Glitchless frequency-adjustable ring oscillator

Abstract

A clock generation circuit includes a reference clock for putting out a stable reference clocking signal. A digital ring oscillator includes a series circuit loop having at least one inverting gate and a programmable delay line of plural delays formed a series of tapped digital transmission gates connected between an output and an input of the inverting gate. A multiplexer selects among the series of taps in accordance with a tap selection signal. A clock monitoring circuit is connected to compare the clock output with a stable reference clocking signal to produce a digital clock cycle count. A programmed microcontroller generates the tap selection value as a function of the digital clock cycle count and a desired clock output frequency set point. And, a synchronization circuit synchronizes tap selection value applied to the multiplexer in relation to the present, adjustable clocking signal, and to a logical state of a successor, adjustable clocking signal to be put out by the digital ring oscillator following the tap selection, in order to avoid glitches and without interrupting oscillation.