Measuring the Distribution of Metastable Upsets over Time

As modern ASICs comprise an increasing number of independently clocked subsystems that need to interact, the accurate reliability assessment of synchronizers becomes crucial. Traditionally the reliability of a synchronizer is characterized by the mean time between upsets (MTBU), and the relevant flip-flop parameters are specified in a way to support MTBU calculation. In this paper we claim that actually a deeper insight into the distribution of upsets over time is needed in order to make a reasonable prediction in the range of the high reliability values that are generally targeted. We present a measurement concept that appropriately extends state-of-the-art approaches so as to allow for an experimental assessment of the upset distribution over time. In this way the distribution function can be studied, and in particular the probability of upsets with low temporal distance -- which is the relevant one for high reliability -- can be identified. We implement our concept on three different FPGA platforms and present the selected results. The distribution function we obtain indicates that the assumption of a uniform or standard normal distribution, which one might be tempted to imply for lack of better information, is definitely not generally useful.