EP3C5 datasheet(88/348 Pages) ALTERA

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Chapter 5: Clock Networks and PLLs in the Cyclone III Device Family

PLL Reconfiguration

Cyclone III Device Handbook

December 2011

Altera Corporation

Volume 1

Figure 5–21 shows a functional simulation of the PLL reconfiguration feature.

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 rbypass bit 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 rselodd bit 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:

Figure 5–21. PLL Reconfiguration Scan Chain

scandata

scanclk

scanclkena

scandataout

configupdate

scandone

areset

Dn_old

D0_old

LSB

EP3C5 Datasheet(PDF) 88 Page - Altera Corporation

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