This slide shows the components which are used to build a clock tree and some basic timing concepts. These concepts will help in selecting the most suitable component for a clock tree. The first part of timing concepts is to distinguish between Free-running clocks versus Synchronous clocks. Free running applications require independent clocks without any special phase-lock or synchronization requirements. Examples include standard processors, memory controllers, SoCs, and peripheral components (e.g., USB, PCI Express switches). Shown on the right of this slide are three examples of free running clock trees. Notice that all clock trees are using a crystal or crystal oscillators as the reference source. The output frequencies in these clock trees are independent and not synchronized.