5–28
Chapter 5: Clock Networks and PLLs in the Cyclone III Device Family
PLL Reconfiguration
Figure 5–21 shows a functional simulation of the PLL reconfiguration feature.
Figure 5–21. PLL Reconfiguration Scan Chain
scandata
Dn
D0
LSB
scanclk
scanclkena
scandataout
Dn_old
D0_old
Dn
configupdate
scandone
areset
1
When reconfiguring the counter clock frequency, the corresponding counter phase
shift settings cannot be reconfigured using the same interface. You can reconfigure
phase shifts in real time using the dynamic phase shift reconfiguration interface. If
you reconfigure the counter frequency, but wish to keep the same non-zero phase shift
setting (for example, 90°) on the clock output, you must reconfigure the phase shift
after reconfiguring the counter clock frequency.
Post-Scale Counters (C0 to C4)
You can configure multiply or divide values and duty cycle of post-scale counters in
real time. Each counter has an 8-bit high time setting and an 8-bit low time setting.
The duty cycle is the ratio of output high or low time to the total cycle time, which is
the sum of the two. Additionally, these counters have two control bits, rbypass, for
bypassing the counter, and rselodd, to select the output clock duty cycle.
When the rbypassbit is set to 1, it bypasses the counter, resulting in a divide by one.
When this bit is set to 0, the PLL computes the effective division of the VCO output
frequency based on the high and low time counters. For example, if the post-scale
divide factor is 10, the high and low count values is set to 5 and 5 respectively, to
achieve a 50–50% duty cycle. The PLL implements this duty cycle by transitioning the
output clock from high-to-low on the rising edge of the VCO output clock. However, a
4 and 6 setting for the high and low count values, respectively, would produce an
output clock with 40–60% duty cycle.
The rseloddbit indicates an odd divide factor for the VCO output frequency with a
50% duty cycle. For example, if the post-scale divide factor is three, the high and low
time count values are 2 and 1, respectively, to achieve this division. This implies a
67%–33% duty cycle. If you need a 50%–50% duty cycle, you must set the rselodd
control bit to 1 to achieve this duty cycle despite an odd division factor. The PLL
implements this duty cycle by transitioning the output clock from high-to-low on a
falling edge of the VCO output clock. When you set rselodd= 1, subtract 0.5 cycles
from the high time and add 0.5 cycles to the low time.
For example:
Cyclone III Device Handbook
Volume 1
July 2012 Altera Corporation
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