5–18
Chapter 5: Clock Networks and PLLs in the Cyclone III Device Family
Hardware Features
If you use the SignalTap® II tool to probe the lockedsignal before the D flip-flop, the
lockedsignal goes low only when aresetis deasserted. If the aresetsignal is not
enabled, the extra logic is not implemented in the ALTPLL megafunction.
f
For more information about the PLL control signals, refer to the Phase-Locked Loop
(ALTPLL) Megafunction User Guide.
Clock Switchover
The clock switchover feature allows the PLL to switch between two reference input
clocks. Use this feature for clock redundancy or for a dual-clock domain application,
such as a system that turns on the redundant clock if the previous clock stops running.
Your design can automatically perform clock switchover when the clock is no longer
toggling, or based on the user control signal, clkswitch
.
Automatic Clock Switchover
Cyclone III device family PLLs support a fully configurable clock switchover
capability.
When the current reference clock is not present, the clock-sense block automatically
switches to the backup clock for PLL reference. The clock switchover circuit also sends
out three status signals—clkbad[0], clkbad[1], and activeclock—from the PLL to
implement a custom switchover circuit. You can select a clock source at the backup
clock by connecting it to the inclk1port of the PLL in your design.
Figure 5–14 shows the block diagram of the switchover circuit built into the PLL.
Figure 5–14. Automatic Clock Switchover Circuit
clkbad0
clkbad1
Activeclock
Switchover
State
Machine
Clock
Sense
clksw
clkswitch
(provides manual
switchover support)
inclk0
inclk1
n Counter
PFD
refclk
muxout
fbclk
Cyclone III Device Handbook
Volume 1
December 2011 Altera Corporation
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