P240 datasheet(79/348 Pages) ALTERA

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

5–19

Hardware Features

December 2011

Altera Corporation

Cyclone III Device Handbook

Volume 1

There are two ways to use the clock switchover feature:

■

Use the switchover circuitry for switching from inclk0 to inclk1 running at the

same frequency. For example, in applications that require a redundant clock with

the same frequency as the reference clock, the switchover state machine generates

a signal that controls the multiplexer select input shown in Figure 5–14. In this

case, inclk1 becomes the reference clock for the PLL. This automatic switchover

can switch back and forth between the inclk0 and inclk1 clocks any number of

times, when one of the two clocks fails and the other clock is available.

■

Use the clkswitch input for user- or system-controlled switch conditions. This is

possible for same-frequency switchover or to switch between inputs of different

frequencies. For example, if inclk0 is 66 MHz and inclk1 is 200 MHz, you must

control the switchover because the automatic clock-sense circuitry cannot monitor

primary and secondary clock frequencies with a frequency difference of more than

20%. This feature is useful when clock sources can originate from multiple cards

on the backplane, requiring a system-controlled switchover between frequencies

of operation. Choose the secondary clock frequency so the VCO operates in the

recommended frequency range. Also, set the M, N, and C counters accordingly to

keep the VCO operating frequency in the recommended range.

Figure 5–15 shows a waveform example of the switchover feature when using

automatic loss of clock detection. Here, the inclk0 signal remains low. After the

inclk0

signal remains low for approximately two clock cycles, the clock-sense

circuitry drives the clkbad[0] signal high. Also, because the reference clock signal is

not toggling, the switchover state machine controls the multiplexer through the clksw

signal to switch to inclk1.

Figure 5–15. Automatic Switchover Upon Clock Loss Detection (1)

Note to Figure 5–15:

(1) Switchover is enabled on the falling edge of inclk0 or inclk1, depending on which clock is available. In this figure,

switchover is enabled on the falling edge of inclk1.

inclk0

inclk1

muxout

clkbad0

clkbad1

(1)

activeclock

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