Circuitry for determining timing characteristics, for example, access time, setup time, hold time, recovery time and removal time, of as-manufactured digital circuit elements, such as latches, flip-flops and memory cells. Each element under test is embodied in variable-loop-path ring oscillator circuitry...http://www.google.de/patents/US8164966?utm_source=gb-gplus-sharePatent US8164966 - Variable-loop-path ring oscillator test circuit and systems and methods utilizing same
