Clock signal testing apparatus for use in a synchronous transmission

Abstract

An apparatus for use in a synchronous transmission system (STS) efficiently tests N clock signals provided from a device incorporated in the STS, wherein N is a positive integer larger than 1 and the N clock signals are represented by a first predetermined clock frequency. The N clock signals are first received by a counting device in response to a reset signal issued by a system controller in the STS to produce an error reference signal for each of the N received clock signals. And then, at a clock generator, a reference clock signal represented by a second predetermined clock frequency is provided. A first set of error detection signals for the N clock signals is derived based on the N clock signals, the reference clock signal and the error reference signals at a first error detection device. Thereafter, at a second error detection device, a second set of error detection signals for the N clock signals is obtained based on the reference clock signal and the error reference signals. Finally, the first and the second sets of error detection signals are logically combined at an error decision device to thereby produce information representing the status of each of the N clock signals.