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We propose a new design for testability approach for testing clock faults of next generation high performance microprocessors. In fact, it has been shown that conventional manufacturing test is unable to guarantee their detection, although they could compromise the effectiveness of delay fault testing, as well as the microprocessor correct operation in the field. These conditions will of course worsen with technology scaling, due to the expected increase in fault likelihood, included clock faults. To deal with these problems we propose a design for testability approach that, by means of simple modifications to conventional clock buffers, allows clock fault detection through any conventional manufacturing test approach. This is achieved at the cost of very low increase in area and power consumption of clock buffers, and with no additional test cost or impact on the microprocessor performance and in-field operation. We then introduce a possible further modification to clock buffers that, at additional limited costs in terms of area and power consumption, allows their calibration after fabrication in order to compensate for parameter variations possibly occurring during manufacturing, thus minimizing the likelihood of either false test fails, or test misses. As an example, we show the application of our approach to the clock distribution network of the Pentium® 4 microprocessor (Other names and brands may be claimed as property of others). However, it can be applied to the clock distribution of any high performance ASIC, or microprocessor.