24.3.2.1.2 External Clock

External clock (XCK) is used in synchronous slave mode operation. The XCK clock input is sampled on the peripheral clock frequency, and the maximum XCK clock frequency (fXCK) is limited by the following:

f X C K < f C L K _ P E R 4

For each high and low period, XCK clock cycles must be sampled twice by the peripheral clock. If the XCK clock has jitter, or if the high/low period duty cycle is not 50/50, the maximum XCK clock speed must be reduced accordingly.