Each half period of the external clock applied must be longer than one system clock cycle to ensure correct
sampling. The external clock must be guaranteed to have less than half the system clock frequency (fExtClk <
fclk_I/O/2) given a 50/50% duty cycle. Since the edge detector uses sampling, the maximum frequency of an
external clock it can detect is half the sampling frequency (Nyquist sampling theorem). However, due to
variation of the system clock frequency and duty cycle caused by Oscillator source (crystal, resonator, and
capacitors) tolerances, it is recommended that maximum frequency of an external clock source is less than
fclk_I/O/2.5.