A circuit separately measures one or both of the rising-edge and falling-edge signal propagation delays through a signal path of interest. The greater of these delays can then be used to establish a worst-case delay for the signal path. The worst-case delay can be used, in turn, to create accurate timing...http://www.google.de/patents/US6134191?utm_source=gb-gplus-sharePatent US6134191 - Oscillator for measuring on-chip delays
