EP2C5T144I8N Datasheet, PDF (218/470 Pages) Altera Corporation – Cyclone II Device Handbook, Volume 1

Conclusion
In addition, the clock networks in the Cyclone II device support dynamic
selection of the clock source and also support a power-down mode where
clock networks that are not being used can easily be turned off, reducing
the overall power consumption of the device.
7–34
Cyclone II Device Handbook, Volume 1
Altera Corporation
February 2007