Finding Optimum Clock Frequencies for Aperiodic Test

Abstract

With scale down in technology, size and complexity of integrated circuits increase. The scan method is the most popular technique of testing sequential circuits today. In this method, ip- ops functionally form one or more shift registers in the test mode. Faults in the combinational logic can be tested by shifting test patterns in and out of the shift register. Larger circuits these days consist of many ip- ops and the scan design has large shift registers, which require many clock cycles for loading and unloading. An ATE is used to test these scan circuits for faults after fabrication. The testing cost of using an ATE increases directly with the time spent in testing the chip and adds to the nal cost of the chip. In general, power constraints do not allow speeding up of the clock during test. For the complex integrated circuits today, long test times are a concern. A recently proposed methodology [37] reduces the test time on ATE using an aperiodic clock with many di erent frequencies. In practice, the ATE generates only a limited number of frequencies and determining an optimum set of frequencies is a discrete optimization problem. This work discusses an algorithm to obtain the optimum set of frequencies for test time reduction. The algorithm is implemented by simulating ISCAS '89 benchmark circuits and veri ed on the Advantest T2000GS ATE located at Auburn University, Alabama. It is observed that by using just four optimum frequencies, that the tester allows, test time reductions of up to 52% can be obtained in some benchmark circuits.