Phase locked loop made operative when stable input sync signal is

Abstract

An oscillator control system includes a phase-locked loop having a programmable frequency divider operative within the loop. A switch is further included within the phase-locked loop to permit the loop to be open or closed in response to an input signal. A counter accumulates oscillator clock signal counts between each successive rising edge portion of the applied sync signal. A pair of shift registers sequentially store successive clock signal counts for the current and previous sync signal intervals. The output counts are compared for consistency by producing a difference signal therebetween which is utilized to control the phase-locked loop switch and close it once the consistency of sync signal has been established. The output counts of the shift registers are combined and used to provide a scaling factor which sets the frequency division of the programmable frequency divider within the phase-locked loop and properly scales the oscillator frequency to the sync signal frequency.