Apparatus for dynamically switching the clock source of a data

Abstract

A data processing system includes a first and a second central system wherein the central processing unit (CPU) of the first central system is operatively connected to the system control unit (SCU) of the second central system and the CPU of the second central system is operatively connected to the SCU of the first central system, each central system including a service processor and a timing subsystem for providing clocking signals. Each central system includes an apparatus for distributing the clocking signals such that each central system is clocked from predetermined ones of the clocking signals in response to control signals from the service processors. The apparatus permits the distribution of the clocking signals to be switched dynamically while the central systems are operating, the switching of the clocking signals being accomplished without affecting the operation of the central systems. The apparatus comprises a generating element for generating local clocking signals. A first logic element generates a stop signal at a predetermined time relative to the local clocking signals in response to predetermined ones of the control signals. A gating element conditionally outputs the local clocking signals in response to the stop signal. A selector which receives remote clocking signals from the timing subsystem of the other central system selectively outputs the local clocking signals or the remote clocking signals in response to the predetermined control signals, the control signals being modified during a time period that the stop signal is active, thereby permitting the selector to select alternate clocking signals before the stop signal becomes inactive thereby switching the source of the clocking signals which are clocking the central systems without perturbing the tasks being executed in the central systems.