15.4.4 Synchronous Clock Operation

When synchronous mode is used (UMSEL = 1), the XCKn pin will be used as either clock input (Slave) or clock output (Master). The dependency between the clock edges and data sampling or data change is the same. The basic principle is that data input (on RxDn) is sampled at the opposite XCKn clock edge of the edge the data output (TxDn) is changed.

The UCPOL bit UCRSC selects which XCKn clock edge is used for data sampling and which is used for data change. As the above timing diagram shows, when UCPOL is zero, the data will be changed at rising XCKn edge and sampled at falling XCKn edge. If UCPOL is set, the data will be changed at falling XCKn edge and sampled at rising XCKn edge.