5–20
Chapter 5: Clock Networks and PLLs in the Cyclone III Device Family
Hardware Features
Manual Override
If you are using the automatic switchover, you must switch input clocks with the
manual override feature with the clkswitchinput.
Figure 5–16 shows an example of a waveform illustrating the switchover feature
when controlled by clkswitch. In this case, both clock sources are functional and
inclk0is selected as the reference clock. A low-to-high transition of the clkswitch
signal starts the switchover sequence. The clkswitchsignal must be high for at least
three clock cycles (at least three of the longer clock period if inclk0and inclk1have
different frequencies). On the falling edge of inclk0, the reference clock of the counter,
muxout, is gated off to prevent any clock glitching. On the falling edge of inclk1, the
reference clock multiplexer switches from inclk0to inclk1as the PLL reference. On
the falling edge of inclk1, the reference clock multiplexer switches from inclk0to
inclk1as the PLL reference, and the activeclocksignal changes to indicate which
clock is currently feeding the PLL.
In this mode, the activeclocksignal mirrors the clkswitchsignal. As both blocks are
still functional during the manual switch, neither clkbadsignals go high. Because the
switchover circuit is positive edge-sensitive, the falling edge of the clkswitchsignal
does not cause the circuit to switch back from inclk1to inclk0. When the clkswitch
signal goes high again, the process repeats. The clkswitchsignal and the automatic
switch only works depending on the availability of the clock that is switched to. If the
clock is unavailable, the state machine waits until the clock is available.
1
If CLKSWITCH = 1, the automatic switchover function is overridden. While the
clkswitchsignal is high, further switchover action is blocked.
(1)
Figure 5–16. Clock Switchover Using the clkswitch Control
inclk0
inclk1
muxout
clkswitch
activeclock
clkbad0
clkbad1
Note to Figure 5–16:
(1) Both inclk0and inclk1must be running when the clkswitchsignal goes high to start a manual clock switchover
event.
Cyclone III Device Handbook
Volume 1
July 2012 Altera Corporation
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