@RobTuDelft The comment in the example is wrong. Maximum is 19455. For an input value between 19456 and 19531 you'll get 19531 Hz. Generally, the precision is low at the higher frequencies. So specifying a value of 19000 give 19011 Hz, 19200 -> 19230. So it looks as if the "real" periods are multiples of a small value like 200 ns.