Mechanism for measurement of time duration between asynchronous events

Abstract

A novel and useful mechanism for measuring the time duration between asynchronous events. The mechanism utilizes two metastability resolvers, one for detecting the rising edge of the input signal and one for detecting its falling edge. The input signal is typically assumed to have some known nominal clock rate, but its exact frequency and phase (timing of transitions) are not known. Each of the two metastability resolvers comprises two branches of cascaded flip flops, each clocked off the rising edge and falling edge of a fast clock. Each metastability resolver functions to output an edge event signal and a clock phase signal indicating which edge of the fast clock the rising (or falling) edge of the data signal was closer to. The edge event signals are used to start and stop a counter clocked off the fast clock. The clock phase is used to correct (i.e. compensate) the counter value depending on which half cycle of the fast clock the rising and falling edge of the data signal arrived in. Thus, this measurement mechanism reduces its maximal timing error from a full cycle of the fast clock to only half of it.