VCO range). Then, the post-scale
counters scale down the VCO frequency for each output port.
There is one pre-scale counter, N, and one multiply counter, M, per PLL, with a range
of 1 to 512 for both M and N. The N counter does not use duty cycle control because
the purpose of this counter is only to calculate frequency division. There are five
generic post-scale counters per PLL that can feed GCLKs or external clock outputs.
These post-scale counters range from 1 to 512 with a 50% duty cycle setting. The
post-scale counters range from 1 to 256 with any non-50% duty cycle setting. The sum
of the high/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.
1
Phase alignment between output counters are determined using the tPLL_PSERR
specification.
July 2012 Altera Corporation
Cyclone III Device Handbook
Volume 1
ALTERA [ ALTERA CORPORATION ]
