EP3C5 datasheet(78/274 Pages) ALTERA

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

Hardware Features

Cyclone III Device Handbook

July 2012

Altera Corporation

Volume 1

If you use the SignalTap® II tool to probe the locked signal before the D flip-flop, the

locked

signal goes low only when areset is deasserted. If the areset signal 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 inclk1 port 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

Switchover

State

Machine

Clock

Sense

n Counter

PFD

clkswitch

(provides manual

switchover support)

Activeclock

clkbad1

clkbad0

muxout

inclk0

inclk1

refclk

fbclk

clksw

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