Embodiments of a clock synchronization unit of an All Digital Phase-Locked Loop (ADPLL), a successive approximation register (SAR) Time-to-Digital Converter (TDC) of an ADPLL and a method for clock synchronization in an ADPLL are disclosed. In one embodiment, a clock synchronization unit of an ADPLL includes a two-flop synchronizer, a phase frequency detector (PFD) connected to the two-flop synchronizer, and a synchronization control circuit configured to control the two-flop synchronizer and the PFD to perform clock synchronization between a reference clock input signal and a divided clock input signal and to control the two-flop synchronizer and the PFD to replace a performance of the clock synchronization between the reference clock input signal and the divided clock input signal with a PFD operation. Other embodiments are also described.