Method of switching between a first and second clock signal by establishing a trigger time in which both signals are considered to be substantially in-phase

Abstract

A system and method for observing the two clocking phase signals, finding a point in time when said signals have a phase coincidence which is good enough for fulfilling a phase difference requirement (e.g. 20 ps), and switching from one clock source to the other. The essential idea is not to compare the phases directly but to generate an auxiliary signal out of the two clock signals which is easier to handle in order to find that desired point in time and which reflects all desired properties of the time dependent phase shift between said clock signals. At a predetermined location in the cycle of both clock signals (e.g. its positive transition) a pulse is generated out of each of the clock signals with matched identical delay elements located very close to each other on the same chip for both signals. As they match they produce exactly the same pulse widths. The absolute length of the pulse width is of minor relevance as long as the length of the pulses is the same within close limits. Both signals are ANDed. Thus, in the resulting signal a pulse emerge at every positive transition of the oscillator clocks when the phase alignment of the clock signals is closer than the width of the transition pulse. When the alignment is bad—no signal will be produced.