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On-chip Clock Testing and Frequency Measurement

This work presents a method to measure the frequency of an on-chip test clock in relation to a reference clock. Frequency measurement is accomplished by counting pulses of both test and reference clocks, albeit adjusting the reference clock pulse count to estimate the number of pulses that the test clock is expected to see. The proposed method places no constraints on the frequency relationship between the test and reference clocks which allows the reference clock frequency to be any multiple δ (1 <; δ ≤ 1) of the test clock frequency. Doing so allows a high degree of flexibility and a wide range of scenarios for which this approach could be deployed to measure the frequency of an unknown clock. Applications of this approach range from calibrating the frequency of on chip at speed test clocks for DFT, measurement of ppm of clocks subject to variations in process, temperature, spread spectrum effects among other considerations. The method also guarantees cycle to cycle accuracy in frequency measurement. Multiple on chips clocks can be tested using one instance of this method when the frequency information of all clocks to be tested is made available in specific register files.