A clock multiplexer (clock MUX) selects one of the several inputs and propagates that signal forward. Renesas offers several types of clock multiplexers that not only include a multiplexing function, but also clock divider and fanout buffer functions integrated on the same device. This not only reduces the total part count in a clock tree, but can also help improve the performance in many cases.
Download: Clock Distribution Overview (PDF | English, 日本語)
Download: Flexible Solutions for Fast Edge Rate and Low Phase Noise Requirements (PDF)