Measuring Deep Metastability and Its Effect on Synchronizer Performance

Present measurement techniques do not allow synchronizer reliability to be measured in the region of most interest, that is, beyond the first half cycle of the synchronizer clock. We describe methods of extending the measurement range, in which the number of metastable events generated is increased by four orders of magnitude and events with long metastable times are selected from the large number of more normal events. The relationship found between input times and the resulting output times is dependent on accurate measurement of input time distributions with deviations of less than 10 ps. We show how the distribution of to clock times at the input can be characterized in the presence of noise and how predictions of failure rates for long synchronizer times can be made. Anomalies such as the increased failure rates in a master-slave synchronizer produced by the back edge of the clock are explained and demonstrated.