The paper deals with an oscillation based built-in self-test (OBIST) technique to test faults in complex CMOS digital circuits (CCDCs). It focuses on stuck-at-faults, open or short faults, parametric gate delay faults. The method converts complex CMOS digital circuit under test (CCDCUT) to an oscillator and the output pulses are measured for fix time duration. Discrepancy in the number of pulses is used to judge circuits with catastrophic faults as well as parametric gate delays beyond the

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... hold limit which is set by designer. The advantage of this method is it does not need external test vector input or complex response analyzer. Universal gates and CCDCs are used to assess and substantiate the usefulness of proposed method. The simulation results show that the proposed method is quite proficient to improve diagnostic accuracy. Fault coverage for catastrophic faults is almost 100%. For ±10% deviation in parameters outside tolerance range limit 100% fault coverage is obtained. The yield loss is around 5% in the tolerance range limit.