On-Chip Measurement of Deep Metastability in Synchronizers

A deep metastability measurement scheme has been implemented on chip using digital circuits with 0.18 mum technology. Compared with previous off-chip implementations using analog circuits, the on-chip implementation allows integration of both the synchronizer circuits and the measurement method, and eliminates high-speed off-chip paths which are a source of inaccuracy. It also makes control at the picosecond level easier because of the inherent stability of digital integrating counters and digital delay lines. Our results show that the digital delay line used to adjust the data to clock times is controllable to an increment of 0.1 ps, and the input time distribution is 5.2 ps compared with 7.6 ps for the analog version. Because of the use of high and low counters, we can control the ratio of high to low outputs so that the actual input distribution can be measured to within better than 1 ps. The metastability time constant tau has been measured down to 10-17 s which corresponds to an mean time between failures (MTBF) of 100 seconds in an experimental time of 10 minutes and can be extended to a lower level by increasing the measurement time. Our results also show that a new synchronizer circuit designed for robustness to variation in Vdd performed at least as well as the Jamb Latch at all values of Vdd, and is more than 20% faster when Vdd was reduced by 25%.