Multiple frequency phase-locked loop clock generator with stable

Abstract

A phase-locked loop (PLL) clock generator circuit which is capable of changing the frequency of its outpt clock signal in a stable fashion. Selection of the frequency of the output clock signal is made by way of a selectable frequency divider coupled between the reference clock signal and an input of the PLL, with another frequency divider in the feedback loop of the PLL; each of these frequency dividers are selectable according to a signal on a select bus, translated by way of a ROM look-up table. The circuit also includes a multiplexer having a first input coupled to the PLL output, and a second input coupled to a stable clock signal, for example to the referenc clock signal or to the output of a fixed frequency PLL. The conrol input of the multiplexer is controlled by a state machine which monitors the select bus. Responsive to detection of a transition of the select bus, indicating a new frequency, the state machine issues a pulse to the control input of the multiplexer to cause it to select the stable clock signal for sufficient time to allow the PLL to acquire and lock onto the new frequency, after which the multiplexer again selects the PLL output as the output clock signal. As a result, the unstable and non-linear behavior at the PLL output does not appear at the output of the circuit, with a stable clock signal at a safe frequency appearing thereat during the PLL transitional cycles.