ep3c120f780c8nes Altera Corporation, ep3c120f780c8nes Datasheet - Page 131

• Manufacturer Part Number: ep3c120f780c8nes
• Description: Cyclone Iii Device Family
• Manufacturer: Altera Corporation

Figure 6–18. Clock Switchover Using the Clkswitch Control
Altera Corporation-Preliminary
March 2007
In this mode, the activeclock signal mirrors the clkswitch signal. As
both blocks are still functional during the manual switch, neither clkbad
signals go high. Since the switchover circuit is edge-sensitive, the falling
edge of the clkswitch signal does not cause the circuit to switch back
from inclk1 to inclk0. When the clkswitch signal goes high again,
the process repeats. The clkswitch signal and automatic switch only
work if the clock being switched to is available. If the clock is not
available, the state machine waits until the clock is available.
Manual Clock Switchover
Cyclone III PLLs support manual switchover, where the clkswitch
signal controls whether inclk0 or inclk1 is the input clock to the PLL.
The characteristics of the manual switchover is similar to the manual
override feature in an automatic clock switchover, where the switchover
circuit is edge-sensitive. When the clkswitch signal goes high, the
switchover sequence starts. The falling edge of the clkswitch signal
does not cause the circuit to switch back to the previous input clock.
Cyclone III Device Handbook, Volume 1
Hardware Features
6–31