Test clock generating apparatus

A test clock generating apparatus is provided in the invention. The test clock generating apparatus includes an at-speed clock generator and a multiplexer. The at-speed clock generator is for receiving a reference clock signal and a scan chain enable signal and outputting an at-speed clock signal. The frequency of the at-speed clock signal is substantially the same with that of the reference clock signal. The multiplexer is for receiving the at-speed clock signal and a scan chain clock signal and outputting a test clock signal according to the scan chain enable signal. The frequency of the reference clock signal is higher than that of the scan chain clock.