A programmable system-on-a-chip integrated circuit device comprises at least one of a crystal oscillator circuit, an RC oscillator circuit, and an external oscillator input. A clock conditioning circuit is selectively coupleable to one of the programmable logic block, the crystal oscillator circuit,...http://www.google.com.hk/patents/US7102391?utm_source=gb-gplus-share專利 US7102391 - Clock-generator architecture for a programmable-logic-based system on a chip
