1. Aug 20071 Data/Clock Synchronization Fourteen ways to fool your synchronizer Ginosar, R.; Asynchronous Circuits and Systems, 2003. Proceedings. Ninth International Symposium on12-15 May 2003 Page(s):89 - 96 Digital Object Identifier 10.1109/ASYNC.2003.1199169

2. Aug 20072 Metastability If a data input to a latch/flip-flop does not meet setup/hold time requirement, output of latch/FF can become metastable (enter state between 0 and 1 for indeterminate period of time). When latching ASYNCHRONOUS data (data not synchronized to your clock), have to protect against this condition

3. Aug 20073 Two-flop Synchronizer Very safe synchronizer, but takes two clocks to receive data (long latency)

4. Aug 20074 Mean Time Between Failures for 2-flop Synchronizer tau – settling time constant of FF T – settling window – time between the two clock inputs to the two flops (clock period generally) Tw – parameter related to time window of susceptibility F A – synchronizer’s clock frequency F D – frequency of pushing data across the clock boundary MTBF is generally many EONS for 2-flop sync.

5. Aug 20075 Metastability numbers For 0.18 u process... Tw = 50 ps, tau = 10 ps, assume Fa = 200 Mhz, and data exchanged every 10 clock periods, then MTBF = e 500 / (2 * 10 5 ) = 10 204 years. The age of the universe is 10 10 years, so you should be safe.....

6. Aug 20076 Ignoring Metastability If you ignore metastablity, how often to does it occur? For 0.18 u process... Enter metastablity at rate of Tw x Fd x Fc Tw is about 50 ps, assume Fc = 200 MHz, and new data is sent every 1000 clock cycles (2000/sec = Fd), then get two metastability events per millisecond!