PLLs in Stratix II and Stratix II GX Devices
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Applications that require a clock switchover feature and a small
frequency drift should use a low-bandwidth PLL. The
low-bandwidth PLL reacts slower than a high-bandwidth PLL to
reference input clock changes. When the switchover happens, a
low-bandwidth PLL propagates the stopping of the clock to the
output slower than a high-bandwidth PLL. A low-bandwidth PLL
filters out jitter on the reference clock. However, be aware that the
low-bandwidth PLL also increases lock time.
■
Stratix II and Stratix II GX device PLLs can use both the automatic
clock switchover and the clkswitchinput simultaneously.
Therefore, the switchover circuitry can automatically switch from the
primary to the secondary clock. Once the primary clock stabilizes
again, the clkswitchsignal can switch back to the primary clock.
During switchover, the PLL_VCOcontinues to run and slows down,
generating frequency drift on the PLL outputs. The clkswitch
signal controls switchover with its rising edge only.
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■
■
If the clock switchover event is glitch-free, after the switch occurs,
there is still a finite resynchronization period to lock onto a new clock
as the VCO ramps up. The exact amount of time it takes for the PLL
to relock is dependent on the PLL configuration. Use the PLL
programmable bandwidth feature to adjust the relock time.
If the phase relationship between the input clock to the PLL and
output clock from the PLL is important in your design, assert
aresetfor 10ns after performing a clock switchover. Wait for the
locked signal (or gated lock) to go high before re-enabling the output
clocks from the PLL.
Figure 1–25 shows how the VCO frequency gradually decreases
when the primary clock is lost and then increases as the VCO locks
on to the secondary clock. After the VCO locks on to the secondary
clock, some overshoot can occur (an over-frequency condition) in the
VCO frequency.
Figure 1–25. VCO Switchover Operating Frequency
Primary Clock Stops Running
Frequency Overshoot
Switchover Occurs
VCO Tracks Secondary Clock
ΔF
vco
Altera Corporation
July 2009
1–43
Stratix II Device Handbook, Volume 2
ALTERA [ ALTERA CORPORATION ]
