A conversion circuit measures individual period lengths for periods of a clock signal. Two of the measured period lengths are selected and compared. The comparison operates to compare a first period length against a threshold set as a function of the second period length. The result of the comparison is indicative of the presence of a clock error. If the threshold is set less than the second period length, the comparison functions to detect a clock glitch. If the threshold is set more than the second period, the comparison functions to detect a loss of clock. The result of the comparison may be used to control further handling of the clock signal by, for example, blocking logic state changes in the clock signal for the length of one period in response to the detection of the clock error.